usb 2.0 english
Universal Serial Bus
Specification
Compaq
Hewlett-Packard
Intel
Lucent
Microsoft
NEC
Philips
Revision 2.0
April 27, 2000
Universal Serial Bus Specification Revision 2.0
Scope of this Revision
The 2.0 revision of the specification is intended for product design. Every attempt has been made to ensure a
consistent and implementable specification. Implementations should ensure compliance with this revision.
Revision History
Revision
Issue Date
Comments
0.7
November 11, 1994
Supersedes 0.6e.
0.8
December 30, 1994
Revisions to Chapters 3-8, 10, and 11. Added
appendixes.
0.9
April 13, 1995
Revisions to all the chapters.
0.99
August 25, 1995
Revisions to all the chapters.
1.0 FDR
November 13, 1995
Revisions to Chapters 1, 2, 5-11.
1.0
January 15, 1996
Edits to Chapters 5, 6, 7, 8, 9, 10, and 11 for
consistency.
1.1
September 23, 1998
Updates to all chapters to fix problems identified.
2.0 (draft 0.79)
October 5, 1999
Revisions to chapters 5, 7, 8, 9, 11 to add high
speed.
2.0 (draft 0.9)
December 21, 1999
Revisions to all chapters to add high speed.
2.0
April 27, 2000
Revisions for high-speed mode.
Universal Serial Bus Specification
Copyright © 2000, Compaq Computer Corporation,
Hewlett-Packard Company, Intel Corporation, Lucent Technologies Inc,
Microsoft Corporation, NEC Corporation, Koninklijke Philips Electronics N.V.
All rights reserved.
INTELLECTUAL PROPERTY DISCLAIMER
THIS SPECIFICATION IS PROVIDED TO YOU “AS IS” WITH NO WARRANTIES WHATSOEVER,
INCLUDING ANY WARRANTY OF MERCHANTABILITY, NON-INFRINGEMENT, OR FITNESS FOR
ANY PARTICULAR PURPOSE. THE AUTHORS OF THIS SPECIFICATION DISCLAIM ALL LIABILITY,
INCLUDING LIABILITY FOR INFRINGEMENT OF ANY PROPRIETARY RIGHTS, RELATING TO USE
OR IMPLEMENTATION OF INFORMATION IN THIS SPECIFICATION. THE PROVISION OF THIS
SPECIFICATION TO YOU DOES NOT PROVIDE YOU WITH ANY LICENSE, EXPRESS OR IMPLIED,
BY ESTOPPEL OR OTHERWISE, TO ANY INTELLECTUAL PROPERTY RIGHTS.
All product names are trademarks, registered trademarks, or servicemarks of their respective owners.
Please send comments via electronic mail to [email protected]
For industry information, refer to the USB Implementers Forum web page at http://www.usb.org
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Universal Serial Bus Specification Revision 2.0
Acknowledgement of USB 2.0 Technical Contribution
The authors of this specification would like to recognize the following people who participated in the USB
2.0 Promoter Group technical working groups. We would also like to thank others in the USB 2.0
Promoter companies and throughout the industry who contributed to the development of this specification.
Hub Working Group
John Garney
Ken Stufflebeam
David Wooten
Matt Nieberger
John Howard
Venkat Iyer
Steve McGowan
Geert Knapen
Zong Liang Wu
Jim Clee
Jim Guziak
Dave Thompson
John Fuller
Nathan Sherman
Mark Williams
Nobuo Furuya
Toshimi Sakurai
Moto Sato
Katsuya Suzuki
Intel Corporation (Chair/Editor)
Compaq Computer Corporation
Compaq Computer Corporation
Hewlett-Packard Company
Intel Corporation
Intel Corporation
Intel Corporation
Royal Philips Electronics
Royal Philips Electronics
Lucent Technologies Inc
Lucent Technologies Inc
Lucent Technologies Inc
Microsoft Corporation
Microsoft Corporation
Microsoft Corporation
NEC Corporation
NEC Corporation
NEC Corporation
NEC Corporation
Electrical Working Group
Jon Lueker
David Wooten
Matt Nieberger
Larry Taugher
Venkat Iyer
Steve McGowan
Mike Pennell
Todd West
Gerrit den Besten
Marq Kole
Zong Liang Wu
Jim Clee
Jim Guziak
Par Parikh
Dave Thompson
Ed Giaimo
Mark Williams
Toshihiko Ohtani
Kugao Ouchi
Katsuya Suzuki
Toshio Tasaki
Intel Corporation (Chair/Editor)
Compaq Computer Corporation
Hewlett-Packard Company
Hewlett-Packard Company
Intel Corporation
Intel Corporation
Intel Corporation
Intel Corporation
Royal Philips Electronics
Royal Philips Electronics
Royal Philips Electronics
Lucent Technologies Inc
Lucent Technologies Inc
Lucent Technologies Inc
Lucent Technologies Inc
Microsoft Corporation
Microsoft Corporation
NEC Corporation
NEC Corporation
NEC Corporation
NEC Corporation
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Universal Serial Bus Specification Revision 2.0
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Universal Serial Bus Specification Revision 2.0
Contents
CHAPTER 1 INTRODUCTION
1.1
Motivation .............................................................................................................................................. 1
1.2
Objective of the Specification ............................................................................................................... 1
1.3
Scope of the Document .......................................................................................................................... 2
1.4
USB Product Compliance ..................................................................................................................... 2
1.5
Document Organization ........................................................................................................................ 2
CHAPTER 2 TERMS AND ABBREVIATIONS
CHAPTER 3 BACKGROUND
3.1
Goals for the Universal Serial Bus ..................................................................................................... 11
3.2
Taxonomy of Application Space......................................................................................................... 12
3.3
Feature List .......................................................................................................................................... 13
CHAPTER 4 ARCHITECTURAL OVERVIEW
4.1 USB System Description ..................................................................................................................... 15
4.1.1 Bus Topology ................................................................................................................................. 16
4.2 Physical Interface ................................................................................................................................ 17
4.2.1 Electrical......................................................................................................................................... 17
4.2.2 Mechanical ..................................................................................................................................... 18
4.3 Power .................................................................................................................................................... 18
4.3.1 Power Distribution ......................................................................................................................... 18
4.3.2 Power Management ........................................................................................................................ 18
4.4
Bus Protocol ......................................................................................................................................... 18
4.5 Robustness............................................................................................................................................ 19
4.5.1 Error Detection ............................................................................................................................... 19
4.5.2 Error Handling................................................................................................................................ 19
4.6 System Configuration.......................................................................................................................... 19
4.6.1 Attachment of USB Devices........................................................................................................... 20
4.6.2 Removal of USB Devices............................................................................................................... 20
4.6.3 Bus Enumeration ............................................................................................................................ 20
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4.7 Data Flow Types ...................................................................................................................................20
4.7.1 Control Transfers.............................................................................................................................21
4.7.2 Bulk Transfers .................................................................................................................................21
4.7.3 Interrupt Transfers...........................................................................................................................21
4.7.4 Isochronous Transfers .....................................................................................................................21
4.7.5 Allocating USB Bandwidth.............................................................................................................21
4.8 USB Devices ..........................................................................................................................................22
4.8.1 Device Characterizations.................................................................................................................22
4.8.2 Device Descriptions ........................................................................................................................22
4.9
USB Host: Hardware and Software...................................................................................................24
4.10 Architectural Extensions......................................................................................................................24
CHAPTER 5 USB DATA FLOW MODEL
5.1
Implementer Viewpoints......................................................................................................................25
5.2 Bus Topology ........................................................................................................................................27
5.2.1 USB Host ........................................................................................................................................27
5.2.2 USB Devices ...................................................................................................................................28
5.2.3 Physical Bus Topology....................................................................................................................29
5.2.4 Logical Bus Topology.....................................................................................................................30
5.2.5 Client Software-to-function Relationship........................................................................................31
5.3 USB Communication Flow ..................................................................................................................31
5.3.1 Device Endpoints ............................................................................................................................33
5.3.2 Pipes ................................................................................................................................................34
5.3.3 Frames and Microframes.................................................................................................................36
5.4 Transfer Types......................................................................................................................................36
5.4.1 Table Calculation Examples............................................................................................................37
5.5 Control Transfers .................................................................................................................................38
5.5.1 Control Transfer Data Format .........................................................................................................38
5.5.2 Control Transfer Direction ..............................................................................................................39
5.5.3 Control Transfer Packet Size Constraints........................................................................................39
5.5.4 Control Transfer Bus Access Constraints........................................................................................40
5.5.5 Control Transfer Data Sequences....................................................................................................43
5.6 Isochronous Transfers..........................................................................................................................44
5.6.1 Isochronous Transfer Data Format..................................................................................................44
5.6.2 Isochronous Transfer Direction.......................................................................................................44
5.6.3 Isochronous Transfer Packet Size Constraints ................................................................................44
5.6.4 Isochronous Transfer Bus Access Constraints ................................................................................47
5.6.5 Isochronous Transfer Data Sequences.............................................................................................47
5.7 Interrupt Transfers ..............................................................................................................................48
5.7.1 Interrupt Transfer Data Format .......................................................................................................48
5.7.2 Interrupt Transfer Direction ............................................................................................................48
5.7.3 Interrupt Transfer Packet Size Constraints......................................................................................48
5.7.4 Interrupt Transfer Bus Access Constraints......................................................................................49
5.7.5 Interrupt Transfer Data Sequences ..................................................................................................52
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5.8 Bulk Transfers ..................................................................................................................................... 52
5.8.1 Bulk Transfer Data Format............................................................................................................. 52
5.8.2 Bulk Transfer Direction.................................................................................................................. 52
5.8.3 Bulk Transfer Packet Size Constraints ........................................................................................... 53
5.8.4 Bulk Transfer Bus Access Constraints ........................................................................................... 53
5.8.5 Bulk Transfer Data Sequences ....................................................................................................... 55
5.9 High-Speed, High Bandwidth Endpoints........................................................................................... 56
5.9.1 High Bandwidth Interrupt Endpoints ............................................................................................. 56
5.9.2 High Bandwidth Isochronous Endpoints ........................................................................................ 57
5.10 Split Transactions ................................................................................................................................ 58
5.11 Bus Access for Transfers..................................................................................................................... 58
5.11.1 Transfer Management..................................................................................................................... 59
5.11.2 Transaction Tracking...................................................................................................................... 61
5.11.3 Calculating Bus Transaction Times................................................................................................ 63
5.11.4 Calculating Buffer Sizes in Functions and Software ...................................................................... 65
5.11.5 Bus Bandwidth Reclamation .......................................................................................................... 65
5.12 Special Considerations for Isochronous Transfers ........................................................................... 65
5.12.1 Example Non-USB Isochronous Application................................................................................. 66
5.12.2 USB Clock Model .......................................................................................................................... 69
5.12.3 Clock Synchronization ................................................................................................................... 71
5.12.4 Isochronous Devices....................................................................................................................... 71
5.12.5 Data Prebuffering ........................................................................................................................... 80
5.12.6 SOF Tracking ................................................................................................................................. 81
5.12.7 Error Handling................................................................................................................................ 81
5.12.8 Buffering for Rate Matching .......................................................................................................... 82
CHAPTER 6 MECHANICAL
6.1
Architectural Overview....................................................................................................................... 85
6.2
Keyed Connector Protocol.................................................................................................................. 85
6.3
Cable ..................................................................................................................................................... 86
6.4 Cable Assembly.................................................................................................................................... 86
6.4.1 Standard Detachable Cable Assemblies ......................................................................................... 86
6.4.2 High-/full-speed Captive Cable Assemblies................................................................................... 88
6.4.3 Low-speed Captive Cable Assemblies ........................................................................................... 90
6.4.4 Prohibited Cable Assemblies.......................................................................................................... 92
6.5 Connector Mechanical Configuration and Material Requirements................................................ 93
6.5.1 USB Icon Location ......................................................................................................................... 93
6.5.2 USB Connector Termination Data ................................................................................................. 94
6.5.3 Series “A” and Series “B” Receptacles .......................................................................................... 94
6.5.4 Series “A” and Series “B” Plugs .................................................................................................... 98
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6.6 Cable Mechanical Configuration and Material Requirements ......................................................102
6.6.1 Description ....................................................................................................................................102
6.6.2 Construction ..................................................................................................................................103
6.6.3 Electrical Characteristics...............................................................................................................105
6.1.4 Cable Environmental Characteristics ............................................................................................106
6.1.5 Listing ...........................................................................................................................................106
6.7 Electrical, Mechanical, and Environmental Compliance Standards .............................................106
6.7.1 Applicable Documents ..................................................................................................................114
6.8
USB Grounding ..................................................................................................................................114
6.9
PCB Reference Drawings...................................................................................................................114
CHAPTER 7 ELECTRICAL
7.1 Signaling ..............................................................................................................................................119
7.1.1 USB Driver Characteristics ...........................................................................................................123
7.1.2 Data Signal Rise and Fall, Eye Patterns ........................................................................................129
7.1.3 Cable Skew....................................................................................................................................139
7.1.4 Receiver Characteristics ................................................................................................................139
7.1.5 Device Speed Identification ..........................................................................................................141
7.1.6 Input Characteristics......................................................................................................................142
7.1.7 Signaling Levels............................................................................................................................144
7.1.8 Data Encoding/Decoding ..............................................................................................................157
7.1.9 Bit Stuffing....................................................................................................................................157
7.1.10 Sync Pattern ..................................................................................................................................159
7.1.11 Data Signaling Rate.......................................................................................................................159
7.1.12 Frame Interval ...............................................................................................................................159
7.1.13 Data Source Signaling ...................................................................................................................160
7.1.14 Hub Signaling Timings .................................................................................................................162
7.1.15 Receiver Data Jitter .......................................................................................................................164
7.1.16 Cable Delay...................................................................................................................................165
7.1.17 Cable Attenuation..........................................................................................................................167
7.1.18 Bus Turn-around Time and Inter-packet Delay.............................................................................168
7.1.19 Maximum End-to-end Signal Delay..............................................................................................168
7.1.20 Test Mode Support ........................................................................................................................169
7.2 Power Distribution .............................................................................................................................171
7.2.1 Classes of Devices.........................................................................................................................171
7.2.2 Voltage Drop Budget ....................................................................................................................175
7.2.3 Power Control During Suspend/Resume.......................................................................................176
7.2.4 Dynamic Attach and Detach..........................................................................................................177
7.3 Physical Layer.....................................................................................................................................178
7.3.1 Regulatory Requirements ..............................................................................................................178
7.3.2 Bus Timing/Electrical Characteristics ...........................................................................................178
7.3.3 Timing Waveforms .......................................................................................................................191
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CHAPTER 8 PROTOCOL LAYER
8.1
Byte/Bit Ordering .............................................................................................................................. 195
8.2
SYNC Field......................................................................................................................................... 195
8.3 Packet Field Formats......................................................................................................................... 195
8.3.1 Packet Identifier Field .................................................................................................................. 195
8.3.2 Address Fields .............................................................................................................................. 197
8.3.3 Frame Number Field..................................................................................................................... 197
8.3.4 Data Field ..................................................................................................................................... 197
8.3.5 Cyclic Redundancy Checks .......................................................................................................... 198
8.4 Packet Formats .................................................................................................................................. 199
8.4.1 Token Packets............................................................................................................................... 199
8.4.2 Split Transaction Special Token Packets...................................................................................... 199
8.4.3 Start-of-Frame Packets ................................................................................................................. 204
8.4.4 Data Packets ................................................................................................................................. 206
8.4.5 Handshake Packets ....................................................................................................................... 206
8.4.6 Handshake Responses .................................................................................................................. 207
8.5 Transaction Packet Sequences.......................................................................................................... 209
8.5.1 NAK Limiting via Ping Flow Control.......................................................................................... 217
8.5.2 Bulk Transactions......................................................................................................................... 221
8.5.3 Control Transfers.......................................................................................................................... 225
8.5.4 Interrupt Transactions................................................................................................................... 228
8.5.5 Isochronous Transactions ............................................................................................................. 229
8.6 Data Toggle Synchronization and Retry ......................................................................................... 232
8.6.1 Initialization via SETUP Token ................................................................................................... 233
8.6.2 Successful Data Transactions ....................................................................................................... 233
8.6.3 Data Corrupted or Not Accepted .................................................................................................. 233
8.6.4 Corrupted ACK Handshake.......................................................................................................... 234
8.6.5 Low-speed Transactions............................................................................................................... 235
8.7 Error Detection and Recovery.......................................................................................................... 236
8.7.1 Packet Error Categories................................................................................................................ 236
8.7.2 Bus Turn-around Timing.............................................................................................................. 237
8.7.3 False EOPs ................................................................................................................................... 237
8.7.4 Babble and Loss of Activity Recovery......................................................................................... 238
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CHAPTER 9 USB DEVICE FRAMEWORK
9.1 USB Device States...............................................................................................................................239
9.1.1 Visible Device States.....................................................................................................................239
9.1.2 Bus Enumeration ...........................................................................................................................243
9.2 Generic USB Device Operations .......................................................................................................244
9.2.1 Dynamic Attachment and Removal...............................................................................................244
9.2.2 Address Assignment......................................................................................................................244
9.2.3 Configuration ................................................................................................................................244
9.2.4 Data Transfer.................................................................................................................................245
9.2.5 Power Management.......................................................................................................................245
9.2.6 Request Processing........................................................................................................................245
9.2.7 Request Error ................................................................................................................................247
9.3 USB Device Requests..........................................................................................................................248
9.3.1 bmRequestType.............................................................................................................................248
9.3.2 bRequest........................................................................................................................................249
9.3.3 wValue ..........................................................................................................................................249
9.3.4 wIndex...........................................................................................................................................249
9.3.5 wLength.........................................................................................................................................249
9.4 Standard Device Requests .................................................................................................................250
9.4.1 Clear Feature .................................................................................................................................252
9.4.2 Get Configuration..........................................................................................................................253
9.4.3 Get Descriptor ...............................................................................................................................253
9.4.4 Get Interface..................................................................................................................................254
9.4.5 Get Status ......................................................................................................................................254
9.4.6 Set Address....................................................................................................................................256
9.4.7 Set Configuration ..........................................................................................................................257
9.4.8 Set Descriptor................................................................................................................................257
9.4.9 Set Feature.....................................................................................................................................258
9.4.10 Set Interface...................................................................................................................................259
9.4.11 Synch Frame..................................................................................................................................260
9.5
Descriptors ..........................................................................................................................................260
9.6 Standard USB Descriptor Definitions...............................................................................................261
9.6.1 Device ...........................................................................................................................................261
9.6.2 Device_Qualifier ...........................................................................................................................264
9.6.3 Configuration ................................................................................................................................264
9.6.4 Other_Speed_Configuration..........................................................................................................266
9.6.5 Interface.........................................................................................................................................267
9.6.6 Endpoint ........................................................................................................................................269
9.6.7 String .............................................................................................................................................273
9.7 Device Class Definitions .....................................................................................................................274
9.7.1 Descriptors ....................................................................................................................................274
9.7.2 Interface(s) and Endpoint Usage ...................................................................................................274
9.7.3 Requests ........................................................................................................................................274
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CHAPTER 10 USB HOST: HARDWARE AND SOFTWARE
10.1 Overview of the USB Host ................................................................................................................ 275
10.1.1 Overview ...................................................................................................................................... 275
10.1.2 Control Mechanisms..................................................................................................................... 278
10.1.3 Data Flow ..................................................................................................................................... 278
10.1.4 Collecting Status and Activity Statistics....................................................................................... 279
10.1.5 Electrical Interface Considerations............................................................................................... 279
10.2 Host Controller Requirements ......................................................................................................... 279
10.2.1 State Handling .............................................................................................................................. 280
10.2.2 Serializer/Deserializer .................................................................................................................. 280
10.2.3 Frame and Microframe Generation .............................................................................................. 280
10.2.4 Data Processing ............................................................................................................................ 281
10.2.5 Protocol Engine ............................................................................................................................ 281
10.2.6 Transmission Error Handling ....................................................................................................... 282
10.2.7 Remote Wakeup ........................................................................................................................... 282
10.2.8 Root Hub ...................................................................................................................................... 282
10.2.9 Host System Interface................................................................................................................... 283
10.3 Overview of Software Mechanisms .................................................................................................. 283
10.3.1 Device Configuration ................................................................................................................... 283
10.3.2 Resource Management ................................................................................................................. 285
10.3.3 Data Transfers .............................................................................................................................. 286
10.3.4 Common Data Definitions............................................................................................................ 286
10.4 Host Controller Driver...................................................................................................................... 287
10.5 Universal Serial Bus Driver .............................................................................................................. 287
10.5.1 USBD Overview........................................................................................................................... 288
10.5.2 USBD Command Mechanism Requirements ............................................................................... 289
10.5.3 USBD Pipe Mechanisms .............................................................................................................. 291
10.5.4 Managing the USB via the USBD Mechanisms........................................................................... 293
10.5.5 Passing USB Preboot Control to the Operating System ............................................................... 295
10.6 Operating System Environment Guides .......................................................................................... 296
CHAPTER 11 HUB SPECIFICATION
11.1 Overview............................................................................................................................................. 297
11.1.1 Hub Architecture .......................................................................................................................... 297
11.1.2 Hub Connectivity.......................................................................................................................... 298
11.2 Hub Frame/Microframe Timer ........................................................................................................ 300
11.2.1 High-speed Microframe Timer Range.......................................................................................... 300
11.2.2 Full-speed Frame Timer Range .................................................................................................... 301
11.2.3 Frame/Microframe Timer Synchronization.................................................................................. 301
11.2.4 Microframe Jitter Related to Frame Jitter..................................................................................... 303
11.2.5 EOF1 and EOF2 Timing Points.................................................................................................... 303
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11.3
Host Behavior at End-of-Frame........................................................................................................306
11.3.1 Full-/low-speed Latest Host Packet...............................................................................................306
11.3.2 Full-/low-speed Packet Nullification.............................................................................................306
11.3.3 Full-/low-speed Transaction Completion Prediction.....................................................................306
11.4 Internal Port .......................................................................................................................................307
11.4.1 Inactive..........................................................................................................................................308
11.4.2 Suspend Delay...............................................................................................................................308
11.4.3 Full Suspend (Fsus) .......................................................................................................................308
11.4.4 Generate Resume (GResume) .......................................................................................................308
11.5 Downstream Facing Ports..................................................................................................................309
11.5.1 Downstream Facing Port State Descriptions .................................................................................312
11.5.2 Disconnect Detect Timer...............................................................................................................315
11.5.3 Port Indicator.................................................................................................................................316
11.6 Upstream Facing Port ........................................................................................................................318
11.6.1 Full-speed......................................................................................................................................318
11.6.2 High-speed ....................................................................................................................................318
11.6.3 Receiver.........................................................................................................................................318
11.6.4 Transmitter ....................................................................................................................................322
11.7 Hub Repeater......................................................................................................................................324
11.7.1 High-speed Packet Connectivity ...................................................................................................324
11.7.2 Hub Repeater State Machine .........................................................................................................327
11.7.3 Wait for Start of Packet from Upstream Port (WFSOPFU) ..........................................................329
11.7.4 Wait for End of Packet from Upstream Port (WFEOPFU) ...........................................................330
11.7.5 Wait for Start of Packet (WFSOP) ................................................................................................330
11.7.6 Wait for End of Packet (WFEOP) .................................................................................................330
11.8 Bus State Evaluation ..........................................................................................................................330
11.8.1 Port Error.......................................................................................................................................330
11.8.2 Speed Detection.............................................................................................................................331
11.8.3 Collision ........................................................................................................................................331
11.8.4 Low-speed Port Behavior..............................................................................................................331
11.9 Suspend and Resume..........................................................................................................................332
11.10 Hub Reset Behavior............................................................................................................................334
11.11 Hub Port Power Control....................................................................................................................335
11.11.1 Multiple Gangs..............................................................................................................................335
11.12 Hub Controller ...................................................................................................................................336
11.12.1 Endpoint Organization ..................................................................................................................336
11.12.2 Hub Information Architecture and Operation ...............................................................................337
11.12.3 Port Change Information Processing.............................................................................................337
11.12.4 Hub and Port Status Change Bitmap .............................................................................................338
11.12.5 Over-current Reporting and Recovery ..........................................................................................339
11.12.6 Enumeration Handling ..................................................................................................................340
11.13 Hub Configuration .............................................................................................................................340
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11.14 Transaction Translator ..................................................................................................................... 342
11.14.1 Overview ...................................................................................................................................... 342
11.14.2 Transaction Translator Scheduling............................................................................................... 344
11.15 Split Transaction Notation Information .......................................................................................... 346
11.16 Common Split Transaction State Machines .................................................................................... 349
11.16.1 Host Controller State Machine ..................................................................................................... 350
11.16.2 Transaction Translator State Machine .......................................................................................... 354
11.17 Bulk/Control Transaction Translation Overview........................................................................... 360
11.17.1 Bulk/Control Split Transaction Sequences................................................................................... 360
11.17.2 Bulk/Control Split Transaction State Machines ........................................................................... 366
11.17.3 Bulk/Control Sequencing ............................................................................................................. 371
11.17.4 Bulk/Control Buffering Requirements ......................................................................................... 372
11.17.5 Other Bulk/Control Details........................................................................................................... 372
11.18 Periodic Split Transaction Pipelining and Buffer Management.................................................... 372
11.18.1 Best Case Full-Speed Budget ....................................................................................................... 373
11.18.2 TT Microframe Pipeline ............................................................................................................... 373
11.18.3 Generation of Full-speed Frames ................................................................................................. 374
11.18.4 Host Split Transaction Scheduling Requirements ........................................................................ 374
11.18.5 TT Response Generation .............................................................................................................. 378
11.18.6 TT Periodic Transaction Handling Requirements ........................................................................ 379
11.18.7 TT Transaction Tracking.............................................................................................................. 380
11.18.8 TT Complete-split Transaction State Searching........................................................................... 381
11.19 Approximate TT Buffer Space Required ........................................................................................ 382
11.20 Interrupt Transaction Translation Overview ................................................................................. 382
11.20.1 Interrupt Split Transaction Sequences .......................................................................................... 383
11.20.2 Interrupt Split Transaction State Machines .................................................................................. 386
11.20.3 Interrupt OUT Sequencing ........................................................................................................... 392
11.20.4 Interrupt IN Sequencing ............................................................................................................... 393
11.21 Isochronous Transaction Translation Overview ............................................................................ 394
11.21.1 Isochronous Split Transaction Sequences .................................................................................... 395
11.21.2 Isochronous Split Transaction State Machines............................................................................. 398
11.21.3 Isochronous OUT Sequencing...................................................................................................... 403
11.21.4 Isochronous IN Sequencing.......................................................................................................... 404
11.22 TT Error Handling............................................................................................................................ 404
11.22.1 Loss of TT Synchronization With HS SOFs ................................................................................ 404
11.22.2 TT Frame and Microframe Timer Synchronization Requirements .............................................. 405
11.23 Descriptors ......................................................................................................................................... 407
11.23.1 Standard Descriptors for Hub Class ............................................................................................. 407
11.23.2 Class-specific Descriptors ............................................................................................................ 417
11.24 Requests.............................................................................................................................................. 419
11.24.1 Standard Requests ........................................................................................................................ 419
11.24.2 Class-specific Requests ................................................................................................................ 420
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APPENDIX A TRANSACTION EXAMPLES
A.1
Bulk/Control OUT and SETUP Transaction Examples..................................................................439
A.2
Bulk/Control IN Transaction Examples...........................................................................................464
A.3
Interrupt OUT Transaction Examples .............................................................................................489
A.4
Interrupt IN Transaction Examples .................................................................................................509
A.5
Isochronous OUT SpAppendix A Transaction Examples
APPENDIX B EXAMPLE DECLARATIONS FOR STATE MACHINES
B.1
Global Declarations ............................................................................................................................555
B.2
Host Controller Declarations.............................................................................................................558
B.3
Transaction Translator Declarations................................................................................................560
APPENDIX C RESET PROTOCOL STATE DIAGRAMS
C.1
Downstream Facing Port State Diagram..........................................................................................565
C.2 Upstream Facing Port State Diagram...............................................................................................567
C.2.1 Reset From Suspended State .........................................................................................................567
C.2.2 Reset From Full-speed Non-suspended State................................................................................570
C.2.3 Reset From High-speed Non-suspended State ..............................................................................570
C.2.4 Reset Handshake ...........................................................................................................................570
INDEX
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Figures
Figure 3-1. Application Space Taxonomy ...........................................................................................................12
Figure 4-1. Bus Topology ....................................................................................................................................16
Figure 4-2. USB Cable.........................................................................................................................................17
Figure 4-3. A Typical Hub...................................................................................................................................23
Figure 4-4. Hubs in a Desktop Computer Environment.......................................................................................23
Figure 5-1. Simple USB Host/Device View ........................................................................................................25
Figure 5-2. USB Implementation Areas...............................................................................................................26
Figure 5-3. Host Composition..............................................................................................................................27
Figure 5-4. Physical Device Composition ...........................................................................................................28
Figure 5-5. USB Physical Bus Topology.............................................................................................................29
Figure 5-6. Multiple Full-speed Buses in a High-speed System ..........................................................................30
Figure 5-7. USB Logical Bus Topology ..............................................................................................................30
Figure 5-8. Client Software-to-function Relationships ........................................................................................31
Figure 5-9. USB Host/Device Detailed View ......................................................................................................32
Figure 5-10. USB Communication Flow .............................................................................................................33
Figure 5-11. Data Phase PID Sequence for Isochronous IN High Bandwidth Endpoints....................................57
Figure 5-12. Data Phase PID Sequence for Isochronous OUT High Bandwidth Endpoints................................58
Figure 5-13. USB Information Conversion From Client Software to Bus...........................................................59
Figure 5-14. Transfers for Communication Flows...............................................................................................62
Figure 5-15. Arrangement of IRPs to Transactions/(Micro)frames .....................................................................63
Figure 5-16. Non-USB Isochronous Example .....................................................................................................67
Figure 5-17. USB Full-speed Isochronous Application .......................................................................................70
Figure 5-18. Example Source/Sink Connectivity.................................................................................................77
Figure 5-19. Data Prebuffering ............................................................................................................................81
Figure 5-20. Packet and Buffer Size Formulas for Rate-matched Isochronous Transfers ...................................83
Figure 6-1. Keyed Connector Protocol ................................................................................................................85
Figure 6-2. USB Standard Detachable Cable Assembly......................................................................................87
Figure 6-3. USB High-/full-speed Hardwired Cable Assembly...........................................................................89
Figure 6-4. USB Low-speed Hardwired Cable Assembly ...................................................................................91
Figure 6-5. USB Icon...........................................................................................................................................93
Figure 6-6. Typical USB Plug Orientation ..........................................................................................................93
Figure 6-7. USB Series "A" Receptacle Interface and Mating Drawing..............................................................95
Figure 6-8. USB Series "B" Receptacle Interface and Mating Drawing..............................................................96
xv
Universal Serial Bus Specification Revision 2.0
Figure 6-9. USB Series "A" Plug Interface Drawing...........................................................................................99
Figure 6-10. USB Series “B” Plug Interface Drawing.......................................................................................100
Figure 6-11. Typical High-/full-speed Cable Construction ...............................................................................102
Figure 6-12. Single Pin-type Series "A" Receptacle..........................................................................................115
Figure 6-13. Dual Pin-type Series "A" Receptacle ............................................................................................116
Figure 6-14. Single Pin-type Series "B" Receptacle ..........................................................................................117
Figure 7-1. Example High-speed Capable Transceiver Circuit .........................................................................120
Figure 7-2. Maximum Input Waveforms for USB Signaling.............................................................................124
Figure 7-3. Example Full-speed CMOS Driver Circuit (non High-speed capable) ...........................................125
Figure 7-4. Full-speed Buffer V/I Characteristics..............................................................................................126
Figure 7-5. Full-speed Buffer V/I Characteristics for High-speed Capable Transceiver...................................127
Figure 7-6. Full-speed Signal Waveforms .........................................................................................................128
Figure 7-7. Low-speed Driver Signal Waveforms.............................................................................................128
Figure 7-8. Data Signal Rise and Fall Time.......................................................................................................130
Figure 7-9. Full-speed Load...............................................................................................................................130
Figure 7-10. Low-speed Port Loads...................................................................................................................131
Figure 7-11. Measurement Planes .....................................................................................................................131
Figure 7-12. Transmitter/Receiver Test Fixture ................................................................................................132
Figure 7-13. Template 1.....................................................................................................................................133
Figure 7-14. Template 2.....................................................................................................................................134
Figure 7-15. Template 3.....................................................................................................................................135
Figure 7-16. Template 4.....................................................................................................................................136
Figure 7-17. Template 5.....................................................................................................................................137
Figure 7-18. Template 6.....................................................................................................................................138
Figure 7-19. Differential Input Sensitivity Range for Low-/full-speed .............................................................140
Figure 7-20. Full-speed Device Cable and Resistor Connections......................................................................141
Figure 7-21. Low-speed Device Cable and Resistor Connections.....................................................................141
Figure 7-22. Placement of Optional Edge Rate Control Capacitors for Low-/full-speed ..................................143
Figure 7-23. Diagram for High-speed Loading Equivalent Circuit ...................................................................143
Figure 7-24. Upstream Facing Full-speed Port Transceiver ..............................................................................146
Figure 7-25. Downstream Facing Low-/full-speed Port Transceiver.................................................................146
Figure 7-26. Low-/full-speed Disconnect Detection..........................................................................................149
Figure 7-27. Full-/high-speed Device Connect Detection .................................................................................149
Figure 7-28. Low-speed Device Connect Detection ..........................................................................................150
Figure 7-29. Power-on and Connection Events Timing.....................................................................................150
Figure 7-30. Low-/full-speed Packet Voltage Levels ........................................................................................152
Figure 7-31. NRZI Data Encoding.....................................................................................................................157
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Universal Serial Bus Specification Revision 2.0
Figure 7-32. Bit Stuffing....................................................................................................................................157
Figure 7-33. Illustration of Extra Bit Preceding EOP (Full-/low-speed) ...........................................................158
Figure 7-34. Flow Diagram for Bit Stuffing ......................................................................................................158
Figure 7-35. Sync Pattern (Low-/full-speed) .....................................................................................................159
Figure 7-36. Data Jitter Taxonomy ....................................................................................................................160
Figure 7-37. SE0 for EOP Width Timing ..........................................................................................................161
Figure 7-38. Hub Propagation Delay of Full-speed Differential Signals...........................................................162
Figure 7-39. Full-speed Cable Delay .................................................................................................................166
Figure 7-40. Low-speed Cable Delay ................................................................................................................166
Figure 7-41. Worst-case End-to-end Signal Delay Model for Low-/full-speed.................................................169
Figure 7-42. Compound Bus-powered Hub .......................................................................................................172
Figure 7-43. Compound Self-powered Hub.......................................................................................................173
Figure 7-44. Low-power Bus-powered Function...............................................................................................174
Figure 7-45. High-power Bus-powered Function ..............................................................................................174
Figure 7-46. Self-powered Function ..................................................................................................................175
Figure 7-47. Worst-case Voltage Drop Topology (Steady State) ......................................................................175
Figure 7-48. Typical Suspend Current Averaging Profile .................................................................................176
Figure 7-49. Differential Data Jitter for Low-/full-speed...................................................................................191
Figure 7-50. Differential-to-EOP Transition Skew and EOP Width for Low-/full-speed .................................191
Figure 7-51. Receiver Jitter Tolerance for Low-/full-speed...............................................................................191
Figure 7-52. Hub Differential Delay, Differential Jitter, and SOP Distortion for Low-/full-speed ...................192
Figure 7-53. Hub EOP Delay and EOP Skew for Low-/full-speed....................................................................193
Figure 8-1. PID Format......................................................................................................................................195
Figure 8-2. ADDR Field ....................................................................................................................................197
Figure 8-3. Endpoint Field.................................................................................................................................197
Figure 8-4. Data Field Format ...........................................................................................................................198
Figure 8-5. Token Format ..................................................................................................................................199
Figure 8-6. Packets in a Start-split Transaction .................................................................................................200
Figure 8-7. Packets in a Complete-split Transaction .........................................................................................200
Figure 8-8. Relationship of Interrupt IN Transaction to High-speed Split Transaction.....................................201
Figure 8-9. Relationship of Interrupt OUT Transaction to High-speed Split OUT Transaction........................202
Figure 8-10. Start-split (SSPLIT) Token ...........................................................................................................202
Figure 8-11. Port Field.......................................................................................................................................203
Figure 8-12. Complete-split (CSPLIT) Transaction Token ...............................................................................204
Figure 8-13. SOF Packet....................................................................................................................................204
Figure 8-14. Relationship between Frames and Microframes ...........................................................................205
Figure 8-15. Data Packet Format .......................................................................................................................206
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Universal Serial Bus Specification Revision 2.0
Figure 8-16. Handshake Packet .........................................................................................................................206
Figure 8-17. Legend for State Machines............................................................................................................210
Figure 8-18. State Machine Context Overview .................................................................................................211
Figure 8-19. Host Controller Top Level Transaction State Machine Hierarchy Overview ...............................211
Figure 8-20. Host Controller Non-split Transaction State Machine Hierarchy Overview.................................212
Figure 8-21. Device Transaction State Machine Hierarchy Overview ..............................................................212
Figure 8-22. Device Top Level State Machine ..................................................................................................213
Figure 8-23. Device_process_Trans State Machine...........................................................................................213
Figure 8-24. Dev_do_OUT State Machine ........................................................................................................214
Figure 8-25. Dev_do_IN State Machine ............................................................................................................215
Figure 8-26. HC_Do_nonsplit State Machine....................................................................................................216
Figure 8-27. Host High-speed Bulk OUT/Control Ping State Machine.............................................................218
Figure 8-28. Dev_HS_ping State Machine ........................................................................................................219
Figure 8-29. Device High-speed Bulk OUT /Control State Machine ................................................................220
Figure 8-30. Bulk Transaction Format...............................................................................................................221
Figure 8-31. Bulk/Control/Interrupt OUT Transaction Host State Machine .....................................................222
Figure 8-32. Bulk/Control/Interrupt OUT Transaction Device State Machine..................................................223
Figure 8-33. Bulk/Control/Interrupt IN Transaction Host State Machine .........................................................224
Figure 8-34. Bulk/Control/Interrupt IN Transaction Device State Machine......................................................225
Figure 8-35. Bulk Reads and Writes..................................................................................................................225
Figure 8-36. Control SETUP Transaction..........................................................................................................226
Figure 8-37. Control Read and Write Sequences...............................................................................................226
Figure 8-38. Interrupt Transaction Format ........................................................................................................229
Figure 8-39. Isochronous Transaction Format...................................................................................................229
Figure 8-40. Isochronous OUT Transaction Host State Machine......................................................................230
Figure 8-41. Isochronous OUT Transaction Device State Machine ..................................................................231
Figure 8-42. Isochronous IN Transaction Host State Machine..........................................................................231
Figure 8-43. Isochronous IN Transaction Device State Machine ......................................................................232
Figure 8-44. SETUP Initialization .....................................................................................................................233
Figure 8-45. Consecutive Transactions..............................................................................................................233
Figure 8-46. NAKed Transaction with Retry.....................................................................................................234
Figure 8-47. Corrupted ACK Handshake with Retry.........................................................................................234
Figure 8-48. Low-speed Transaction .................................................................................................................235
Figure 8-49. Bus Turn-around Timer Usage......................................................................................................237
Figure 9-1. Device State Diagram .....................................................................................................................240
Figure 9-2. wIndex Format when Specifying an Endpoint ................................................................................249
Figure 9-3. wIndex Format when Specifying an Interface ................................................................................249
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Universal Serial Bus Specification Revision 2.0
Figure 9-4. Information Returned by a GetStatus() Request to a Device ..........................................................255
Figure 9-5. Information Returned by a GetStatus() Request to an Interface......................................................255
Figure 9-6. Information Returned by a GetStatus() Request to an Endpoint .....................................................256
Figure 9-7. Example of Feedback Endpoint Numbers.......................................................................................272
Figure 9-8. Example of Feedback Endpoint Relationships................................................................................272
Figure 10-1. Interlayer Communications Model................................................................................................275
Figure 10-2. Host Communications ...................................................................................................................276
Figure 10-3. Frame and Microframe Creation ...................................................................................................281
Figure 10-4. Configuration Interactions.............................................................................................................284
Figure 10-5. Universal Serial Bus Driver Structure...........................................................................................288
Figure 11-1. Hub Architecture ...........................................................................................................................298
Figure 11-2. Hub Signaling Connectivity ..........................................................................................................299
Figure 11-3. Resume Connectivity ....................................................................................................................299
Figure 11-4. Example High-speed EOF Offsets Due to Propagation Delay Without EOF Advancement ........302
Figure 11-5. Example High-speed EOF Offsets Due to Propagation Delay With EOF Advancement..............302
Figure 11-6. High-speed EOF2 Timing Point....................................................................................................304
Figure 11-7. High-speed EOF1 Timing Point....................................................................................................304
Figure 11-8. Full-speed EOF Timing Points......................................................................................................304
Figure 11-9. Internal Port State Machine...........................................................................................................308
Figure 11-10. Downstream Facing Hub Port State Machine .............................................................................310
Figure 11-11. Port Indicator State Diagram .......................................................................................................317
Figure 11-12. Upstream Facing Port Receiver State Machine...........................................................................319
Figure 11-13. Upstream Facing Port Transmitter State Machine ......................................................................322
Figure 11-14. Example Hub Repeater Organization..........................................................................................324
Figure 11-15. High-speed Port Selector State Machine .....................................................................................326
Figure 11-16. Hub Repeater State Machine.......................................................................................................328
Figure 11-17. Example Remote-wakeup Resume Signaling With Full-/low-speed Device ..............................333
Figure 11-18. Example Remote-wakeup Resume Signaling With High-speed Device .....................................334
Figure 11-19. Example Hub Controller Organization........................................................................................336
Figure 11-20. Relationship of Status, Status Change, and Control Information to Device States .....................337
Figure 11-21. Port Status Handling Method ......................................................................................................338
Figure 11-22. Hub and Port Status Change Bitmap ...........................................................................................339
Figure 11-23. Example Hub and Port Change Bit Sampling .............................................................................339
Figure 11-24. Transaction Translator Overview ................................................................................................342
Figure 11-25. Periodic and Non-periodic Buffer Sections of TT.......................................................................343
Figure 11-26. TT Microframe Pipeline for Periodic Split Transactions ............................................................344
Figure 11-27. TT Nonperiodic Buffering...........................................................................................................345
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Universal Serial Bus Specification Revision 2.0
Figure 11-28. Example Full-/low-speed Handler Scheduling for Start-splits....................................................346
Figure 11-29. Flow Sequence Legend ...............................................................................................................346
Figure 11-30. Legend for State Machines..........................................................................................................347
Figure 11-31. State Machine Context Overview ...............................................................................................348
Figure 11-32. Host Controller Split Transaction State Machine Hierarchy Overview ......................................349
Figure 11-33. Transaction Translator State Machine Hierarchy Overview .......................................................350
Figure 11-34. Host Controller............................................................................................................................350
Figure 11-35. HC_Process_Command ..............................................................................................................351
Figure 11-36. HC_Do_Start...............................................................................................................................352
Figure 11-37. HC_Do_Complete.......................................................................................................................353
Figure 11-38. Transaction Translator ................................................................................................................354
Figure 11-39. TT_Process_Packet .....................................................................................................................355
Figure 11-40. TT_Do_Start ...............................................................................................................................356
Figure 11-41. TT_Do_Complete .......................................................................................................................357
Figure 11-42. TT_BulkSS..................................................................................................................................357
Figure 11-43. TT_BulkCS .................................................................................................................................358
Figure 11-44. TT_IntSS.....................................................................................................................................358
Figure 11-45. TT_IntCS ....................................................................................................................................359
Figure 11-46. TT_IsochSS.................................................................................................................................359
Figure 11-47. Sample Algorithm for Compare_buffs........................................................................................361
Figure 11-48. Bulk/Control OUT Start-split Transaction Sequence..................................................................362
Figure 11-49. Bulk/Control OUT Complete-split Transaction Sequence..........................................................363
Figure 11-50. Bulk/Control IN Start-split Transaction Sequence......................................................................364
Figure 11-51. Bulk/Control IN Complete-split Transaction Sequence..............................................................365
Figure 11-52. Bulk/Control OUT Start-split Transaction Host State Machine..................................................366
Figure 11-53. Bulk/Control OUT Complete-split Transaction Host State Machine..........................................367
Figure 11-54. Bulk/Control OUT Start-split Transaction TT State Machine ....................................................368
Figure 11-55. Bulk/Control OUT Complete-split Transaction TT State Machine ............................................368
Figure 11-56. Bulk/Control IN Start-split Transaction Host State Machine......................................................369
Figure 11-57. Bulk/Control IN Complete-split Transaction Host State Machine..............................................370
Figure 11-58. Bulk/Control IN Start-split Transaction TT State Machine ........................................................371
Figure 11-59. Bulk/Control IN Complete-split Transaction TT State Machine ................................................371
Figure 11-60. Best Case Budgeted Full-speed Wire Time With No Bit Stuffing..............................................373
Figure 11-61. Scheduling of TT Microframe Pipeline.......................................................................................374
Figure 11-62. Isochronous OUT Example That Avoids a Start-split-end With Zero Data................................375
Figure 11-63. End of Frame TT Pipeline Scheduling Example.........................................................................376
Figure 11-64. Isochronous IN Complete-split Schedule Example at L=Y6 .......................................................377
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Universal Serial Bus Specification Revision 2.0
Figure 11-65. Isochronous IN Complete-split Schedule Example at L=Y7 .......................................................377
Figure 11-66. Microframe Pipeline....................................................................................................................380
Figure 11-67. Advance_Pipeline Pseudocode....................................................................................................381
Figure 11-68. Interrupt OUT Start-split Transaction Sequence .........................................................................383
Figure 11-69. Interrupt OUT Complete-split Transaction Sequence .................................................................384
Figure 11-70. Interrupt IN Start-split Transaction Sequence .............................................................................385
Figure 11-71. Interrupt IN Complete-split Transaction Sequence .....................................................................385
Figure 11-72. Interrupt OUT Start-split Transaction Host State Machine .........................................................386
Figure 11-73. Interrupt OUT Complete-split Transaction Host State Machine .................................................387
Figure 11-74. Interrupt OUT Start-split Transaction TT State Machine............................................................388
Figure 11-75. Interrupt OUT Complete-split Transaction TT State Machine....................................................389
Figure 11-76. Interrupt IN Start-split Transaction Host State Machine.............................................................389
Figure 11-77. Interrupt IN Complete-split Transaction Host State Machine .....................................................390
Figure 11-78. HC_Data_or_Error State Machine ..............................................................................................391
Figure 11-79. Interrupt IN Start-split Transaction TT State Machine................................................................391
Figure 11-80. Interrupt IN Complete-split Transaction TT State Machine........................................................392
Figure 11-81. Example of CRC16 Handling for Interrupt OUT........................................................................393
Figure 11-82. Example of CRC16 Handling for Interrupt IN............................................................................394
Figure 11-83. Isochronous OUT Start-split Transaction Sequence....................................................................395
Figure 11-84. Isochronous IN Start-split Transaction Sequence .......................................................................396
Figure 11-85. Isochronous IN Complete-split Transaction Sequence................................................................397
Figure 11-86. Isochronous OUT Start-split Transaction Host State Machine ...................................................398
Figure 11-87. Isochronous OUT Start-split Transaction TT State Machine ......................................................399
Figure 11-88. Isochronous IN Start-split Transaction Host State Machine .......................................................400
Figure 11-89. Isochronous IN Complete-split Transaction Host State Machine ...............................................401
Figure 11-90. Isochronous IN Start-split Transaction TT State Machine ..........................................................402
Figure 11-91. Isochronous IN Complete-split Transaction TT State Machine ..................................................402
Figure 11-92. Example of CRC16 Isochronous OUT Data Packet Handling ....................................................403
Figure 11-93. Example of CRC16 Isochronous IN Data Packet Handling ........................................................404
Figure 11-94. Example Frame/Microframe Synchronization Events.................................................................406
Figure A-1. Normal No Smash ..........................................................................................................................441
Figure A-2. Normal HS DATA0/1 Smash.........................................................................................................442
Figure A-3. Normal HS DATA0/1 3 Strikes Smash..........................................................................................443
Figure A-4. Normal HS ACK(S) Smash(case 1) ...............................................................................................444
Figure A-5. Normal HS ACK(S) Smash(case 2) ...............................................................................................445
Figure A-6. Normal HS ACK(S) 3 Strikes Smash.............................................................................................446
Figure A-7. Normal HS CSPLIT Smash............................................................................................................447
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Universal Serial Bus Specification Revision 2.0
Figure A-8. Normal HS CSPLIT 3 Strikes Smash.............................................................................................448
Figure A-9. Normal HS ACK(C) Smash ...........................................................................................................449
Figure A-10. Normal S ACK(C) 3 Strikes Smash .............................................................................................450
Figure A-11. Normal FS/LS DATA0/1 Smash..................................................................................................451
Figure A-12. Normal FS/LS DATA0/1 3 Strikes Smash...................................................................................452
Figure A-13. Normal FS/LS ACK Smash .........................................................................................................453
Figure A-14. Normal FS/LS ACK 3 Strikes Smash ..........................................................................................454
Figure A-15. No buffer Available No Smash (HS NAK(S)) .............................................................................455
Figure A-16. No Buffer Available HS NAK(S) Smash.....................................................................................456
Figure A-17. No Buffer Available HS NAK(S) 3 Strikes Smash......................................................................457
Figure A-18. CS Earlier No Smash (HS NYET) ...............................................................................................458
Figure A-19. CS Earlier HS NYET Smash(case 1) ...........................................................................................459
Figure A-20. CS Earlier HS NYET Smash(case 2) ...........................................................................................460
Figure A-21. CS Earlier HS NYET 3 Strikes Smash.........................................................................................461
Figure A-22. Device Busy No Smash(FS/LS NAK) .........................................................................................462
Figure A-23. Device Stall No Smash(FS/LS STALL).......................................................................................463
Figure A-24. Normal No Smash ........................................................................................................................466
Figure A-25. Normal HS SSPLIT Smash ..........................................................................................................467
Figure A-26. Normal SSPLIT 3 Strikes Smash .................................................................................................468
Figure A-27. Normal HS ACK(S) Smash(case 1) .............................................................................................469
Figure A-28. Normal HS ACK(S) Smash(case 2) .............................................................................................470
Figure A-29. Normal HS ACK(S) 3 Strikes Smash...........................................................................................471
Figure A-30. Normal HS CSPLIT Smash..........................................................................................................472
Figure A-31. Normal HS CSPLIT 3 Strikes Smash...........................................................................................473
Figure A-32. Normal HS DATA0/1 Smash.......................................................................................................474
Figure A-33. Normal HS DATA0/1 3 Strikes Smash........................................................................................475
Figure A-34. Normal FS/LS IN Smash..............................................................................................................476
Figure A-35. Normal FS/LS IN 3 Strikes Smash...............................................................................................477
Figure A-36. Normal FS/LS DATA0/1 Smash..................................................................................................478
Figure A-37. Normal FS/LS DATA0/1 3 Strikes Smash...................................................................................479
Figure A-38. Normal FS/LS ACK Smash .........................................................................................................480
Figure A-39. No Buffer Available No Smash(HS NAK(S)) .............................................................................481
Figure A-40. No Buffer Available HS NAK(S) Smash.....................................................................................482
Figure A-41. No Buffer Available HS NAK(S) 3 Strikes Smash......................................................................483
Figure A-42. CS Earlier No Smash(HS NYET) ................................................................................................484
Figure A-43. CS Earlier HS NYET Smash(case 1) ...........................................................................................485
Figure A-44. CS Earlier HS NYET Smash(case 2) ...........................................................................................486
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Universal Serial Bus Specification Revision 2.0
Figure A-45. Device Busy No Smash(FS/LS NAK)..........................................................................................487
Figure A-46. Device Stall No Smash(FS/LS STALL).......................................................................................488
Figure A-47. Normal No Smash(FS/LS Handshake Packet is Done by M+1) ..................................................492
Figure A-48. Normal HS DATA0/1 Smash.......................................................................................................493
Figure A-49. Normal HS CSPLIT Smash..........................................................................................................494
Figure A-50. Normal HS CSPLIT 3 Strikes Smash...........................................................................................495
Figure A-51. Normal HS ACK(C) Smash .........................................................................................................496
Figure A-52. Normal HS ACK(C) 3 Strikes Smash ..........................................................................................497
Figure A-53. Normal FS/LS DATA0/1 Smash..................................................................................................498
Figure A-54. Normal FS/LS ACK Smash..........................................................................................................499
Figure A-55. Searching No Smash ....................................................................................................................500
Figure A-56. CS Earlier No Smash(HS NYET and FS/LS Handshake Packet is Done by M+2) .....................501
Figure A-57. CS Earlier No Smash(HS NYET and FS/LS Handshake Packet is Done by M+3) .....................502
Figure A-58. CS Earlier HS NYET Smash........................................................................................................503
Figure A-59. CS Earlier HS NYET 3 Strikes Smash.........................................................................................504
Figure A-60. Abort and Free Abort(FS/LS Transaction is Continued at End of M+3) .....................................505
Figure A-61. Abort and Free Free(FS/LS Transaction is not Started at End of M+3) .......................................506
Figure A-62. Device Busy No Smash(FS/LS NAK)..........................................................................................507
Figure A-63. Device Stall No Smash(FS/LS STALL).......................................................................................508
Figure A-64. Normal No Smash(FS/LS Data Packet is on M+1) ......................................................................512
Figure A-65. Normal HS SSPLIT Smash ..........................................................................................................513
Figure A-66. Normal HS CSPLIT Smash..........................................................................................................514
Figure A-67. Normal HS CSPLIT 3 Strikes Smash...........................................................................................515
Figure A-68. Normal HS DATA0/1 Smash.......................................................................................................516
Figure A-69. Normal HS DATA0/1 3 Strikes Smash........................................................................................517
Figure A-70. Normal FS/LS IN Smash..............................................................................................................518
Figure A-71. Normal FS/LS DATA0/1 Smash..................................................................................................519
Figure A-72. Normal FS/LS ACK Smash..........................................................................................................520
Figure A-73. Searching No Smash ....................................................................................................................521
Figure A-74. CS Earlier No Smash(HS MDATA and FS/LS Data Packet is on M+1 and M+2)......................522
Figure A-75. CS Earlier No Smash(HS NYET and FS/LS Data Packet is on M+2) .........................................523
Figure A-76. CS Earlier No Smash(HS NYET and MDATA and FS/LS Data Packet is on M+2 and M+3) ...524
Figure A-77. CS Earlier No Smash(HS NYET and FS/LS Data Packet is on M+3) .........................................525
Figure A-78. CS Earlier HS NYET Smash........................................................................................................526
Figure A-79. CS Earlier HS NYET 3 Strikes Smash.........................................................................................527
Figure A-80. Abort and Free Abort(HS NYET and FS/LS Transaction is Continued at End of M+3) .............528
Figure A-81. Abort and Free Free(HS NYET and FS/LS Transaction is not Started at End of M+3)...............529
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Universal Serial Bus Specification Revision 2.0
Figure A-82. Device Busy No Smash(FS/LS NAK) .........................................................................................530
Figure A-83. Device Stall No Smash(FS/LS STALL).......................................................................................531
Figure C-1. Downstream Facing Port Reset Protocol State Diagram ................................................................566
Figure C-2. Upstream Facing Port Reset Detection State Diagram ...................................................................568
Figure C-3. Upstream Facing Port Reset Handshake State Diagram.................................................................569
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Universal Serial Bus Specification Revision 2.0
Tables
Table 5-1. Low-speed Control Transfer Limits ...................................................................................................41
Table 5-2. Full-speed Control Transfer Limits ....................................................................................................42
Table 5-3. High-speed Control Transfer Limits...................................................................................................43
Table 5-4. Full-speed Isochronous Transaction Limits........................................................................................45
Table 5-5. High-speed Isochronous Transaction Limits ......................................................................................46
Table 5-6. Low-speed Interrupt Transaction Limits ............................................................................................49
Table 5-7. Full-speed Interrupt Transaction Limits .............................................................................................50
Table 5-8. High-speed Interrupt Transaction Limits............................................................................................51
Table 5-9. Full-speed Bulk Transaction Limits ...................................................................................................54
Table 5-10. High-speed Bulk Transaction Limits................................................................................................55
Table 5-11. wMaxPacketSize Field of Endpoint Descriptor ................................................................................56
Table 5-12. Synchronization Characteristics .......................................................................................................72
Table 5-13. Connection Requirements.................................................................................................................79
Table 6-1. USB Connector Termination Assignment ..........................................................................................94
Table 6-2. Power Pair ........................................................................................................................................103
Table 6-3. Signal Pair ........................................................................................................................................104
Table 6-4. Drain Wire Signal Pair .....................................................................................................................104
Table 6-5. Nominal Cable Diameter ..................................................................................................................105
Table 6-6. Conductor Resistance .......................................................................................................................105
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards ........................................106
Table 7-1. Description of Functional Elements in the Example Shown in Figure 7-1.......................................122
Table 7-2. Low-/full-speed Signaling Levels.....................................................................................................145
Table 7-3. High-speed Signaling Levels............................................................................................................147
Table 7-4. Full-speed Jitter Budget....................................................................................................................164
Table 7-5. Low-speed Jitter Budget...................................................................................................................165
Table 7-6. Maximum Allowable Cable Loss.....................................................................................................167
Table 7-7. DC Electrical Characteristics............................................................................................................178
Table 7-8. High-speed Source Electrical Characteristics...................................................................................180
Table 7-9. Full-speed Source Electrical Characteristics ....................................................................................181
Table 7-10. Low-speed Source Electrical Characteristics..................................................................................182
Table 7-11. Hub/Repeater Electrical Characteristics .........................................................................................183
Table 7-12. Cable Characteristics (Note 14)......................................................................................................185
Table 7-13. Hub Event Timings.........................................................................................................................186
Table 7-14. Device Event Timings ....................................................................................................................188
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Universal Serial Bus Specification Revision 2.0
Table 8-1. PID Types.........................................................................................................................................196
Table 8-2. Isochronous OUT Payload Continuation Encoding..........................................................................203
Table 8-3. Endpoint Type Values in Split Special Token..................................................................................204
Table 8-4. Function Responses to IN Transactions ...........................................................................................208
Table 8-5. Host Responses to IN Transactions ..................................................................................................208
Table 8-6. Function Responses to OUT Transactions in Order of Precedence..................................................209
Table 8-7. Status Stage Responses.....................................................................................................................227
Table 8-8. Packet Error Types ...........................................................................................................................236
Table 9-1. Visible Device States........................................................................................................................241
Table 9-2. Format of Setup Data........................................................................................................................248
Table 9-3. Standard Device Requests ................................................................................................................250
Table 9-4. Standard Request Codes ...................................................................................................................251
Table 9-5. Descriptor Types ..............................................................................................................................251
Table 9-6. Standard Feature Selectors ...............................................................................................................252
Table 9-7. Test Mode Selectors .........................................................................................................................259
Table 9-8. Standard Device Descriptor..............................................................................................................262
Table 9-9. Device_Qualifier Descriptor ............................................................................................................264
Table 9-10. Standard Configuration Descriptor.................................................................................................265
Table 9-11. Other_Speed_Configuration Descriptor .........................................................................................267
Table 9-12. Standard Interface Descriptor.........................................................................................................268
Table 9-13. Standard Endpoint Descriptor ........................................................................................................269
Table 9-14. Allowed wMaxPacketSize Values for Different Numbers of Transactions per Microframe .........273
Table 9-15. String Descriptor Zero, Specifying Languages Supported by the Device ......................................273
Table 9-16. UNICODE String Descriptor..........................................................................................................274
Table 11-1. High-speed Microframe Timer Range Contributions.....................................................................300
Table 11-2. Full-speed Frame Timer Range Contributions ...............................................................................301
Table 11-3. Hub and Host EOF1/EOF2 Timing Points .....................................................................................303
Table 11-4. Internal Port Signal/Event Definitions............................................................................................308
Table 11-5. Downstream Facing Port Signal/Event Definitions........................................................................311
Table 11-6. Automatic Port State to Port Indicator Color Mapping ..................................................................316
Table 11-7. Port Indicator Color Definitions .....................................................................................................317
Table 11-8. Upstream Facing Port Receiver Signal/Event Definitions..............................................................320
Table 11-9. Upstream Facing Port Transmit Signal/Event Definitions .............................................................323
Table 11-10. High-speed Port Selector Signal/Event Definitions......................................................................326
Table 11-11. Hub Repeater Signal/Event Definitions........................................................................................329
Table 11-12. Hub Power Operating Mode Summary ........................................................................................341
Table 11-13. Hub Descriptor .............................................................................................................................417
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Universal Serial Bus Specification Revision 2.0
Table 11-14. Hub Responses to Standard Device Requests...............................................................................419
Table 11-15. Hub Class Requests ......................................................................................................................420
Table 11-16. Hub Class Request Codes.............................................................................................................421
Table 11-17. Hub Class Feature Selectors .........................................................................................................421
Table 11-18. wValue Field for Clear_TT_Buffer ..............................................................................................424
Table 11-19. Hub Status Field, wHubStatus ......................................................................................................425
Table 11-20. Hub Change Field, wHubChange .................................................................................................426
Table 11-21. Port Status Field, wPortStatus ......................................................................................................427
Table 11-22. Port Change Field, wPortChange .................................................................................................431
Table 11-23. Format of Returned TT State........................................................................................................432
Table 11-24. Test Mode Selector Codes ............................................................................................................436
Table 11-25. Port Indicator Selector Codes .......................................................................................................437
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Chapter 1
Introduction
1.1 Motivation
The original motivation for the Universal Serial Bus (USB) came from three interrelated considerations:
•
Connection of the PC to the telephone
It is well understood that the merge of computing and communication will be the basis for the next
generation of productivity applications. The movement of machine-oriented and human-oriented data
types from one location or environment to another depends on ubiquitous and cheap connectivity.
Unfortunately, the computing and communication industries have evolved independently. The USB
provides a ubiquitous link that can be used across a wide range of PC-to-telephone interconnects.
•
Ease-of-use
The lack of flexibility in reconfiguring the PC has been acknowledged as the Achilles’ heel to its
further deployment. The combination of user-friendly graphical interfaces and the hardware and
software mechanisms associated with new-generation bus architectures have made computers less
confrontational and easier to reconfigure. However, from the end user’s point of view, the PC’s I/O
interfaces, such as serial/parallel ports, keyboard/mouse/joystick interfaces, etc., do not have the
attributes of plug-and-play.
•
Port expansion
The addition of external peripherals continues to be constrained by port availability. The lack of a bidirectional, low-cost, low-to-mid speed peripheral bus has held back the creative proliferation of
peripherals such as telephone/fax/modem adapters, answering machines, scanners, PDA’s, keyboards,
mice, etc. Existing interconnects are optimized for one or two point products. As each new function or
capability is added to the PC, a new interface has been defined to address this need.
The more recent motivation for USB 2.0 stems from the fact that PCs have increasingly higher performance
and are capable of processing vast amounts of data. At the same time, PC peripherals have added more
performance and functionality. User applications such as digital imaging demand a high performance
connection between the PC and these increasingly sophisticated peripherals. USB 2.0 addresses this need
by adding a third transfer rate of 480 Mb/s to the 12 Mb/s and 1.5 Mb/s originally defined for USB.
USB 2.0 is a natural evolution of USB, delivering the desired bandwidth increase while preserving the
original motivations for USB and maintaining full compatibility with existing peripherals.
Thus, USB continues to be the answer to connectivity for the PC architecture. It is a fast, bi-directional,
isochronous, low-cost, dynamically attachable serial interface that is consistent with the requirements of the
PC platform of today and tomorrow.
1.2 Objective of the Specification
This document defines an industry-standard USB. The specification describes the bus attributes, the
protocol definition, types of transactions, bus management, and the programming interface required to
design and build systems and peripherals that are compliant with this standard.
The goal is to enable such devices from different vendors to interoperate in an open architecture. The
specification is intended as an enhancement to the PC architecture, spanning portable, business desktop, and
home environments. It is intended that the specification allow system OEMs and peripheral developers
adequate room for product versatility and market differentiation without the burden of carrying obsolete
interfaces or losing compatibility.
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Universal Serial Bus Specification Revision 2.0
1.3 Scope of the Document
The specification is primarily targeted to peripheral developers and system OEMs, but provides valuable
information for platform operating system/ BIOS/ device driver, adapter IHVs/ISVs, and platform/adapter
controller vendors. This specification can be used for developing new products and associated software.
1.4 USB Product Compliance
Adopters of the USB 2.0 specification have signed the USB 2.0 Adopters Agreement, which provides them
access to a reciprocal royalty-free license from the Promoters and other Adopters to certain intellectual
property contained in products that are compliant with the USB 2.0 specification. Adopters can demonstrate
compliance with the specification through the testing program as defined by the USB Implementers Forum.
Products that demonstrate compliance with the specification will be granted certain rights to use the USB
Implementers Forum logo as defined in the logo license.
1.5 Document Organization
Chapters 1 through 5 provide an overview for all readers, while Chapters 6 through 11 contain detailed
technical information defining the USB.
•
Peripheral implementers should particularly read Chapters 5 through 11.
•
USB Host Controller implementers should particularly read Chapters 5 through 8, 10, and 11.
•
USB device driver implementers should particularly read Chapters 5, 9, and 10.
This document is complemented and referenced by the Universal Serial Bus Device Class Specifications.
Device class specifications exist for a wide variety of devices. Please contact the USB Implementers
Forum for further details.
Readers are also requested to contact operating system vendors for operating system bindings specific to the
USB.
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Universal Serial Bus Specification Revision 2.0
Chapter 2
Terms and Abbreviations
This chapter lists and defines terms and abbreviations used throughout this specification.
ACK
Handshake packet indicating a positive acknowledgment.
Active Device
A device that is powered and is not in the Suspend state.
Asynchronous Data
Data transferred at irregular intervals with relaxed latency requirements.
Asynchronous RA
The incoming data rate, Fsi, and the outgoing data rate, Fso, of the RA process
are independent (i.e., there is no shared master clock). See also rate
adaptation.
Asynchronous SRC
The incoming sample rate, Fsi, and outgoing sample rate, Fso, of the SRC
process are independent (i.e., there is no shared master clock). See also sample
rate conversion.
Audio Device
A device that sources or sinks sampled analog data.
AWG#
The measurement of a wire’s cross section, as defined by the American Wire
Gauge standard.
Babble
Unexpected bus activity that persists beyond a specified point in a
(micro)frame.
Bandwidth
The amount of data transmitted per unit of time, typically bits per second (b/s)
or bytes per second (B/s).
Big Endian
A method of storing data that places the most significant byte of multiple-byte
values at a lower storage address. For example, a 16-bit integer stored in big
endian format places the least significant byte at the higher address and the
most significant byte at the lower address. See also little endian.
Bit
A unit of information used by digital computers. Represents the smallest piece
of addressable memory within a computer. A bit expresses the choice between
two possibilities and is typically represented by a logical one (1) or zero (0).
Bit Stuffing
Insertion of a “0” bit into a data stream to cause an electrical transition on the
data wires, allowing a PLL to remain locked.
b/s
Transmission rate expressed in bits per second.
B/s
Transmission rate expressed in bytes per second.
Buffer
Storage used to compensate for a difference in data rates or time of occurrence
of events, when transmitting data from one device to another.
Bulk Transfer
One of the four USB transfer types. Bulk transfers are non-periodic, large
bursty communication typically used for a transfer that can use any available
bandwidth and can also be delayed until bandwidth is available. See also
transfer type.
Bus Enumeration
Detecting and identifying USB devices.
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Universal Serial Bus Specification Revision 2.0
Byte
A data element that is eight bits in size.
Capabilities
Those attributes of a USB device that are administrated by the host.
Characteristics
Those qualities of a USB device that are unchangeable; for example, the device
class is a device characteristic.
Client
Software resident on the host that interacts with the USB System Software to
arrange data transfer between a function and the host. The client is often the
data provider and consumer for transferred data.
Configuring
Software
Software resident on the host software that is responsible for configuring a
USB device. This may be a system configurator or software specific to the
device.
Control Endpoint
A pair of device endpoints with the same endpoint number that are used by a
control pipe. Control endpoints transfer data in both directions and, therefore,
use both endpoint directions of a device address and endpoint number
combination. Thus, each control endpoint consumes two endpoint addresses.
Control Pipe
Same as a message pipe.
Control Transfer
One of the four USB transfer types. Control transfers support
configuration/command/status type communications between client and
function. See also transfer type.
CRC
See Cyclic Redundancy Check.
CTI
Computer Telephony Integration.
Cyclic Redundancy
Check (CRC)
A check performed on data to see if an error has occurred in transmitting,
reading, or writing the data. The result of a CRC is typically stored or
transmitted with the checked data. The stored or transmitted result is
compared to a CRC calculated for the data to determine if an error has
occurred.
Default Address
An address defined by the USB Specification and used by a USB device when
it is first powered or reset. The default address is 00H.
Default Pipe
The message pipe created by the USB System Software to pass control and
status information between the host and a USB device’s endpoint zero.
Device
A logical or physical entity that performs a function. The actual entity
described depends on the context of the reference. At the lowest level, device
may refer to a single hardware component, as in a memory device. At a higher
level, it may refer to a collection of hardware components that perform a
particular function, such as a USB interface device. At an even higher level,
device may refer to the function performed by an entity attached to the USB;
for example, a data/FAX modem device. Devices may be physical, electrical,
addressable, and logical.
When used as a non-specific reference, a USB device is either a hub or a
function.
Device Address
4
A seven-bit value representing the address of a device on the USB. The device
address is the default address (00H) when the USB device is first powered or
the device is reset. Devices are assigned a unique device address by the USB
System Software.
Universal Serial Bus Specification Revision 2.0
Device Endpoint
A uniquely addressable portion of a USB device that is the source or sink of
information in a communication flow between the host and device. See also
endpoint address.
Device Resources
Resources provided by USB devices, such as buffer space and endpoints. See
also Host Resources and Universal Serial Bus Resources.
Device Software
Software that is responsible for using a USB device. This software may or
may not also be responsible for configuring the device for use.
Downstream
The direction of data flow from the host or away from the host. A downstream
port is the port on a hub electrically farthest from the host that generates
downstream data traffic from the hub. Downstream ports receive upstream
data traffic.
Driver
When referring to hardware, an I/O pad that drives an external load. When
referring to software, a program responsible for interfacing to a hardware
device, that is, a device driver.
DWORD
Double word. A data element that is two words (i.e., four bytes or 32 bits) in
size.
Dynamic Insertion
and Removal
The ability to attach and remove devices while the host is in operation.
2
E PROM
See Electrically Erasable Programmable Read Only Memory.
EEPROM
See Electrically Erasable Programmable Read Only Memory.
Electrically
Erasable
Programmable
Read Only Memory
(EEPROM)
Non-volatile rewritable memory storage technology.
End User
The user of a host.
Endpoint
See device endpoint.
Endpoint Address
The combination of an endpoint number and an endpoint direction on a USB
device. Each endpoint address supports data transfer in one direction.
Endpoint Direction
The direction of data transfer on the USB. The direction can be either IN or
OUT. IN refers to transfers to the host; OUT refers to transfers from the host.
Endpoint Number
A four-bit value between 0H and FH, inclusive, associated with an endpoint on
a USB device.
Envelope detector
An electronic circuit inside a USB device that monitors the USB data lines and
detects certain voltage related signal characteristics.
EOF
End-of-(micro)Frame.
EOP
End-of-Packet.
External Port
See port.
Eye pattern
A representation of USB signaling that provides minimum and maximum
voltage levels as well as signal jitter.
False EOP
A spurious, usually noise-induced event that is interpreted by a packet receiver
as an EOP.
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6
Frame
A 1 millisecond time base established on full-/low-speed buses.
Frame Pattern
A sequence of frames that exhibit a repeating pattern in the number of samples
transmitted per frame. For a 44.1 kHz audio transfer, the frame pattern could
be nine frames containing 44 samples followed by one frame containing
45 samples.
Fs
See sample rate.
Full-duplex
Computer data transmission occurring in both directions simultaneously.
Full-speed
USB operation at 12 Mb/s. See also low-speed and high-speed.
Function
A USB device that provides a capability to the host, such as an ISDN
connection, a digital microphone, or speakers.
Handshake Packet
A packet that acknowledges or rejects a specific condition. For examples, see
ACK and NAK.
High-bandwidth
endpoint
A high-speed device endpoint that transfers more than 1024 bytes and less than
3073 bytes per microframe.
High-speed
USB operation at 480 Mb/s. See also low-speed and full-speed.
Host
The host computer system where the USB Host Controller is installed. This
includes the host hardware platform (CPU, bus, etc.) and the operating system
in use.
Host Controller
The host’s USB interface.
Host Controller
Driver (HCD)
The USB software layer that abstracts the Host Controller hardware. The Host
Controller Driver provides an SPI for interaction with a Host Controller. The
Host Controller Driver hides the specifics of the Host Controller hardware
implementation.
Host Resources
Resources provided by the host, such as buffer space and interrupts. See also
Device Resources and Universal Serial Bus Resources.
Hub
A USB device that provides additional connections to the USB.
Hub Tier
One plus the number of USB links in a communication path between the host
and a function. See Figure 4-1.
Interrupt Request
(IRQ)
A hardware signal that allows a device to request attention from a host. The
host typically invokes an interrupt service routine to handle the condition that
caused the request.
Interrupt Transfer
One of the four USB transfer types. Interrupt transfer characteristics are small
data, non-periodic, low-frequency, and bounded-latency. Interrupt transfers
are typically used to handle service needs. See also transfer type.
I/O Request Packet
An identifiable request by a software client to move data between itself (on the
host) and an endpoint of a device in an appropriate direction.
IRP
See I/O Request Packet.
IRQ
See Interrupt Request.
Isochronous Data
A stream of data whose timing is implied by its delivery rate.
Isochronous Device
An entity with isochronous endpoints, as defined in the USB Specification, that
sources or sinks sampled analog streams or synchronous data streams.
Universal Serial Bus Specification Revision 2.0
Isochronous Sink
Endpoint
An endpoint that is capable of consuming an isochronous data stream that is
sent by the host.
Isochronous Source
Endpoint
An endpoint that is capable of producing an isochronous data stream and
sending it to the host.
Isochronous
Transfer
One of the four USB transfer types. Isochronous transfers are used when
working with isochronous data. Isochronous transfers provide periodic,
continuous communication between host and device. See also transfer type.
Jitter
A tendency toward lack of synchronization caused by mechanical or electrical
changes. More specifically, the phase shift of digital pulses over a
transmission medium.
kb/s
Transmission rate expressed in kilobits per second.
kB/s
Transmission rate expressed in kilobytes per second.
Little Endian
Method of storing data that places the least significant byte of multiple-byte
values at lower storage addresses. For example, a 16-bit integer stored in little
endian format places the least significant byte at the lower address and the
most significant byte at the next address. See also big endian.
LOA
Loss of bus activity characterized by an SOP without a corresponding EOP.
Low-speed
USB operation at 1.5 Mb/s. See also full-speed and high-speed.
LSb
Least significant bit.
LSB
Least significant byte.
Mb/s
Transmission rate expressed in megabits per second.
MB/s
Transmission rate expressed in megabytes per second.
Message Pipe
A bi-directional pipe that transfers data using a request/data/status paradigm.
The data has an imposed structure that allows requests to be reliably identified
and communicated.
Microframe
A 125 microsecond time base established on high-speed buses.
MSb
Most significant bit.
MSB
Most significant byte.
NAK
Handshake packet indicating a negative acknowledgment.
Non Return to Zero
Invert (NRZI)
A method of encoding serial data in which ones and zeroes are represented by
opposite and alternating high and low voltages where there is no return to zero
(reference) voltage between encoded bits. Eliminates the need for clock
pulses.
NRZI
See Non Return to Zero Invert.
Object
Host software or data structure representing a USB entity.
Packet
A bundle of data organized in a group for transmission. Packets typically
contain three elements: control information (e.g., source, destination, and
length), the data to be transferred, and error detection and correction bits.
Packet Buffer
The logical buffer used by a USB device for sending or receiving a single
packet. This determines the maximum packet size the device can send or
receive.
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Universal Serial Bus Specification Revision 2.0
8
Packet ID (PID)
A field in a USB packet that indicates the type of packet, and by inference, the
format of the packet and the type of error detection applied to the packet.
Phase
A token, data, or handshake packet. A transaction has three phases.
Phase Locked Loop
(PLL)
A circuit that acts as a phase detector to keep an oscillator in phase with an
incoming frequency.
Physical Device
A device that has a physical implementation; e.g., speakers, microphones, and
CD players.
PID
See Packet ID.
Pipe
A logical abstraction representing the association between an endpoint on a
device and software on the host. A pipe has several attributes; for example, a
pipe may transfer data as streams (stream pipe) or messages (message pipe).
See also stream pipe and message pipe.
PLL
See Phase Locked Loop.
Polling
Asking multiple devices, one at a time, if they have any data to transmit.
POR
See Power On Reset.
Port
Point of access to or from a system or circuit. For the USB, the point where a
USB device is attached.
Power On Reset
(POR)
Restoring a storage device, register, or memory to a predetermined state when
power is applied.
Programmable
Data Rate
Either a fixed data rate (single-frequency endpoints), a limited number of data
rates (32 kHz, 44.1 kHz, 48 kHz, …), or a continuously programmable data
rate. The exact programming capabilities of an endpoint must be reported in
the appropriate class-specific endpoint descriptors.
Protocol
A specific set of rules, procedures, or conventions relating to format and timing
of data transmission between two devices.
RA
See rate adaptation.
Rate Adaptation
The process by which an incoming data stream, sampled at Fsi, is converted to
an outgoing data stream, sampled at Fso,with a certain loss of quality,
determined by the rate adaptation algorithm. Error control mechanisms are
required for the process. Fsi and Fso can be different and asynchronous. Fsi is
the input data rate of the RA; Fso is the output data rate of the RA.
Request
A request made to a USB device contained within the data portion of a SETUP
packet.
Retire
The action of completing service for a transfer and notifying the appropriate
software client of the completion.
Root Hub
A USB hub directly attached to the Host Controller. This hub (tier 1) is
attached to the host.
Root Port
The downstream port on a Root Hub.
Sample
The smallest unit of data on which an endpoint operates; a property of an
endpoint.
Sample Rate (Fs)
The number of samples per second, expressed in Hertz (Hz).
Universal Serial Bus Specification Revision 2.0
Sample Rate
Conversion (SRC)
A dedicated implementation of the RA process for use on sampled analog data
streams. The error control mechanism is replaced by interpolating techniques.
Service
A procedure provided by a System Programming Interface (SPI).
Service Interval
The period between consecutive requests to a USB endpoint to send or receive
data.
Service Jitter
The deviation of service delivery from its scheduled delivery time.
Service Rate
The number of services to a given endpoint per unit time.
SOF
See Start-of-Frame.
SOP
Start-of-Packet.
SPI
See System Programming Interface.
Split transaction
A transaction type supported by host controllers and hubs. This transaction
type allows full- and low-speed devices to be attached to hubs operating at
high-speed.
SRC
See Sample Rate Conversion.
Stage
One part of the sequence composing a control transfer; stages include the Setup
stage, the Data stage, and the Status stage.
Start-of-Frame
(SOF)
The first transaction in each (micro)frame. An SOF allows endpoints to
identify the start of the (micro)frame and synchronize internal endpoint clocks
to the host.
Stream Pipe
A pipe that transfers data as a stream of samples with no defined USB
structure.
Synchronization
Type
A classification that characterizes an isochronous endpoint’s capability to
connect to other isochronous endpoints.
Synchronous RA
The incoming data rate, Fsi, and the outgoing data rate, Fso, of the RA process
are derived from the same master clock. There is a fixed relation between Fsi
and Fso.
Synchronous SRC
The incoming sample rate, Fsi, and outgoing sample rate, Fso, of the SRC
process are derived from the same master clock. There is a fixed relation
between Fsi and Fso.
System
Programming
Interface (SPI)
A defined interface to services provided by system software.
TDM
See Time Division Multiplexing.
TDR
See Time Domain Reflectometer.
Termination
Passive components attached at the end of cables to prevent signals from being
reflected or echoed.
Time Division
Multiplexing
(TDM)
A method of transmitting multiple signals (data, voice, and/or video)
simultaneously over one communications medium by interleaving a piece of
each signal one after another.
Time Domain
Reflectometer
(TDR)
An instrument capable of measuring impedance characteristics of the USB
signal lines.
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Universal Serial Bus Specification Revision 2.0
10
Timeout
The detection of a lack of bus activity for some predetermined interval.
Token Packet
A type of packet that identifies what transaction is to be performed on the bus.
Transaction
The delivery of service to an endpoint; consists of a token packet, optional data
packet, and optional handshake packet. Specific packets are allowed/required
based on the transaction type.
Transaction
translator
A functional component of a USB hub. The Transaction Translator responds to
special high-speed transactions and translates them to full/low-speed
transactions with full/low-speed devices attached on downstream facing ports.
Transfer
One or more bus transactions to move information between a software client
and its function.
Transfer Type
Determines the characteristics of the data flow between a software client and
its function. Four standard transfer types are defined: control, interrupt, bulk,
and isochronous.
Turn-around Time
The time a device needs to wait to begin transmitting a packet after a packet
has been received to prevent collisions on the USB. This time is based on the
length and propagation delay characteristics of the cable and the location of the
transmitting device in relation to other devices on the USB.
Universal Serial
Bus Driver (USBD)
The host resident software entity responsible for providing common services to
clients that are manipulating one or more functions on one or more Host
Controllers.
Universal Serial
Bus Resources
Resources provided by the USB, such as bandwidth and power. See also
Device Resources and Host Resources.
Upstream
The direction of data flow towards the host. An upstream port is the port on a
device electrically closest to the host that generates upstream data traffic from
the hub. Upstream ports receive downstream data traffic.
USBD
See Universal Serial Bus Driver.
USB-IF
USB Implementers Forum, Inc. is a nonprofit corporation formed to facilitate
the development of USB compliant products and promote the technology.
Virtual Device
A device that is represented by a software interface layer. An example of a
virtual device is a hard disk with its associated device driver and client
software that makes it able to reproduce an audio .WAV file.
Word
A data element that is two bytes (16 bits) in size.
Universal Serial Bus Specification Revision 2.0
Chapter 3
Background
This chapter presents a brief description of the background of the Universal Serial Bus (USB), including
design goals, features of the bus, and existing technologies.
3.1
Goals for the Universal Serial Bus
The USB is specified to be an industry-standard extension to the PC architecture with a focus on PC
peripherals that enable consumer and business applications. The following criteria were applied in defining
the architecture for the USB:
•
Ease-of-use for PC peripheral expansion
•
Low-cost solution that supports transfer rates up to 480 Mb/s
•
Full support for real-time data for voice, audio, and video
•
Protocol flexibility for mixed-mode isochronous data transfers and asynchronous messaging
•
Integration in commodity device technology
•
Comprehension of various PC configurations and form factors
•
Provision of a standard interface capable of quick diffusion into product
•
Enabling new classes of devices that augment the PC’s capability
•
Full backward compatibility of USB 2.0 for devices built to previous versions of the specification
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Universal Serial Bus Specification Revision 2.0
3.2
Taxonomy of Application Space
Figure 3-1 describes a taxonomy for the range of data traffic workloads that can be serviced over a USB.
As can be seen, a 480 Mb/s bus comprehends the high-speed, full-speed, and low-speed data ranges.
Typically, high-speed and full-speed data types may be isochronous, while low-speed data comes from
interactive devices. The USB is primarily a PC bus but can be readily applied to other host-centric
computing devices. The software architecture allows for future extension of the USB by providing support
for multiple USB Host Controllers.
PERFORMANCE
APPLICATIONS
LOW-SPEED
Keyboard, Mouse
Stylus
Game Peripherals
Virtual Reality Peripherals
• Interactive Devices
• 10 – 100 kb/s
FULL-SPEED
• Phone, Audio,
Compressed Video
• 500 kb/s – 10 Mb/s
HIGH-SPEED
• Video, Storage
• 25 – 400 Mb/s
POTS
Broadband
Audio
Microphone
Video
Storage
Imaging
Broadband
ATTRIBUTES
Lowest Cost
Ease-of-Use
Dynamic Attach-Detach
Multiple Peripherals
Lower Cost
Ease-of-Use
Dynamic Attach-Detach
Multiple Peripherals
Guaranteed Bandwidth
Guaranteed Latency
Low Cost
Ease-of-Use
Dynamic Attach-Detach
Multiple Peripherals
Guaranteed Bandwidth
Guaranteed Latency
High Bandwidth
Figure 3-1. Application Space Taxonomy
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Universal Serial Bus Specification Revision 2.0
3.3
Feature List
The USB Specification provides a selection of attributes that can achieve multiple price/performance
integration points and can enable functions that allow differentiation at the system and component level.
Features are categorized by the following benefits:
Easy to use for end user
•
Single model for cabling and connectors
•
Electrical details isolated from end user (e.g., bus terminations)
•
Self-identifying peripherals, automatic mapping of function to driver and configuration
•
Dynamically attachable and reconfigurable peripherals
Wide range of workloads and applications
•
Suitable for device bandwidths ranging from a few kb/s to several hundred Mb/s
•
Supports isochronous as well as asynchronous transfer types over the same set of wires
•
Supports concurrent operation of many devices (multiple connections)
•
Supports up to 127 physical devices
•
Supports transfer of multiple data and message streams between the host and devices
•
Allows compound devices (i.e., peripherals composed of many functions)
•
Lower protocol overhead, resulting in high bus utilization
Isochronous bandwidth
•
Guaranteed bandwidth and low latencies appropriate for telephony, audio, video, etc.
Flexibility
•
Supports a wide range of packet sizes, which allows a range of device buffering options
•
Allows a wide range of device data rates by accommodating packet buffer size and latencies
•
Flow control for buffer handling is built into the protocol
Robustness
•
Error handling/fault recovery mechanism is built into the protocol
•
Dynamic insertion and removal of devices is identified in user-perceived real-time
•
Supports identification of faulty devices
Synergy with PC industry
•
Protocol is simple to implement and integrate
•
Consistent with the PC plug-and-play architecture
•
Leverages existing operating system interfaces
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Universal Serial Bus Specification Revision 2.0
Low-cost implementation
•
Low-cost subchannel at 1.5 Mb/s
•
Optimized for integration in peripheral and host hardware
•
Suitable for development of low-cost peripherals
•
Low-cost cables and connectors
•
Uses commodity technologies
Upgrade path
•
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Architecture upgradeable to support multiple USB Host Controllers in a system
Universal Serial Bus Specification Revision 2.0
Chapter 4
Architectural Overview
This chapter presents an overview of the Universal Serial Bus (USB) architecture and key concepts. The
USB is a cable bus that supports data exchange between a host computer and a wide range of
simultaneously accessible peripherals. The attached peripherals share USB bandwidth through a hostscheduled, token-based protocol. The bus allows peripherals to be attached, configured, used, and detached
while the host and other peripherals are in operation.
Later chapters describe the various components of the USB in greater detail.
4.1 USB System Description
A USB system is described by three definitional areas:
•
USB interconnect
•
USB devices
•
USB host
The USB interconnect is the manner in which USB devices are connected to and communicate with the
host. This includes the following:
•
Bus Topology: Connection model between USB devices and the host.
•
Inter-layer Relationships: In terms of a capability stack, the USB tasks that are performed at each layer
in the system.
•
Data Flow Models: The manner in which data moves in the system over the USB between producers
and consumers.
•
USB Schedule: The USB provides a shared interconnect. Access to the interconnect is scheduled in
order to support isochronous data transfers and to eliminate arbitration overhead.
USB devices and the USB host are described in detail in subsequent sections.
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Universal Serial Specification Revision 2.0
4.1.1 Bus Topology
The USB connects USB devices with the USB host. The USB physical interconnect is a tiered star
topology. A hub is at the center of each star. Each wire segment is a point-to-point connection between the
host and a hub or function, or a hub connected to another hub or function. Figure 4-1 illustrates the
topology of the USB.
Due to timing constraints allowed for hub and cable propagation times, the maximum number of tiers
allowed is seven (including the root tier). Note that in seven tiers, five non-root hubs maximum can be
supported in a communication path between the host and any device. A compound device (see Figure 4-1)
occupies two tiers; therefore, it cannot be enabled if attached at tier level seven. Only functions can be
enabled in tier seven.
Host
RootHub
Host (Tier 1)
Tier 2
Hub 1
Tier 3
Func
Hub 2
Func
Tier 4
Hub 3
Hub 4
Func
Func
Tier 5
Func
Hub 5
Hub 6
Func
Tier 6
Compound Device
Hub 7
Tier 7
Func
Figure 4-1. Bus Topology
4.1.1.1 USB Host
There is only one host in any USB system. The USB interface to the host computer system is referred to as
the Host Controller. The Host Controller may be implemented in a combination of hardware, firmware, or
software. A root hub is integrated within the host system to provide one or more attachment points.
Additional information concerning the host may be found in Section 4.9 and in Chapter 10.
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4.1.1.2 USB Devices
USB devices are one of the following:
•
Hubs, which provide additional attachment points to the USB
•
Functions, which provide capabilities to the system, such as an ISDN connection, a digital joystick, or
speakers
USB devices present a standard USB interface in terms of the following:
•
Their comprehension of the USB protocol
•
Their response to standard USB operations, such as configuration and reset
•
Their standard capability descriptive information
Additional information concerning USB devices may be found in Section 4.8 and in Chapter 9.
4.2 Physical Interface
The physical interface of the USB is described in the electrical (Chapter 7) and mechanical (Chapter 6)
specifications for the bus.
4.2.1 Electrical
The USB transfers signal and power over a four-wire cable, shown in Figure 4-2. The signaling occurs over
two wires on each point-to-point segment.
There are three data rates:
•
The USB high-speed signaling bit rate is 480 Mb/s.
•
The USB full-speed signaling bit rate is 12 Mb/s.
•
A limited capability low-speed signaling mode is also defined at 1.5 Mb/s.
USB 2.0 host controllers and hubs provide capabilities so that full-speed and low-speed data can be
transmitted at high-speed between the host controller and the hub, but transmitted between the hub and the
device at full-speed or low-speed. This capability minimizes the impact that full-speed and low-speed
devices have upon the bandwidth available for high-speed devices.
The low-speed mode is defined to support a limited number of low-bandwidth devices, such as mice,
because more general use would degrade bus utilization.
The clock is transmitted, encoded along with the differential data. The clock encoding scheme is NRZI
with bit stuffing to ensure adequate transitions. A SYNC field precedes each packet to allow the receiver(s)
to synchronize their bit recovery clocks.
VBUS
D+
DGND
...
...
VBUS
D+
DGND
Figure 4-2. USB Cable
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Universal Serial Specification Revision 2.0
The cable also carries VBUS and GND wires on each segment to deliver power to devices. VBUS is
nominally +5 V at the source. The USB allows cable segments of variable lengths, up to several meters, by
choosing the appropriate conductor gauge to match the specified IR drop and other attributes such as device
power budget and cable flexibility. In order to provide guaranteed input voltage levels and proper
termination impedance, biased terminations are used at each end of the cable. The terminations also permit
the detection of attach and detach at each port and differentiate between high/full-speed and low-speed
devices.
4.2.2 Mechanical
The mechanical specifications for cables and connectors are provided in Chapter 6. All devices have an
upstream connection. Upstream and downstream connectors are not mechanically interchangeable, thus
eliminating illegal loopback connections at hubs. The cable has four conductors: a twisted signal pair of
standard gauge and a power pair in a range of permitted gauges. The connector is four-position, with
shielded housing, specified robustness, and ease of attach-detach characteristics.
4.3 Power
The specification covers two aspects of power:
•
Power distribution over the USB deals with the issues of how USB devices consume power provided by
the host over the USB.
•
Power management deals with how the USB System Software and devices fit into the host-based power
management system.
4.3.1 Power Distribution
Each USB segment provides a limited amount of power over the cable. The host supplies power for use by
USB devices that are directly connected. In addition, any USB device may have its own power supply.
USB devices that rely totally on power from the cable are called bus-powered devices. In contrast, those
that have an alternate source of power are called self-powered devices. A hub also supplies power for its
connected USB devices. The architecture permits bus-powered hubs within certain constraints of topology
that are discussed later in Chapter 11.
4.3.2 Power Management
A USB host may have a power management system that is independent of the USB. The USB System
Software interacts with the host’s power management system to handle system power events such as
suspend or resume. Additionally, USB devices typically implement additional power management features
that allow them to be power managed by system software.
The power distribution and power management features of the USB allow it to be designed into powersensitive systems such as battery-based notebook computers.
4.4 Bus Protocol
The USB is a polled bus. The Host Controller initiates all data transfers.
Most bus transactions involve the transmission of up to three packets. Each transaction begins when the
Host Controller, on a scheduled basis, sends a USB packet describing the type and direction of transaction,
the USB device address, and endpoint number. This packet is referred to as the “token packet.” The USB
device that is addressed selects itself by decoding the appropriate address fields. In a given transaction, data
is transferred either from the host to a device or from a device to the host. The direction of data transfer is
specified in the token packet. The source of the transaction then sends a data packet or indicates it has no
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Universal Serial Bus Specification Revision 2.0
data to transfer. The destination, in general, responds with a handshake packet indicating whether the
transfer was successful.
Some bus transactions between host controllers and hubs involve the transmission of four packets. These
types of transactions are used to manage the data transfers between the host and full-/low- speed devices.
The USB data transfer model between a source or destination on the host and an endpoint on a device is
referred to as a pipe. There are two types of pipes: stream and message. Stream data has no USB-defined
structure, while message data does. Additionally, pipes have associations of data bandwidth, transfer
service type, and endpoint characteristics like directionality and buffer sizes. Most pipes come into
existence when a USB device is configured. One message pipe, the Default Control Pipe, always exists
once a device is powered, in order to provide access to the device’s configuration, status, and control
information.
The transaction schedule allows flow control for some stream pipes. At the hardware level, this prevents
buffers from underrun or overrun situations by using a NAK handshake to throttle the data rate. When
NAKed, a transaction is retried when bus time is available. The flow control mechanism permits the
construction of flexible schedules that accommodate concurrent servicing of a heterogeneous mix of stream
pipes. Thus, multiple stream pipes can be serviced at different intervals and with packets of different sizes.
4.5 Robustness
There are several attributes of the USB that contribute to its robustness:
•
Signal integrity using differential drivers, receivers, and shielding
•
CRC protection over control and data fields
•
Detection of attach and detach and system-level configuration of resources
•
Self-recovery in protocol, using timeouts for lost or corrupted packets
•
Flow control for streaming data to ensure isochrony and hardware buffer management
•
Data and control pipe constructs for ensuring independence from adverse interactions between
functions
4.5.1 Error Detection
The core bit error rate of the USB medium is expected to be close to that of a backplane and any glitches
will very likely be transient in nature. To provide protection against such transients, each packet includes
error protection fields. When data integrity is required, such as with lossless data devices, an error recovery
procedure may be invoked in hardware or software.
The protocol includes separate CRCs for control and data fields of each packet. A failed CRC is considered
to indicate a corrupted packet. The CRC gives 100% coverage on single- and double-bit errors.
4.5.2 Error Handling
The protocol allows for error handling in hardware or software. Hardware error handling includes reporting
and retry of failed transfers. A USB Host Controller will try a transmission that encounters errors up to
three times before informing the client software of the failure. The client software can recover in an
implementation-specific way.
4.6 System Configuration
The USB supports USB devices attaching to and detaching from the USB at any time. Consequently,
system software must accommodate dynamic changes in the physical bus topology.
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4.6.1 Attachment of USB Devices
All USB devices attach to the USB through ports on specialized USB devices known as hubs. Hubs have
status bits that are used to report the attachment or removal of a USB device on one of its ports. The host
queries the hub to retrieve these bits. In the case of an attachment, the host enables the port and addresses
the USB device through the device’s control pipe at the default address.
The host assigns a unique USB address to the device and then determines if the newly attached USB device
is a hub or a function. The host establishes its end of the control pipe for the USB device using the assigned
USB address and endpoint number zero.
If the attached USB device is a hub and USB devices are attached to its ports, then the above procedure is
followed for each of the attached USB devices.
If the attached USB device is a function, then attachment notifications will be handled by host software that
is appropriate for the function.
4.6.2 Removal of USB Devices
When a USB device has been removed from one of a hub’s ports, the hub disables the port and provides an
indication of device removal to the host. The removal indication is then handled by appropriate USB
System Software. If the removed USB device is a hub, the USB System Software must handle the removal
of both the hub and of all of the USB devices that were previously attached to the system through the hub.
4.6.3 Bus Enumeration
Bus enumeration is the activity that identifies and assigns unique addresses to devices attached to a bus.
Because the USB allows USB devices to attach to or detach from the USB at any time, bus enumeration is
an on-going activity for the USB System Software. Additionally, bus enumeration for the USB also
includes the detection and processing of removals.
4.7 Data Flow Types
The USB supports functional data and control exchange between the USB host and a USB device as a set of
either uni-directional or bi-directional pipes. USB data transfers take place between host software and a
particular endpoint on a USB device. Such associations between the host software and a USB device
endpoint are called pipes. In general, data movement though one pipe is independent from the data flow in
any other pipe. A given USB device may have many pipes. As an example, a given USB device could have
an endpoint that supports a pipe for transporting data to the USB device and another endpoint that supports a
pipe for transporting data from the USB device.
The USB architecture comprehends four basic types of data transfers:
•
Control Transfers: Used to configure a device at attach time and can be used for other device-specific
purposes, including control of other pipes on the device.
•
Bulk Data Transfers: Generated or consumed in relatively large and bursty quantities and have wide
dynamic latitude in transmission constraints.
•
Interrupt Data Transfers: Used for timely but reliable delivery of data, for example, characters or
coordinates with human-perceptible echo or feedback response characteristics.
•
Isochronous Data Transfers: Occupy a prenegotiated amount of USB bandwidth with a prenegotiated
delivery latency. (Also called streaming real time transfers).
A pipe supports only one of the types of transfers described above for any given device configuration. The
USB data flow model is described in more detail in Chapter 5.
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4.7.1 Control Transfers
Control data is used by the USB System Software to configure devices when they are first attached. Other
driver software can choose to use control transfers in implementation-specific ways. Data delivery is
lossless.
4.7.2 Bulk Transfers
Bulk data typically consists of larger amounts of data, such as that used for printers or scanners. Bulk data
is sequential. Reliable exchange of data is ensured at the hardware level by using error detection in
hardware and invoking a limited number of retries in hardware. Also, the bandwidth taken up by bulk data
can vary, depending on other bus activities.
4.7.3 Interrupt Transfers
A limited-latency transfer to or from a device is referred to as interrupt data. Such data may be presented
for transfer by a device at any time and is delivered by the USB at a rate no slower than is specified by the
device.
Interrupt data typically consists of event notification, characters, or coordinates that are organized as one or
more bytes. An example of interrupt data is the coordinates from a pointing device. Although an explicit
timing rate is not required, interactive data may have response time bounds that the USB must support.
4.7.4 Isochronous Transfers
Isochronous data is continuous and real-time in creation, delivery, and consumption. Timing-related
information is implied by the steady rate at which isochronous data is received and transferred. Isochronous
data must be delivered at the rate received to maintain its timing. In addition to delivery rate, isochronous
data may also be sensitive to delivery delays. For isochronous pipes, the bandwidth required is typically
based upon the sampling characteristics of the associated function. The latency required is related to the
buffering available at each endpoint.
A typical example of isochronous data is voice. If the delivery rate of these data streams is not maintained,
drop-outs in the data stream will occur due to buffer or frame underruns or overruns. Even if data is
delivered at the appropriate rate by USB hardware, delivery delays introduced by software may degrade
applications requiring real-time turn-around, such as telephony-based audio conferencing.
The timely delivery of isochronous data is ensured at the expense of potential transient losses in the data
stream. In other words, any error in electrical transmission is not corrected by hardware mechanisms such
as retries. In practice, the core bit error rate of the USB is expected to be small enough not to be an issue.
USB isochronous data streams are allocated a dedicated portion of USB bandwidth to ensure that data can
be delivered at the desired rate. The USB is also designed for minimal delay of isochronous data transfers.
4.7.5 Allocating USB Bandwidth
USB bandwidth is allocated among pipes. The USB allocates bandwidth for some pipes when a pipe is
established. USB devices are required to provide some buffering of data. It is assumed that USB devices
requiring more bandwidth are capable of providing larger buffers. The goal for the USB architecture is to
ensure that buffering-induced hardware delay is bounded to within a few milliseconds.
The USB’s bandwidth capacity can be allocated among many different data streams. This allows a wide
range of devices to be attached to the USB. Further, different device bit rates, with a wide dynamic range,
can be concurrently supported.
The USB Specification defines the rules for how each transfer type is allowed access to the bus.
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Universal Serial Specification Revision 2.0
4.8 USB Devices
USB devices are divided into device classes such as hub, human interface, printer, imaging, or mass storage
device. The hub device class indicates a specially designated USB device that provides additional USB
attachment points (refer to Chapter 11). USB devices are required to carry information for selfidentification and generic configuration. They are also required at all times to display behavior consistent
with defined USB device states.
4.8.1 Device Characterizations
All USB devices are accessed by a USB address that is assigned when the device is attached and
enumerated. Each USB device additionally supports one or more pipes through which the host may
communicate with the device. All USB devices must support a specially designated pipe at endpoint zero to
which the USB device’s USB control pipe will be attached. All USB devices support a common access
mechanism for accessing information through this control pipe.
Associated with the control pipe at endpoint zero is the information required to completely describe the
USB device. This information falls into the following categories:
•
Standard: This is information whose definition is common to all USB devices and includes items such
as vendor identification, device class, and power management capability. Device, configuration,
interface, and endpoint descriptions carry configuration-related information about the device. Detailed
information about these descriptors can be found in Chapter 9.
•
Class: The definition of this information varies, depending on the device class of the USB device.
•
USB Vendor: The vendor of the USB device is free to put any information desired here. The format,
however, is not determined by this specification.
Additionally, each USB device carries USB control and status information.
4.8.2 Device Descriptions
Two major divisions of device classes exist: hubs and functions. Only hubs have the ability to provide
additional USB attachment points. Functions provide additional capabilities to the host.
4.8.2.1 Hubs
Hubs are a key element in the plug-and-play architecture of the USB. Figure 4-3 shows a typical hub. Hubs
serve to simplify USB connectivity from the user’s perspective and provide robustness at relatively low cost
and complexity.
Hubs are wiring concentrators and enable the multiple attachment characteristics of the USB. Attachment
points are referred to as ports. Each hub converts a single attachment point into multiple attachment points.
The architecture supports concatenation of multiple hubs.
The upstream port of a hub connects the hub towards the host. Each of the downstream ports of a hub
allows connection to another hub or function. Hubs can detect attach and detach at each downstream port
and enable the distribution of power to downstream devices. Each downstream port can be individually
enabled and attached to either high-, full- or low-speed devices.
A USB 2.0 hub consists of three portions: the Hub Controller, the Hub Repeater, and the Transaction
Translator. The Hub Repeater is a protocol-controlled switch between the upstream port and downstream
ports. It also has hardware support for reset and suspend/resume signaling. The Host Controller provides
the communication to/from the host. Hub-specific status and control commands permit the host to
configure a hub and to monitor and control its ports. The Transaction Translator provides the mechanisms
that support full-/low-speed devices behind the hub, while transmitting all device data between the host and
the hub at high-speed.
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Universal Serial Bus Specification Revision 2.0
Port
#1
Upstream
Port
Port
#2
Port
#3
HUB
Port
#7
Port
#6
Port
#4
Port
#5
Figure 4-3. A Typical Hub
Figure 4-4 illustrates how hubs provide connectivity in a typical desktop computer environment.
USB
TYPICAL USB ARCHITECTURAL
CONFIGURATION
Hub/Function
Hub/Function
KBD
Pen
Function
Host/Hub
Monitor
Mouse
Speaker
Function
Function
PC
Mic
Function
Phone
Function
Hub
Hub
Figure 4-4. Hubs in a Desktop Computer Environment
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Universal Serial Specification Revision 2.0
4.8.2.2 Functions
A function is a USB device that is able to transmit or receive data or control information over the bus. A
function is typically implemented as a separate peripheral device with a cable that plugs into a port on a
hub. However, a physical package may implement multiple functions and an embedded hub with a single
USB cable. This is known as a compound device. A compound device appears to the host as a hub with
one or more non-removable USB devices.
Each function contains configuration information that describes its capabilities and resource requirements.
Before a function can be used, it must be configured by the host. This configuration includes allocating
USB bandwidth and selecting function-specific configuration options.
Examples of functions include the following:
•
A human interface device such as a mouse, keyboard, tablet, or game controller
•
An imaging device such as a scanner, printer, or camera
•
A mass storage device such as a CD-ROM drive, floppy drive, or DVD drive
4.9 USB Host: Hardware and Software
The USB host interacts with USB devices through the Host Controller. The host is responsible for the
following:
•
Detecting the attachment and removal of USB devices
•
Managing control flow between the host and USB devices
•
Managing data flow between the host and USB devices
•
Collecting status and activity statistics
•
Providing power to attached USB devices
The USB System Software on the host manages interactions between USB devices and host-based device
software. There are five areas of interactions between the USB System Software and device software:
•
Device enumeration and configuration
•
Isochronous data transfers
•
Asynchronous data transfers
•
Power management
•
Device and bus management information
4.10 Architectural Extensions
The USB architecture comprehends extensibility at the interface between the Host Controller Driver and
USB Driver. Implementations with multiple Host Controllers, and associated Host Controller Drivers, are
possible.
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Universal Serial Bus Specification Revision 2.0
Chapter 5
USB Data Flow Model
This chapter presents information about how data is moved across the USB. The information in this chapter
affects all implementers. The information presented is at a level above the signaling and protocol
definitions of the system. Consult Chapter 7 and Chapter 8 for more details about their respective parts of
the USB system. This chapter provides framework information that is further expanded in Chapters 9
through 11. All implementers should read this chapter so they understand the key concepts of the USB.
5.1 Implementer Viewpoints
The USB provides communication services between a host and attached USB devices. However, the simple
view an end user sees of attaching one or more USB devices to a host, as in Figure 5-1, is in fact a little
more complicated to implement than is indicated by the figure. Different views of the system are required
to explain specific USB requirements from the perspective of different implementers. Several important
concepts and features must be supported to provide the end user with the reliable operation demanded from
today’s personal computers. The USB is presented in a layered fashion to ease explanation and allow
implementers of particular USB products to focus on the details related to their product.
USB Host
USB Device
Figure 5-1. Simple USB Host/Device View
Figure 5-2 shows a deeper overview of the USB, identifying the different layers of the system that will be
described in more detail in the remainder of the specification. In particular, there are four focus
implementation areas:
•
USB Physical Device: A piece of hardware on the end of a USB cable that performs some useful end
user function.
•
Client Software: Software that executes on the host, corresponding to a USB device. This client
software is typically supplied with the operating system or provided along with the USB device.
•
USB System Software: Software that supports the USB in a particular operating system. The USB
System Software is typically supplied with the operating system, independently of particular USB
devices or client software.
•
USB Host Controller (Host Side Bus Interface): The hardware and software that allows USB devices
to be attached to a host.
There are shared rights and responsibilities between the four USB system components. The remainder of
this specification describes the details required to support robust, reliable communication flows between a
function and its client.
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Universal Serial Bus Specification Revision 2.0
Host
Interconnect
Physical Device
Client SW
Function
USB System
SW
USB Logical
Device
USB Host
Controller
USB Bus
Interface
Function Layer
USB Device
Layer
USB Bus
Interface Layer
Actual communications flow
Logical communications flow
Implementation Focus Area
Figure 5-2. USB Implementation Areas
As shown in Figure 5-2, the simple connection of a host to a device requires interaction between a number
of layers and entities. The USB Bus Interface layer provides physical/signaling/packet connectivity
between the host and a device. The USB Device layer is the view the USB System Software has for
performing generic USB operations with a device. The Function layer provides additional capabilities to
the host via an appropriate matched client software layer. The USB Device and Function layers each have a
view of logical communication within their layer that actually uses the USB Bus Interface layer to
accomplish data transfer.
The physical view of USB communication as described in Chapters 6, 7, and 8 is related to the logical
communication view presented in Chapters 9 and 10. This chapter describes those key concepts that affect
USB implementers and should be read by all before proceeding to the remainder of the specification to find
those details most relevant to their product.
To describe and manage USB communication, the following concepts are important:
26
•
Bus Topology: Section 5.2 presents the primary physical and logical components of the USB and how
they interrelate.
•
Communication Flow Models: Sections 5.3 through 5.8 describe how communication flows between
the host and devices through the USB and defines the four USB transfer types.
•
Bus Access Management: Section 5.11 describes how bus access is managed within the host to support
a broad range of communication flows by USB devices.
•
Special Consideration for Isochronous Transfers: Section 5.12 presents features of the USB specific to
devices requiring isochronous data transfers. Device implementers for non-isochronous devices do not
need to read Section 5.12.
Universal Serial Bus Specification Revision 2.0
5.2 Bus Topology
There are four main parts to USB topology:
•
Host and Devices: The primary components of a USB system
•
Physical Topology: How USB elements are connected
•
Logical Topology: The roles and responsibilities of the various USB elements and how the USB
appears from the perspective of the host and a device
•
Client Software-to-function Relationships: How client software and its related function interfaces on a
USB device view each other
5.2.1 USB Host
The host’s logical composition is shown in Figure 5-3 and includes the following:
•
USB Host Controller
•
Aggregate USB System Software (USB Driver, Host Controller Driver, and host software)
•
Client
Host
Client SW
USB System SW
USB Host
Controller
Actual communications flow
Logical communications flow
Figure 5-3. Host Composition
The USB host occupies a unique position as the coordinating entity for the USB. In addition to its special
physical position, the host has specific responsibilities with regard to the USB and its attached devices. The
host controls all access to the USB. A USB device gains access to the bus only by being granted access by
the host. The host is also responsible for monitoring the topology of the USB.
For a complete discussion of the host and its duties, refer to Chapter 10.
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5.2.2 USB Devices
A USB physical device’s logical composition is shown in Figure 5-4 and includes the following:
•
USB bus interface
•
USB logical device
•
Function
USB physical devices provide additional functionality to the host. The types of functionality provided by
USB devices vary widely. However, all USB logical devices present the same basic interface to the host.
This allows the host to manage the USB-relevant aspects of different USB devices in the same manner.
To assist the host in identifying and configuring USB devices, each device carries and reports configurationrelated information. Some of the information reported is common among all logical devices. Other
information is specific to the functionality provided by the device. The detailed format of this information
varies, depending on the device class of the device.
For a complete discussion of USB devices, refer to Chapter 9.
Physical Device
Function
USB Logical
Device
USB Bus
Interface
Actual communications flow
Logical communications flow
Figure 5-4. Physical Device Composition
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5.2.3 Physical Bus Topology
Devices on the USB are physically connected to the host via a tiered star topology, as illustrated in
Figure 5-5. USB attachment points are provided by a special class of USB device known as a hub. The
additional attachment points provided by a hub are called ports. A host includes an embedded hub called
the root hub. The host provides one or more attachment points via the root hub. USB devices that provide
additional functionality to the host are known as functions. To prevent circular attachments, a tiered
ordering is imposed on the star topology of the USB. This results in the tree-like configuration illustrated in
Figure 5-5.
Host
Device
Device
Root Hub
Compound Device
Hub
Device
Hub
Device
Device
Device
Device
Figure 5-5. USB Physical Bus Topology
Multiple functions may be packaged together in what appears to be a single physical device. For example, a
keyboard and a trackball might be combined in a single package. Inside the package, the individual
functions are permanently attached to a hub and it is the internal hub that is connected to the USB. When
multiple functions are combined with a hub in a single package, they are referred to as a compound device.
The hub and each function attached to the hub within the compound device is assigned its own device
address. A device that has multiple interfaces controlled independently of each other is referred to as a
composite device. A composite device has only a single device address. From the host’s perspective, a
compound device is the same as a separate hub with multiple functions attached. Figure 5-5 also illustrates
a compound device.
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USB 2.0 Host
Controller
High Speed Only
HS Hub
FS/ LS
Device
USB 1.1 Hub
HS Device
FS/ LS
Device
Full/Low Speed
(2 x 12 Mb/s
Capacity)
Figure 5-6. Multiple Full-speed Buses in a High-speed System
The hub plays a special role in a high-speed system. The hub isolates the full-/low-speed signaling
environment from the high-speed signaling environment. Figure 5-6 shows a hub operating in high speed
supporting a high-speed attached device. The hub also allows USB1.1 hubs to attach and operate at full/low-speed along with other full-/low-speed only devices. The host controller also directly supports
attaching full-/low-speed only devices. Chapter 11 describes the details of how the hub accomplishes the
isolation of the two signaling environments.
Each high-speed operating hub essentially adds one (or more) additional full-/low-speed buses; i.e., each
hub supports additional (optionally multiple) 12 Mb/s of USB full-/low-speed bandwidth. This allows more
full-/low-speed buses to be attached without requiring additional host controllers in a system. Even though
there can be several 12 Mb/s full-/low-speed buses, there are only at most 127 USB devices attached to any
single host controller.
5.2.4 Logical Bus Topology
While devices physically attach to the USB in a tiered, star topology, the host communicates with each
logical device as if it were directly connected to the root port. This creates the logical view illustrated in
Figure 5-7 that corresponds to the physical topology shown in Figure 5-5. Hubs are logical devices also but
are not shown in Figure 5-7 to simplify the picture. Even though most host/logical device activities use this
logical perspective, the host maintains an awareness of the physical topology to support processing the
removal of hubs. When a hub is removed, all of the devices attached to the hub must be removed from the
host’s view of the logical topology. A more complete discussion of hubs can be found in Chapter 11.
Host
Logical
Device
Logical
Device
Logical
Device
Logical
Device
Logical
Device
Logical
Device
Logical
Device
Figure 5-7. USB Logical Bus Topology
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Universal Serial Bus Specification Revision 2.0
5.2.5 Client Software-to-function Relationship
Even though the physical and logical topology of the USB reflects the shared nature of the bus, client
software (CSw) manipulating a USB function interface is presented with the view that it deals only with its
interface(s) of interest. Client software for USB functions must use USB software programming interfaces
to manipulate their functions as opposed to directly manipulating their functions via memory or I/O accesses
as with other buses (e.g., PCI, EISA, PCMCIA, etc.). During operation, client software should be
independent of other devices that may be connected to the USB. This allows the designer of the device and
client software to focus on the hardware/software interaction design details. Figure 5-8 illustrates a device
designer’s perspective of the relationships of client software and USB functions with respect to the USB
logical topology of Figure 5-7.
Client
Software
Func
CSw
CSw
CSw
CSw
CSw
CSw
Func
Func
Func
Func
Func
Func
Figure 5-8. Client Software-to-function Relationships
5.3 USB Communication Flow
The USB provides a communication service between software on the host and its USB function. Functions
can have different communication flow requirements for different client-to-function interactions. The USB
provides better overall bus utilization by allowing the separation of the different communication flows to a
USB function. Each communication flow makes use of some bus access to accomplish communication
between client and function. Each communication flow is terminated at an endpoint on a device. Device
endpoints are used to identify aspects of each communication flow.
Figure 5-9 shows a more detailed view of Figure 5-2. The complete definition of the actual communication
flows of Figure 5-2 supports the logical device and function layer communication flows. These actual
communication flows cross several interface boundaries. Chapters 6 through 8 describe the mechanical,
electrical, and protocol interface definitions of the USB “wire.” Chapter 9 describes the USB device
programming interface that allows a USB device to be manipulated from the host side of the wire.
Chapter 10 describes two host side software interfaces:
•
Host Controller Driver (HCD): The software interface between the USB Host Controller and USB
System Software. This interface allows a range of Host Controller implementations without requiring
all host software to be dependent on any particular implementation. One USB Driver can support
different Host Controllers without requiring specific knowledge of a Host Controller implementation.
A Host Controller implementer provides an HCD implementation that supports the Host Controller.
•
USB Driver (USBD): The interface between the USB System Software and the client software. This
interface provides clients with convenient functions for manipulating USB devices.
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Universal Serial Bus Specification Revision 2.0
Host
Interconnect
Physical Device
Function
Client SW
a collection of
interfaces
Interface x
manages an interface
Pipe Bundle
to an interface
Buffers
Interfacespecific
No USB
Format
No USB
Format
Endpoint
Zero
USB System SW
manages devices
Default Pipe
USB Logical
Device
a collection of
endpoints
USB Device
to Endpoint Zero
Transfers
USB
Framed
Data
USB Bus
Interface
USB Host
(Chapter 10)
Data Per
Endpoint
USB
Framed
Data
(Chapter 9)
USB Bus
Interface
Host
Controller USB Framed
Data
SIE
Transactions
SIE
USB Wire
Pipe: represents connection abstraction
between two horizontal entities
Mechanical,
Data transport mechanism
Electrical,
Protocol
USB-relevant format of transported data
(Chapter 6, 7, 8)
Figure 5-9. USB Host/Device Detailed View
A USB logical device appears to the USB system as a collection of endpoints. Endpoints are grouped into
endpoint sets that implement an interface. Interfaces are views to the function. The USB System Software
manages the device using the Default Control Pipe. Client software manages an interface using pipe
bundles (associated with an endpoint set). Client software requests that data be moved across the USB
between a buffer on the host and an endpoint on the USB device. The Host Controller (or USB device,
depending on transfer direction) packetizes the data to move it over the USB. The Host Controller also
coordinates when bus access is used to move the packet of data over the USB.
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Universal Serial Bus Specification Revision 2.0
Figure 5-10 illustrates how communication flows are carried over pipes between endpoints and host side
memory buffers. The following sections describe endpoints, pipes, and communication flows in more
detail.
Host
Client
Software
Buffers
Communication
Flows
Pipes
Endpoints
USB Logical Device
Interface
Figure 5-10. USB Communication Flow
Software on the host communicates with a logical device via a set of communication flows. The set of
communication flows are selected by the device software/hardware designer(s) to efficiently match the
communication requirements of the device to the transfer characteristics provided by the USB.
5.3.1 Device Endpoints
An endpoint is a uniquely identifiable portion of a USB device that is the terminus of a communication flow
between the host and device. Each USB logical device is composed of a collection of independent
endpoints. Each logical device has a unique address assigned by the system at device attachment time.
Each endpoint on a device is given at design time a unique device-determined identifier called the endpoint
number. Each endpoint has a device-determined direction of data flow. The combination of the device
address, endpoint number, and direction allows each endpoint to be uniquely referenced. Each endpoint is a
simplex connection that supports data flow in one direction: either input (from device to host) or output
(from host to device).
An endpoint has characteristics that determine the type of transfer service required between the endpoint
and the client software. An endpoint describes itself by:
•
Bus access frequency/latency requirement
•
Bandwidth requirement
•
Endpoint number
•
Error handling behavior requirements
•
Maximum packet size that the endpoint is capable of sending or receiving
•
The transfer type for the endpoint (refer to Section 5.4 for details)
•
The direction in which data is transferred between the endpoint and the host
Endpoints other than those with endpoint number zero are in an unknown state before being configured and
may not be accessed by the host before being configured.
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Universal Serial Bus Specification Revision 2.0
5.3.1.1 Endpoint Zero Requirements
All USB devices are required to implement a default control method that uses both the input and output
endpoints with endpoint number zero. The USB System Software uses this default control method to
initialize and generically manipulate the logical device (e.g., to configure the logical device) as the Default
Control Pipe (see Section 5.3.2). The Default Control Pipe provides access to the device’s configuration
information and allows generic USB status and control access. The Default Control Pipe supports control
transfers as defined in Section 5.5. The endpoints with endpoint number zero are always accessible once a
device is attached, powered, and has received a bus reset.
A USB device that is capable of operating at high-speed must have a minimum level of support for
operating at full-speed. When the device is attached to a hub operating in full-speed, the device must:
•
Be able to reset successfully at full-speed
•
Respond successfully to standard requests: set_address, set_configuration, get_descriptor for device and
configuration descriptors, and return appropriate information
The high-speed device may or may not be able to support its intended functionality when operating at fullspeed.
5.3.1.2 Non-endpoint Zero Requirements
Functions can have additional endpoints as required for their implementation. Low-speed functions are
limited to two optional endpoints beyond the two required to implement the Default Control Pipe. Fullspeed devices can have additional endpoints only limited by the protocol definition (i.e., a maximum of 15
additional input endpoints and 15 additional output endpoints).
Endpoints other than those for the Default Control Pipe cannot be used until the device is configured as a
normal part of the device configuration process (refer to Chapter 9).
5.3.2 Pipes
A USB pipe is an association between an endpoint on a device and software on the host. Pipes represent the
ability to move data between software on the host via a memory buffer and an endpoint on a device. There
are two mutually exclusive pipe communication modes:
•
Stream: Data moving through a pipe has no USB-defined structure
•
Message: Data moving through a pipe has some USB-defined structure
The USB does not interpret the content of data it delivers through a pipe. Even though a message pipe
requires that data be structured according to USB definitions, the content of the data is not interpreted by the
USB.
Additionally, pipes have the following associated with them:
•
A claim on USB bus access and bandwidth usage.
•
A transfer type.
•
The associated endpoint’s characteristics, such as directionality and maximum data payload sizes. The
data payload is the data that is carried in the data field of a data packet within a bus transaction (as
defined in Chapter 8).
The pipe that consists of the two endpoints with endpoint number zero is called the Default Control Pipe.
This pipe is always available once a device is powered and has received a bus reset. Other pipes come into
existence when a USB device is configured. The Default Control Pipe is used by the USB System Software
to determine device identification and configuration requirements and to configure the device. The Default
Control Pipe can also be used by device-specific software after the device is configured. The USB System
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Universal Serial Bus Specification Revision 2.0
Software retains “ownership” of the Default Control Pipe and mediates use of the pipe by other client
software.
A software client normally requests data transfers via I/O Request Packets (IRPs) to a pipe and then either
waits or is notified when they are completed. Details about IRPs are defined in an operating systemspecific manner. This specification uses the term to simply refer to an identifiable request by a software
client to move data between itself (on the host) and an endpoint of a device in an appropriate direction. A
software client can cause a pipe to return all outstanding IRPs if it desires. The software client is notified
that an IRP has completed when the bus transactions associated with it have completed either successfully
or due to errors.
If there are no IRPs pending or in progress for a pipe, the pipe is idle and the Host Controller will take no
action with regard to the pipe; i.e., the endpoint for such a pipe will not see any bus transactions directed to
it. The only time bus activity is present for a pipe is when IRPs are pending for that pipe.
If a non-isochronous pipe encounters a condition that causes it to send a STALL to the host (refer to
Chapter 8) or three bus errors are encountered on any packet of an IRP, the IRP is aborted/retired, all
outstanding IRPs are also retired, and no further IRPs are accepted until the software client recovers from
the condition (in an implementation-dependent way) and acknowledges the halt or error condition via a
USBD call. An appropriate status informs the software client of the specific IRP result for error versus halt
(refer to Chapter 10). Isochronous pipe behavior is described in Section 5.6.
An IRP may require multiple data payloads to move the client data over the bus. The data payloads for such
a multiple data payload IRP are expected to be of the maximum packet size until the last data payload that
contains the remainder of the overall IRP. See the description of each transfer type for more details. For
such an IRP, short packets (i.e., less than maximum-sized data payloads) on input that do not completely fill
an IRP data buffer can have one of two possible meanings, depending upon the expectations of a client:
•
A client can expect a variable-sized amount of data in an IRP. In this case, a short packet that does not
fill an IRP data buffer can be used simply as an in-band delimiter to indicate “end of unit of data.” The
IRP should be retired without error and the Host Controller should advance to the next IRP.
•
A client can expect a specific-sized amount of data. In this case, a short packet that does not fill an IRP
data buffer is an indication of an error. The IRP should be retired, the pipe should be stalled, and any
pending IRPs associated with the pipe should also be retired.
Because the Host Controller must behave differently in the two cases and cannot know on its own which
way to behave for a given IRP; it is possible to indicate per IRP which behavior the client desires.
An endpoint can inform the host that it is busy by responding with NAK. NAKs are not used as a retire
condition for returning an IRP to a software client. Any number of NAKs can be encountered during the
processing of a given IRP. A NAK response to a transaction does not constitute an error and is not counted
as one of the three errors described above.
5.3.2.1 Stream Pipes
Stream pipes deliver data in the data packet portion of bus transactions with no USB-required structure on
the data content. Data flows in at one end of a stream pipe and out the other end in the same order. Stream
pipes are always uni-directional in their communication flow.
Data flowing through a stream pipe is expected to interact with what the USB believes is a single client.
The USB System Software is not required to provide synchronization between multiple clients that may be
using the same stream pipe. Data presented to a stream pipe is moved through the pipe in sequential order:
first-in, first-out.
A stream pipe to a device is bound to a single device endpoint number in the appropriate direction (i.e.,
corresponding to an IN or OUT token as defined by the protocol layer). The device endpoint number for the
opposite direction can be used for some other stream pipe to the device.
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Universal Serial Bus Specification Revision 2.0
Stream pipes support bulk, isochronous, and interrupt transfer types, which are explained in later sections.
5.3.2.2 Message Pipes
Message pipes interact with the endpoint in a different manner than stream pipes. First, a request is sent to
the USB device from the host. This request is followed by data transfer(s) in the appropriate direction.
Finally, a Status stage follows at some later time. In order to accommodate the request/data/status
paradigm, message pipes impose a structure on the communication flow that allows commands to be
reliably identified and communicated. Message pipes allow communication flow in both directions,
although the communication flow may be predominately one way. The Default Control Pipe is always a
message pipe.
The USB System Software ensures that multiple requests are not sent to a message pipe concurrently. A
device is required to service only a single message request at a time per message pipe. Multiple software
clients on the host can make requests via the Default Control Pipe, but they are sent to the device in a firstin, first-out order. A device can control the flow of information during the Data and Status stages based on
its ability to respond to the host transactions (refer to Chapter 8 for more details).
A message pipe will not normally be sent the next message from the host until the current message’s
processing at the device has been completed. However, there are error conditions whereby a message
transfer can be aborted by the host and the message pipe can be sent a new message transfer prematurely
(from the device’s perspective). From the perspective of the software manipulating a message pipe, an error
on some part of an IRP retires the current IRP and all queued IRPs. The software client that requested the
IRP is notified of the IRP completion with an appropriate error indication.
A message pipe to a device requires a single device endpoint number in both directions (IN and OUT
tokens). The USB does not allow a message pipe to be associated with different endpoint numbers for each
direction.
Message pipes support the control transfer type, which is explained in Section 5.5.
5.3.3 Frames and Microframes
USB establishes a 1 millisecond time base called a frame on a full-/low-speed bus and a 125 µs time base
called a microframe on a high-speed bus. A (micro)frame can contain several transactions. Each transfer
type defines what transactions are allowed within a (micro)frame for an endpoint. Isochronous and interrupt
endpoints are given opportunities to the bus every N (micro)frames. The values of N and other details about
isochronous and interrupt transfers are described in Sections 5.6 and 5.7.
5.4 Transfer Types
The USB transports data through a pipe between a memory buffer associated with a software client on the
host and an endpoint on the USB device. Data transported by message pipes is carried in a USB-defined
structure, but the USB allows device-specific structured data to be transported within the USB-defined
message data payload. The USB also defines that data moved over the bus is packetized for any pipe
(stream or message), but ultimately the formatting and interpretation of the data transported in the data
payload of a bus transaction is the responsibility of the client software and function using the pipe.
However, the USB provides different transfer types that are optimized to more closely match the service
requirements of the client software and function using the pipe. An IRP uses one or more bus transactions
to move information between a software client and its function.
Each transfer type determines various characteristics of the communication flow including the following:
36
•
Data format imposed by the USB
•
Direction of communication flow
•
Packet size constraints
Universal Serial Bus Specification Revision 2.0
•
Bus access constraints
•
Latency constraints
•
Required data sequences
•
Error handling
The designers of a USB device choose the capabilities for the device’s endpoints. When a pipe is
established for an endpoint, most of the pipe’s transfer characteristics are determined and remain fixed for
the lifetime of the pipe. Transfer characteristics that can be modified are described for each transfer type.
The USB defines four transfer types:
•
Control Transfers: Bursty, non-periodic, host software-initiated request/response communication,
typically used for command/status operations.
•
Isochronous Transfers: Periodic, continuous communication between host and device, typically used
for time-relevant information. This transfer type also preserves the concept of time encapsulated in the
data. This does not imply, however, that the delivery needs of such data is always time-critical.
•
Interrupt Transfers: Low-frequency, bounded-latency communication.
•
Bulk Transfers: Non-periodic, large-packet bursty communication, typically used for data that can use
any available bandwidth and can also be delayed until bandwidth is available.
Each transfer type is described in detail in the following four major sections. The data for any IRP is
carried by the data field of the data packet as described in Section 8.3.4. Chapter 8 also describes details of
the protocol that are affected by use of each particular transfer type.
5.4.1 Table Calculation Examples
The following sections describe each of the USB transfer types. In these sections, there are tables that
illustrate the maximum number of transactions that can be expected to be contained in a (micro)frame.
These tables can be used to determine the maximum performance behavior possible for a specific transfer
type. Actual performance may vary with specific system implementation details.
Each table shows:
•
The protocol overhead required for the specific transfer type (and speed)
•
For some sample data payload sizes:
o
The maximum sustained bandwidth possible for this case
o
The percentage of a (micro)frame that each transaction requires
o
The maximum number of transactions in a (micro)frame for the specific case
o
The remaining bytes in a (micro)frame that would not be required for the specific case
o
The total number of data bytes transported in a single (micro)frame for the specific case
A transaction of a particular transfer type typically requires multiple packets. The protocol overhead for
each transaction includes:
•
A SYNC field for each packet: either 8 bits (full-/low-speed) or 32 bits (high-speed)
•
A PID byte for each packet: includes PID and PID invert (check) bits
•
An EOP for each packet: 3 bits (full-/low-speed) or 8 bits (high-speed)
•
In a token packet, the endpoint number, device address, and CRC5 fields (16 bits total)
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Universal Serial Bus Specification Revision 2.0
•
In a data packet, CRC16 fields (16 bits total)
•
In a data packet, any data field (8 bits per byte)
•
For transaction with multiple packets, the inter packet gap or bus turnaround time required.
For these calculations, there is assumed to be no bit-stuffing required.
Using the low speed interrupt OUT as an example, there are 5 packets in the transaction:
•
A PRE special packet
•
A token packet
•
A PRE special packet
•
A data packet
•
A handshake packet
There is one bus turnaround between the data and handshake packets. The protocol overhead is therefore:
5 SYNC, 5 PID, Endpoint + CRC5, CRC16, 5 EOPs and interpacket delay (one bus turnaround, 1 delay
between packets, and 2 hub setup times).
5.5 Control Transfers
Control transfers allow access to different parts of a device. Control transfers are intended to support
configuration/command/status type communication flows between client software and its function. A
control transfer is composed of a Setup bus transaction moving request information from host to function,
zero or more Data transactions sending data in the direction indicated by the Setup transaction, and a Status
transaction returning status information from function to host. The Status transaction returns “success”
when the endpoint has successfully completed processing the requested operation. Section 8.5.3 describes
the details of what packets, bus transactions, and transaction sequences are used to accomplish a control
transfer. Chapter 9 describes the details of the defined USB command codes.
Each USB device is required to implement the Default Control Pipe as a message pipe. This pipe is used by
the USB System Software. The Default Control Pipe provides access to the USB device’s configuration,
status, and control information. A function can, but is not required to, provide endpoints for additional
control pipes for its own implementation needs.
The USB device framework (refer to Chapter 9) defines standard, device class, or vendor-specific requests
that can be used to manipulate a device’s state. Descriptors are also defined that can be used to contain
different information on the device. Control transfers provide the transport mechanism to access device
descriptors and make requests of a device to manipulate its behavior.
Control transfers are carried only through message pipes. Consequently, data flows using control transfers
must adhere to USB data structure definitions as described in Section 5.5.1.
The USB system will make a “best effort” to support delivery of control transfers between the host and
devices. A function and its client software cannot request specific bus access frequency or bandwidth for
control transfers. The USB System Software may restrict the bus access and bandwidth that a device may
desire for control transfers. These restrictions are defined in Section 5.5.3 and Section 5.5.4.
5.5.1 Control Transfer Data Format
The Setup packet has a USB-defined structure that accommodates the minimum set of commands required
to enable communication between the host and a device. The structure definition allows vendor-specific
extensions for device specific commands. The Data transactions following Setup have a USB-defined
structure except when carrying vendor-specific information. The Status transaction also has a USB-defined
structure. Specific control transfer Setup/Data definitions are described in Section 8.5.3 and Chapter 9.
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Universal Serial Bus Specification Revision 2.0
5.5.2 Control Transfer Direction
Control transfers are supported via bi-directional communication flow over message pipes. As a
consequence, when a control pipe is configured, it uses both the input and output endpoint with the specified
endpoint number.
5.5.3 Control Transfer Packet Size Constraints
An endpoint for control transfers specifies the maximum data payload size that the endpoint can accept from
or transmit to the bus. The allowable maximum control transfer data payload sizes for full-speed devices is
8, 16, 32, or 64 bytes; for high-speed devices, it is 64 bytes and for low-speed devices, it is 8 bytes. This
maximum applies to the data payloads of the Data packets following a Setup; i.e., the size specified is for
the data field of the packet as defined in Chapter 8, not including other information that is required by the
protocol. A Setup packet is always eight bytes. A control pipe (including the Default Control Pipe) always
uses its wMaxPacketSize value for data payloads.
An endpoint reports in its configuration information the value for its maximum data payload size. The USB
does not require that data payloads transmitted be exactly the maximum size; i.e., if a data payload is less
than the maximum, it does not need to be padded to the maximum size.
All Host Controllers are required to have support for 8-, 16-, 32-, and 64-byte maximum data payload sizes
for full-speed control endpoints, only 8-byte maximum data payload sizes for low-speed control endpoints,
and only 64-byte maximum data payload size for high-speed control endpoints. No Host Controller is
required to support larger or smaller maximum data payload sizes.
In order to determine the maximum packet size for the Default Control Pipe, the USB System Software
reads the device descriptor. The host will read the first eight bytes of the device descriptor. The device
always responds with at least these initial bytes in a single packet. After the host reads the initial part of the
device descriptor, it is guaranteed to have read this default pipe’s wMaxPacketSize field (byte 7 of the
device descriptor). It will then allow the correct size for all subsequent transactions. For all other control
endpoints, the maximum data payload size is known after configuration so that the USB System Software
can ensure that no data payload will be sent to the endpoint that is larger than the supported size.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
wMaxPacketSize (refer to Chapter 9). When a control transfer involves more data than can fit in one data
payload of the currently established maximum size, all data payloads are required to be maximum-sized
except for the last data payload, which will contain the remaining data.
The Data stage of a control transfer from an endpoint to the host is complete when the endpoint does one of
the following:
•
Has transferred exactly the amount of data specified during the Setup stage
•
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
When a Data stage is complete, the Host Controller advances to the Status stage instead of continuing on
with another data transaction. If the Host Controller does not advance to the Status stage when the Data
stage is complete, the endpoint halts the pipe as was outlined in Section 5.3.2. If a larger-than-expected data
payload is received from the endpoint, the IRP for the control transfer will be aborted/retired.
The Data stage of a control transfer from the host to an endpoint is complete when all of the data has been
transferred. If the endpoint receives a larger-than-expected data payload from the host, it halts the pipe.
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Universal Serial Bus Specification Revision 2.0
5.5.4 Control Transfer Bus Access Constraints
Control transfers can be used by high-speed, full-speed, and low-speed USB devices.
An endpoint has no way to indicate a desired bus access frequency for a control pipe. The USB balances
the bus access requirements of all control pipes and the specific IRPs that are pending to provide “best
effort” delivery of data between client software and functions.
The USB requires that part of each (micro)frame be reserved to be available for use by control transfers as
follows:
•
If the control transfers that are attempted (in an implementation-dependent fashion) consume less than
10% of the frame time for full-/low-speed endpoints or less than 20% of a microframe for high-speed
endpoints, the remaining time can be used to support bulk transfers (refer to Section 5.8).
•
A control transfer that has been attempted and needs to be retried can be retried in the current or a
future (micro)frame; i.e., it is not required to be retried in the same (micro)frame.
•
If there are more control transfers than reserved time, but there is additional (micro)frame time that is
not being used for isochronous or interrupt transfers, a Host Controller may move additional control
transfers as they are available.
•
If there are too many pending control transfers for the available (micro)frame time, control transfers are
selected to be moved over the bus as appropriate.
•
If there are control transfers pending for multiple endpoints, control transfers for the different endpoints
are selected according to a fair access policy that is Host Controller implementation-dependent.
•
A transaction of a control transfer that is frequently being retried should not be expected to consume an
unfair share of the bus time.
High-speed control endpoints must support the PING flow control protocol for OUT transactions. The
details of this protocol are described in Section 8.5.1.
These requirements allow control transfers between host and devices to be regularly moved over the bus
with “best effort.”
The USB System Software can, at its discretion, vary the rate of control transfers to a particular endpoint.
An endpoint and its client software cannot assume a specific rate of service for control transfers. A control
endpoint may see zero or more transfers in a single (micro)frame. Bus time made available to a software
client and its endpoint can be changed as other devices are inserted into and removed from the system or
also as control transfers are requested for other device endpoints.
The bus frequency and (micro)frame timing limit the maximum number of successful control transfers
within a (micro)frame for any USB system. For full-/low-speed buses, the number of successful control
transfers per frame is limited to less than 29 full-speed eight-byte data payloads or less than four low-speed
eight-byte data payloads. For high-speed buses, the number of control transfers is limited to less than
32 high-speed 64-byte data payloads per microframe.
Table 5-1 lists information about different-sized low-speed packets and the maximum number of packets
possible in a frame. The table does not include the overhead associated with bit stuffing.
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Universal Serial Bus Specification Revision 2.0
Table 5-1. Low-speed Control Transfer Limits
Max
Protocol Overhead (63 bytes)
(15 SYNC bytes, 15 PID bytes, 6 Endpoint + CRC bytes,
6 CRC bytes, 8 Setup data bytes, and a 13-byte interpacket
delay (EOP, etc.))
Data
Payload
Frame
Bandwidth
per
Transfer
Max
Transfers
Bytes
Remaining
Max Bandwidth
(bytes/second)
Bytes/Frame
Useful Data
1
3000
26%
3
40
3
2
6000
27%
3
37
6
4
12000
28%
3
31
12
8
24000
30%
3
19
24
187500
187
For all speeds, because a control transfer is composed of several packets, the packets can be spread over
several (micro)frames to spread the bus time required across several (micro)frames.
The 10% frame reservation for full-/low-speed non-periodic transfers means that in a system with bus time
fully allocated, all full-speed control transfers in the system contend for a nominal three control transfers per
frame. Because the USB system uses control transfers for configuration purposes in addition to whatever
other control transfers other client software may be requesting, a given software client and its function
should not expect to be able to make use of this full bandwidth for its own control purposes. Host
Controllers are also free to determine how the individual bus transactions for specific control transfers are
moved over the bus within and across frames. An endpoint could see all bus transactions for a control
transfer within the same frame or spread across several noncontiguous frames. A Host Controller, for
various implementation reasons, may not be able to provide the theoretical maximum number of control
transfers per frame.
For high-speed endpoints, the 20% microframe reservation for non-periodic transfers means that all high
speed control transfers are contending for nominally six control transfers per microframe. High-speed
control transfers contend for microframe time along with split-transactions (see Sections 11.15-11.21 for
more information about split transactions) for full- and low-speed control transfers. Both full-speed and
low-speed control transfers contend for the same available frame time. However, high-speed control
transfers for some endpoints can occur simultaneously with full- and low-speed control transfers for other
endpoints. Low-speed control transfers simply take longer to transfer.
41
Universal Serial Bus Specification Revision 2.0
Table 5-2 lists information about different-sized full-speed control transfers and the maximum number of
transfers possible in a frame. This table was generated assuming that there is one Data stage transaction and
that the Data stage has a zero-length status phase. The table illustrates the possible power of two data
payloads less than or equal to the allowable maximum data payload sizes. The table does not include the
overhead associated with bit stuffing.
Table 5-2. Full-speed Control Transfer Limits
Max
42
Protocol Overhead (45 bytes)
(9 SYNC bytes, 9 PID bytes, 6 Endpoint + CRC bytes,
6 CRC bytes, 8 Setup data bytes, and a 7-byte interpacket
delay (EOP, etc.))
Data
Payload
Frame
Bandwidth
per
Transfer
Max
Transfers
Bytes
Remaining
Max Bandwidth
(bytes/second)
Bytes/Frame
Useful Data
1
32000
3%
32
23
32
2
62000
3%
31
43
62
4
120000
3%
30
30
120
8
224000
4%
28
16
224
16
384000
4%
24
36
384
32
608000
5%
19
37
608
64
832000
7%
13
83
832
1500000
1500
Universal Serial Bus Specification Revision 2.0
Table 5-3 lists information about different-sized high-speed control transfers and the maximum number of
transfers possible in a microframe. This table was generated assuming that there is one Data stage
transaction and that the Data stage has a zero-length status stage. The table illustrates the possible power of
two data payloads less than or equal to the allowable maximum data payload size. The table does not
include the overhead associated with bit stuffing.
Table 5-3. High-speed Control Transfer Limits
(Based on 480Mb/s and 8 bit interpacket gap, 88 bit min bus
turnaround, 32 bit sync, 8 bit EOP: (9x4 SYNC bytes,
9 PID bytes, 6 EP/ADDR+CRC,6 CRC16, 8 Setup data,
9x(1+11) byte interpacket delay (EOP, etc.))
Protocol Overhead
(173 bytes)
Max
Data
Payload
Max Bandwidth
(bytes/second)
Microframe
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
1
344000
2%
43
18
43
2
672000
2%
42
150
84
4
1344000
2%
42
66
168
8
2624000
2%
41
79
328
16
4992000
3%
39
129
624
32
9216000
3%
36
120
1152
64
15872000
3%
31
153
1984
60000000
Bytes/
Microframe
Useful Data
7500
5.5.5 Control Transfer Data Sequences
Control transfers require that a Setup bus transaction be sent from the host to a device to describe the type of
control access that the device should perform. The Setup transaction is followed by zero or more control
Data transactions that carry the specific information for the requested access. Finally, a Status transaction
completes the control transfer and allows the endpoint to return the status of the control transfer to the client
software. After the Status transaction for a control transfer is completed, the host can advance to the next
control transfer for the endpoint. As described in Section 5.5.4, each control transaction and the next
control transfer will be moved over the bus at some Host Controller implementation-defined time.
The endpoint can be busy for a device-specific time during the Data and Status transactions of the control
transfer. During these times when the endpoint indicates it is busy (refer to Chapter 8 and Chapter 9 for
details), the host will retry the transaction at a later time.
If a Setup transaction is received by an endpoint before a previously initiated control transfer is completed,
the device must abort the current transfer/operation and handle the new control Setup transaction. A Setup
transaction should not normally be sent before the completion of a previous control transfer. However, if a
transfer is aborted, for example, due to errors on the bus, the host can send the next Setup transaction
prematurely from the endpoint’s perspective.
43
Universal Serial Bus Specification Revision 2.0
After a halt condition is encountered or an error is detected by the host, a control endpoint is allowed to
recover by accepting the next Setup PID; i.e., recovery actions via some other pipe are not required for
control endpoints. For the Default Control Pipe, a device reset will ultimately be required to clear the halt
or error condition if the next Setup PID is not accepted.
The USB provides robust error detection and recovery/retransmission for errors that occur during control
transfers. Transmitters and receivers can remain synchronized with regard to where they are in a control
transfer and recover with minimum effort. Retransmission of Data and Status packets can be detected by a
receiver via data retry indicators in the packet. A transmitter can reliably determine that its corresponding
receiver has successfully accepted a transmitted packet by information returned in a handshake to the
packet. The protocol allows for distinguishing a retransmitted packet from its original packet except for a
control Setup packet. Setup packets may be retransmitted due to a transmission error; however, Setup
packets cannot indicate that a packet is an original or a retried transmission.
5.6 Isochronous Transfers
In non-USB environments, isochronous transfers have the general implication of constant-rate, errortolerant transfers. In the USB environment, requesting an isochronous transfer type provides the requester
with the following:
•
Guaranteed access to USB bandwidth with bounded latency
•
Guaranteed constant data rate through the pipe as long as data is provided to the pipe
•
In the case of a delivery failure due to error, no retrying of the attempt to deliver the data
While the USB isochronous transfer type is designed to support isochronous sources and destinations, it is
not required that software using this transfer type actually be isochronous in order to use the transfer type.
Section 5.12 presents more detail on special considerations for handling isochronous data on the USB.
5.6.1 Isochronous Transfer Data Format
The USB imposes no data content structure on communication flows for isochronous pipes.
5.6.2 Isochronous Transfer Direction
An isochronous pipe is a stream pipe and is, therefore, always uni-directional. An endpoint description
identifies whether a given isochronous pipe’s communication flow is into or out of the host. If a device
requires bi-directional isochronous communication flow, two isochronous pipes must be used, one in each
direction.
5.6.3 Isochronous Transfer Packet Size Constraints
An endpoint in a given configuration for an isochronous pipe specifies the maximum size data payload that
it can transmit or receive. The USB System Software uses this information during configuration to ensure
that there is sufficient bus time to accommodate this maximum data payload in each (micro)frame. If there
is sufficient bus time for the maximum data payload, the configuration is established; if not, the
configuration is not established.
The USB limits the maximum data payload size to 1,023 bytes for each full-speed isochronous endpoint.
High-speed endpoints are allowed up to 1024-byte data payloads. A high speed, high bandwidth endpoint
specifies whether it requires two or three transactions per microframe. Table 5-4 lists information about
different-sized full-speed isochronous transactions and the maximum number of transactions possible in a
frame. The table is shaded to indicate that a full-speed isochronous endpoint (with a non-zero wMaxpacket
size) must not be part of a default interface setting. The table does not include the overhead associated with
bit stuffing.
44
Universal Serial Bus Specification Revision 2.0
Table 5-4. Full-speed Isochronous Transaction Limits
Protocol Overhead (9 bytes)
Max
Data
Payload
Max
Bandwidth(bytes/
second)
1
(2 SYNC bytes, 2 PID bytes, 2 Endpoint + CRC bytes,
2 CRC bytes, and a 1-byte interpacket delay)
Frame
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
150000
1%
150
0
150
2
272000
1%
136
4
272
4
460000
1%
115
5
460
8
704000
1%
88
4
704
16
960000
2%
60
0
960
32
1152000
3%
36
24
1152
64
1280000
5%
20
40
1280
128
1280000
9%
10
130
1280
256
1280000
18%
5
175
1280
512
1024000
35%
2
458
1024
1023
1023000
69%
1
468
1023
1500000
Bytes/Frame
Useful Data
1500
45
Universal Serial Bus Specification Revision 2.0
Table 5-5 lists information about different-sized high-speed isochronous transactions and the maximum
number of transactions possible in a microframe. The table is shaded to indicate that a high-speed
isochronous endpoint must not be part of a default interface setting. The table does not include the overhead
associated with bit stuffing.
Any given transaction for an isochronous pipe need not be exactly the maximum size specified for the
endpoint. The size of a data payload is determined by the transmitter (client software or function) and can
vary as required from transaction to transaction. The USB ensures that whatever size is presented to the
Host Controller is delivered on the bus. The actual size of a data payload is determined by the data
transmitter and may be less than the prenegotiated maximum size. Bus errors can change the actual packet
size seen by the receiver. However, these errors can be detected by either CRC on the data or by knowledge
the receiver has about the expected size for any transaction.
Table 5-5. High-speed Isochronous Transaction Limits
(Based on 480Mb/s and 8 bit interpacket gap, 88 bit min bus
turnaround, 32 bit sync, 8 bit EOP: (2x4 SYNC bytes, 2 PID
bytes, 2 EP/ADDR+addr+CRC5, 2 CRC16, and a 2x(1+11))
byte interpacket delay (EOP, etc.))
Protocol Overhead
Data
Payload
Max
46
Max
Bandwidth
(bytes/second)
Microframe
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
Bytes/
MicroFrame
Useful Data
1
1536000
1%
192
12
192
2
2992000
1%
187
20
374
4
5696000
1%
178
24
712
8
10432000
1%
163
2
1304
16
17664000
1%
138
48
2208
32
27392000
1%
107
10
3424
64
37376000
1%
73
54
4672
128
46080000
2%
45
30
5760
256
51200000
4%
25
150
6400
512
53248000
7%
13
350
6656
1024
57344000
14%
7
66
7168
2048
49152000
28%
3
1242
6144
3072
49152000
41%
2
1280
6144
60000000
7500
Universal Serial Bus Specification Revision 2.0
All device default interface settings must not include any isochronous endpoints with non-zero data payload
sizes (specified via wMaxPacketSize in the endpoint descriptor). Alternate interface settings may specify
non-zero data payload sizes for isochronous endpoints. If the isochronous endpoints have a large data
payload size, it is recommended that additional alternate configurations or interface settings be used to
specify a range of data payload sizes. This increases the chance that the device can be used successfully in
combination with other USB devices.
5.6.4 Isochronous Transfer Bus Access Constraints
Isochronous transfers can only be used by full-speed and high-speed devices.
The USB requires that no more than 90% of any frame be allocated for periodic (isochronous and interrupt)
transfers for full-speed endpoints. High-speed endpoints can allocate at most 80% of a microframe for
periodic transfers.
An isochronous endpoint must specify its required bus access period. Full-/high-speed endpoints must
bInterval-1
) x F, where bInterval is in the range one to (and including) 16 and F is
specify a desired period as (2
125 µs for high-speed and 1ms for full-speed. This allows full-/high-speed isochronous transfers to have
rates slower than one transaction per (micro)frame. However, an isochronous endpoint must be prepared to
handle poll rates faster than the one specified. A host must not issue more than 1 transaction in a
(micro)frame for an isochronous endpoint unless the endpoint is high-speed, high-bandwidth (see below).
An isochronous IN endpoint must return a zero-length packet whenever data is requested at a faster interval
than the specified interval and data is not available.
A high-speed endpoint can move up to 3072 bytes per microframe (or 192 Mb/s). A high-speed
isochronous endpoint that requires more than 1024 bytes per period is called a high-bandwidth endpoint. A
high-bandwidth endpoint uses multiple transactions per microframe. A high-bandwidth endpoint must
specify a period of 1x125 µs (i.e., a bInterval value of 1). See Section 5.9 for more information about the
details of multiple transactions per microframe for high-bandwidth high-speed endpoints.
Errors on the bus or delays in operating system scheduling of client software can result in no packet being
transferred for a (micro)frame. An error indication should be returned as status to the client software in
such a case. A device can also detect this situation by tracking SOF tokens and noticing a disturbance in the
specified bus access period pattern.
The bus frequency and (micro)frame timing limit the maximum number of successful isochronous
transactions within a (micro)frame for any USB system to less than 151 full-speed one-byte data payloads
and less than 193 high-speed one-byte data payloads. A Host Controller, for various implementation
reasons, may not be able to provide the theoretical maximum number of isochronous transactions per
(micro)frame.
5.6.5 Isochronous Transfer Data Sequences
Isochronous transfers do not support data retransmission in response to errors on the bus. A receiver can
determine that a transmission error occurred. The low-level USB protocol does not allow handshakes to be
returned to the transmitter of an isochronous pipe. Normally, handshakes would be returned to tell the
transmitter whether a packet was successfully received or not. For isochronous transfers, timeliness is more
important than correctness/retransmission, and, given the low error rates expected on the bus, the protocol is
optimized by assuming transfers normally succeed. Isochronous receivers can determine whether they
missed data during a (micro)frame. Also, a receiver can determine how much data was lost. Section 5.12
describes these USB mechanisms in more detail.
An endpoint for isochronous transfers never halts because there is no handshake to report a halt condition.
Errors are reported as status associated with the IRP for an isochronous transfer, but the isochronous pipe is
not halted in an error case. If an error is detected, the host continues to process the data associated with the
next (micro)frame of the transfer. Only limited error detection is possible because the protocol for
isochronous transactions does not allow per-transaction handshakes.
47
Universal Serial Bus Specification Revision 2.0
5.7 Interrupt Transfers
The interrupt transfer type is designed to support those devices that need to send or receive data infrequently
but with bounded service periods. Requesting a pipe with an interrupt transfer type provides the requester
with the following:
•
Guaranteed maximum service period for the pipe
•
Retry of transfer attempts at the next period, in the case of occasional delivery failure due to error on
the bus
5.7.1 Interrupt Transfer Data Format
The USB imposes no data content structure on communication flows for interrupt pipes.
5.7.2 Interrupt Transfer Direction
An interrupt pipe is a stream pipe and is therefore always uni-directional. An endpoint description identifies
whether a given interrupt pipe’s communication flow is into or out of the host.
5.7.3 Interrupt Transfer Packet Size Constraints
An endpoint for an interrupt pipe specifies the maximum size data payload that it will transmit or receive.
The maximum allowable interrupt data payload size is 64 bytes or less for full-speed. High-speed endpoints
are allowed maximum data payload sizes up to 1024 bytes. A high speed, high bandwidth endpoint
specifies whether it requires two or three transactions per microframe. Low-speed devices are limited to
eight bytes or less maximum data payload size. This maximum applies to the data payloads of the data
packets; i.e., the size specified is for the data field of the packet as defined in Chapter 8, not including other
protocol-required information. The USB does not require that data packets be exactly the maximum size;
i.e., if a data packet is less than the maximum, it does not need to be padded to the maximum size.
All Host Controllers are required to support maximum data payload sizes from 0 to 64 bytes for full-speed
interrupt endpoints, from 0 to 8 bytes for low-speed interrupt endpoints, and from 0 to 1024 bytes for highspeed interrupt endpoints. See Section 5.9 for more information about the details of multiple transactions
per microframe for high bandwidth high-speed endpoints. No Host Controller is required to support larger
maximum data payload sizes.
The USB System Software determines the maximum data payload size that will be used for an interrupt
pipe during device configuration. This size remains constant for the lifetime of a device’s configuration.
The USB System Software uses the maximum data payload size determined during configuration to ensure
that there is sufficient bus time to accommodate this maximum data payload in its assigned period. If there
is sufficient bus time, the pipe is established; if not, the pipe is not established. However, the actual size of
a data payload is still determined by the data transmitter and may be less than the maximum size.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
wMaxPacketSize value. A device can move data via an interrupt pipe that is larger than wMaxPacketSize.
A software client can accept this data via an IRP for the interrupt transfer that requires multiple bus
transactions without requiring an IRP-complete notification per transaction. This can be achieved by
specifying a buffer that can hold the desired data size. The size of the buffer is a multiple of
wMaxPacketSize with some remainder. The endpoint must transfer each transaction except the last as
wMaxPacketSize and the last transaction is the remainder. The multiple data transactions are moved over
the bus at the period established for the pipe.
When an interrupt transfer involves more data than can fit in one data payload of the currently established
maximum size, all data payloads are required to be maximum-sized except for the last data payload, which
will contain the remaining data. An interrupt transfer is complete when the endpoint does one of the
following:
48
Universal Serial Bus Specification Revision 2.0
•
Has transferred exactly the amount of data expected
•
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
When an interrupt transfer is complete, the Host Controller retires the current IRP and advances to the next
IRP. If a data payload is received that is larger than expected, the interrupt IRP will be aborted/retired and
the pipe will stall future IRPs until the condition is corrected and acknowledged.
All high-speed device default interface settings must not include any interrupt endpoints with a data payload
size (specified via wMaxPacketSize in the endpoint descriptor) greater than 64 bytes. Alternate interface
settings may specify larger data payload sizes for interrupt endpoints. If the interrupt endpoints have a large
data payload size, it is recommended that additional configurations or alternate interface settings be used to
specify a range of data payload sizes. This increases the chances that the device can be used successfully in
combination with other USB devices.
5.7.4 Interrupt Transfer Bus Access Constraints
Interrupt transfers can be used by low-speed, full-speed, and high-speed devices. High-speed endpoints can
be allocated at most 80% of a microframe for periodic transfers. The USB requires that no more than 90%
of any frame be allocated for periodic (isochronous and interrupt) full-/low-speed transfers.
The bus frequency and (micro)frame timing limit the maximum number of successful interrupt transactions
within a (micro)frame for any USB system to less than 108 full-speed one-byte data payloads, or less than
10 low-speed one-byte data payloads, or to less than 134 high-speed one-byte data payloads. A Host
Controller, for various implementation reasons, may not be able to provide the above maximum number of
interrupt transactions per (micro)frame.
Table 5-6 lists information about different low-speed interrupt transactions and the maximum number of
transactions possible in a frame. Table 5-7 lists similar information for full-speed interrupt transactions.
Table 5-8 lists similar information for high-speed interrupt transactions. The shaded portion of Table 5-8
indicates the data payload sizes of a high-speed interrupt endpoint that must not be part of a default interface
setting. The tables do not include the overhead associated with bit stuffing.
Table 5-6. Low-speed Interrupt Transaction Limits
(5 SYNC bytes, 5 PID bytes, 2 Endpoint + CRC bytes,
2 CRC bytes, and a 5-byte interpacket delay)
Protocol Overhead
(19 bytes)
Max
Data
Payload
Max Bandwidth
(bytes/second)
Frame
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
1
9000
11%
9
7
9
2
16000
11%
8
19
16
4
32000
12%
8
3
32
8
48000
14%
6
25
48
187500
Bytes/Frame
Useful Data
187
49
Universal Serial Bus Specification Revision 2.0
Table 5-7. Full-speed Interrupt Transaction Limits
Max
50
Protocol Overhead (13 bytes)
(3 SYNC bytes, 3 PID bytes, 2 Endpoint + CRC bytes,
2 CRC bytes, and a 3-byte interpacket delay)
Data
Payload
Frame
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
Max
Bandwidth
(bytes/second)
Bytes/Frame
Useful Data
1
107000
1%
107
2
107
2
200000
1%
100
0
200
4
352000
1%
88
4
352
8
568000
1%
71
9
568
16
816000
2%
51
21
816
32
1056000
3%
33
15
1056
64
1216000
5%
19
37
1216
1500000
1500
Universal Serial Bus Specification Revision 2.0
Table 5-8. High-speed Interrupt Transaction Limits
Max
Protocol Overhead
(Based on 480Mb/s and 8 bit interpacket gap, 88 bit min
bus turnaround, 32 bit sync, 8 bit EOP: (3x4 SYNC bytes,
3 PID bytes, 2 EP/ADDR+CRC bytes, 2 CRC16 and a
3x(1+11) byte interpacket delay(EOP, etc.))
Data
Payload
Microframe
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
Max
Bandwidth
(bytes/second)
Bytes/
Microframe
Useful Data
1
1064000
1%
133
52
133
2
2096000
1%
131
33
262
4
4064000
1%
127
7
508
8
7616000
1%
119
3
952
16
13440000
1%
105
45
1680
32
22016000
1%
86
18
2752
64
32256000
2%
63
3
4032
128
40960000
2%
40
180
5120
256
49152000
4%
24
36
6144
512
53248000
8%
13
129
6656
1024
49152000
14%
6
1026
6144
2048
49152000
28%
3
1191
6144
3072
49152000
42%
2
1246
6144
60000000
7500
An endpoint for an interrupt pipe specifies its desired bus access period. A full-speed endpoint can specify
a desired period from 1 ms to 255 ms. Low-speed endpoints are limited to specifying only 10 ms to 255 ms.
bInterval-1
)x125 µs, where bInterval is in the range 1 to
High-speed endpoints can specify a desired period (2
(including) 16. The USB System Software will use this information during configuration to determine a
period that can be sustained. The period provided by the system may be shorter than that desired by the
device up to the shortest period defined by the USB (125 µs microframe or 1 ms frame). The client
software and device can depend only on the fact that the host will ensure that the time duration between two
transaction attempts with the endpoint will be no longer than the desired period. Note that errors on the bus
can prevent an interrupt transaction from being successfully delivered over the bus and consequently exceed
the desired period. Also, the endpoint is only polled when the software client has an IRP for an interrupt
transfer pending. If the bus time for performing an interrupt transfer arrives and there is no IRP pending,
the endpoint will not be given an opportunity to transfer data at that time. Once an IRP is available, its data
will be transferred at the next allocated period.
51
Universal Serial Bus Specification Revision 2.0
A high-speed endpoint can move up to 3072 bytes per microframe (or 192 Mb/s). A high-speed interrupt
endpoint that requires more than 1024 bytes per period is called a high-bandwidth endpoint. A highbandwidth endpoint uses multiple transactions per microframe. A high-bandwidth endpoint must specify a
period of 1x125 µs (i.e., a bInterval value of 1). See Section 5.9 for more information about the details of
multiple transactions per microframe for high-bandwidth high-speed endpoints.
Interrupt transfers are moved over the USB by accessing an interrupt endpoint every specified period. For
input interrupt endpoints, the host has no way to determine whether an endpoint will source an interrupt
without accessing the endpoint and requesting an interrupt transfer. If the endpoint has no interrupt data to
transmit when accessed by the host, it responds with NAK. An endpoint should only provide interrupt data
when it has an interrupt pending to avoid having a software client erroneously notified of IRP complete. A
zero-length data payload is a valid transfer and may be useful for some implementations.
5.7.5 Interrupt Transfer Data Sequences
Interrupt transactions may use either alternating data toggle bits, such that the bits are toggled only upon
successful transfer completion or a continuously toggling of data toggle bits. The host in any case must
assume that the device is obeying full handshake/retry rules as defined in Chapter 8. A device may choose
to always toggle DATA0/DATA1 PIDs so that it can ignore handshakes from the host. However, in this
case, the client software can miss some data packets when an error occurs, because the Host Controller
interprets the next packet as a retry of a missed packet.
If a halt condition is detected on an interrupt pipe due to transmission errors or a STALL handshake being
returned from the endpoint, all pending IRPs are retired. Removal of the halt condition is achieved via
software intervention through a separate control pipe. This recovery will reset the data toggle bit to DATA0
for the endpoint on both the host and the device. Interrupt transactions are retried due to errors detected on
the bus that affect a given transfer.
5.8 Bulk Transfers
The bulk transfer type is designed to support devices that need to communicate relatively large amounts of
data at highly variable times where the transfer can use any available bandwidth. Requesting a pipe with a
bulk transfer type provides the requester with the following:
•
Access to the USB on a bandwidth-available basis
•
Retry of transfers, in the case of occasional delivery failure due to errors on the bus
•
Guaranteed delivery of data but no guarantee of bandwidth or latency
Bulk transfers occur only on a bandwidth-available basis. For a USB with large amounts of free bandwidth,
bulk transfers may happen relatively quickly; for a USB with little bandwidth available, bulk transfers may
trickle out over a relatively long period of time.
5.8.1 Bulk Transfer Data Format
The USB imposes no data content structure on communication flows for bulk pipes.
5.8.2 Bulk Transfer Direction
A bulk pipe is a stream pipe and, therefore, always has communication flowing either into or out of the host
for a given pipe. If a device requires bi-directional bulk communication flow, two bulk pipes must be used,
one in each direction.
52
Universal Serial Bus Specification Revision 2.0
5.8.3 Bulk Transfer Packet Size Constraints
An endpoint for bulk transfers specifies the maximum data payload size that the endpoint can accept from
or transmit to the bus. The USB defines the allowable maximum bulk data payload sizes to be only 8, 16,
32, or 64 bytes for full-speed endpoints and 512 bytes for high-speed endpoints. A low-speed device must
not have bulk endpoints. This maximum applies to the data payloads of the data packets; i.e., the size
specified is for the data field of the packet as defined in Chapter 8, not including other protocol-required
information.
A bulk endpoint is designed to support a maximum data payload size. A bulk endpoint reports in its
configuration information the value for its maximum data payload size. The USB does not require that data
payloads transmitted be exactly the maximum size; i.e., if a data payload is less than the maximum, it does
not need to be padded to the maximum size.
All Host Controllers are required to have support for 8-, 16-, 32-, and 64-byte maximum packet sizes for
full-speed bulk endpoints and 512 bytes for high-speed bulk endpoints. No Host Controller is required to
support larger or smaller maximum packet sizes.
During configuration, the USB System Software reads the endpoint’s maximum data payload size and
ensures that no data payload will be sent to the endpoint that is larger than the supported size.
An endpoint must always transmit data payloads with a data field less than or equal to the endpoint’s
reported wMaxPacketSize value. When a bulk IRP involves more data than can fit in one maximum-sized
data payload, all data payloads are required to be maximum size except for the last data payload, which will
contain the remaining data. A bulk transfer is complete when the endpoint does one of the following:
•
Has transferred exactly the amount of data expected
•
Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
When a bulk transfer is complete, the Host Controller retires the current IRP and advances to the next IRP.
If a data payload is received that is larger than expected, all pending bulk IRPs for that endpoint will be
aborted/retired.
5.8.4 Bulk Transfer Bus Access Constraints
Only full-speed and high-speed devices can use bulk transfers.
An endpoint has no way to indicate a desired bus access frequency for a bulk pipe. The USB balances the
bus access requirements of all bulk pipes and the specific IRPs that are pending to provide “good effort”
delivery of data between client software and functions. Moving control transfers over the bus has priority
over moving bulk transfers.
There is no time guaranteed to be available for bulk transfers as there is for control transfers. Bulk transfers
are moved over the bus only on a bandwidth-available basis. If there is bus time that is not being used for
other purposes, bulk transfers will be moved over the bus. If there are bulk transfers pending for multiple
endpoints, bulk transfers for the different endpoints are selected according to a fair access policy that is Host
Controller implementation-dependent.
All bulk transfers pending in a system contend for the same available bus time. Because of this, the USB
System Software at its discretion can vary the bus time made available for bulk transfers to a particular
endpoint. An endpoint and its client software cannot assume a specific rate of service for bulk transfers.
Bus time made available to a software client and its endpoint can be changed as other devices are inserted
into and removed from the system or also as bulk transfers are requested for other device endpoints. Client
software cannot assume ordering between bulk and control transfers; i.e., in some situations, bulk transfers
can be delivered ahead of control transfers.
High-speed bulk OUT endpoints must support the PING flow control protocol. The details of this protocol
are described in Section 8.5.1.
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Universal Serial Bus Specification Revision 2.0
The bus frequency and (micro)frame timing limit the maximum number of successful bulk transactions
within a (micro)frame for any USB system to less than 72 full-speed eight-byte data payloads or less than
14 high-speed 512-byte data payloads. Table 5-9 lists information about different-sized full-speed bulk
transactions and the maximum number of transactions possible in a frame. The table does not include the
overhead associated with bit stuffing. Table 5-10 lists similar information for high-speed bulk transactions.
Table 5-9. Full-speed Bulk Transaction Limits
Max
54
Protocol Overhead (13 bytes)
(3 SYNC bytes, 3 PID bytes, 2 Endpoint + CRC bytes,
2 CRC bytes, and a 3-byte interpacket delay)
Data
Payload
Frame
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
Max Bandwidth
(bytes/second)
Bytes/Frame
Useful Data
1
107000
1%
107
2
107
2
200000
1%
100
0
200
4
352000
1%
88
4
352
8
568000
1%
71
9
568
16
816000
2%
51
21
816
32
1056000
3%
33
15
1056
64
1216000
5%
19
37
1216
1500000
1500
Universal Serial Bus Specification Revision 2.0
Table 5-10. High-speed Bulk Transaction Limits
Max
Protocol Overhead (55 bytes)
(3x4 SYNC bytes, 3 PID bytes, 2 EP/ADDR+CRC bytes,
2 CRC16, and a 3x(1+11) byte interpacket delay (EOP, etc.))
Data
Payload
Microframe
Bandwidth
per Transfer
Max
Transfers
Bytes
Remaining
Max Bandwidth
(bytes/second)
Bytes/
Microframe
Useful Data
1
1064000
1%
133
52
133
2
2096000
1%
131
33
262
4
4064000
1%
127
7
508
8
7616000
1%
119
3
952
16
13440000
1%
105
45
1680
32
22016000
1%
86
18
2752
64
32256000
2%
63
3
4032
128
40960000
2%
40
180
5120
256
49152000
4%
24
36
6144
512
53248000
8%
13
129
6656
60000000
7500
Host Controllers are free to determine how the individual bus transactions for specific bulk transfers are
moved over the bus within and across (micro)frames. An endpoint could see all bus transactions for a bulk
transfer within the same (micro)frame or spread across several (micro)frames. A Host Controller, for
various implementation reasons, may not be able to provide the above maximum number of transactions per
(micro)frame.
5.8.5 Bulk Transfer Data Sequences
Bulk transactions use data toggle bits that are toggled only upon successful transaction completion to
preserve synchronization between transmitter and receiver when transactions are retried due to errors. Bulk
transactions are initialized to DATA0 when the endpoint is configured by an appropriate control transfer.
The host will also start the first bulk transaction with DATA0. If a halt condition is detected on a bulk pipe
due to transmission errors or a STALL handshake being returned from the endpoint, all pending IRPs are
retired. Removal of the halt condition is achieved via software intervention through a separate control pipe.
This recovery will reset the data toggle bit to DATA0 for the endpoint on both the host and the device.
Bulk transactions are retried due to errors detected on the bus that affect a given transaction.
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5.9 High-Speed, High Bandwidth Endpoints
USB supports individual high-speed interrupt or isochronous endpoints that require data rates up to
192 Mb/s (i.e., 3072 data bytes per microframe). One, two, or three high-speed transactions are allowed in
a single microframe to support high-bandwidth endpoints.
A high-speed interrupt or isochronous endpoint indicates that it requires more than 1024 bytes per
microframe when bits 12..11 of the wMaxPacketSize field of the endpoint descriptor (see Table 5-11) are
non-zero. The lower 11 bits of wMaxPacketSize indicate the size of the data payload for each individual
transaction while bits 12..11 indicate the maximum number of required transactions possible. See
Section 9.6.6 for restrictions on the allowed combinations of values for bits 12..11 and bits 10..0.
Table 5-11. wMaxPacketSize Field of Endpoint Descriptor
Bits
Field
15..13
Reserved,
must be
set to zero
12..11
Number of transactions
per microframe
10..0
Maximum size of data
payload in bytes
Note: This representation means that endpoints requesting two transactions per microframe will specify a
total data payload size in the microframe that is a multiple of two bytes. Also endpoints requesting three
transactions per microframe will specify a total data payload size that is a multiple of three bytes. In any
case, any number of bytes can actually be transferred in a microframe.
The host controller must issue an appropriate number of high-speed transactions per microframe. Errors in
the host or on the bus can result in the host controller issuing fewer transactions than requested for the
endpoint. The first transaction(s) must have a data payload(s) as specified by the lower 11 bits of
wMaxPacketSize if enough data is available, while the last transaction has any remaining data less than or
equal to the maximum size specified. The host controller may issue transactions for the same endpoint one
immediately after the other (as required for the actual data provided) or may issue transactions for other
endpoints in between the transactions for a high bandwidth endpoint.
5.9.1 High Bandwidth Interrupt Endpoints
For interrupt transactions, if the endpoint NAKs a transaction during a microframe, the host controller must
not issue further transactions for that endpoint until the next period.
If the endpoint times-out a transaction, the host controller must retry the transaction. The endpoint specifies
the maximum number of desired transactions per microframe. If the maximum number of transactions per
microframe has not been reached, the host controller may immediately retry the transaction during the
current microframe. Host controllers are recommended to do an immediate retry since this minimizes
impact on devices that are bandwidth sensitive. If the maximum number of transactions per microframe has
been reached, the host controller must retry the transaction at the next period for the endpoint.
A host controller is allowed to issue less than the maximum number of transactions to an endpoint per
microframe only if more than a single memory buffer is required for the transactions within the microframe.
Normal DATA0/DATA1 data toggle sequencing is used for each interrupt transaction during a microframe.
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5.9.2 High Bandwidth Isochronous Endpoints
For isochronous transactions, if an IN endpoint provides less than a maximum data payload as specified by
its endpoint descriptor, the host must not issue further transactions for that endpoint for that microframe.
For an isochronous OUT endpoint, the host controller must issue the number of transactions as required for
the actual data provided, not exceeding the maximum number specified by the endpoint descriptor. The
transactions issued must adhere to the maximum payload sizes as specified in the endpoint descriptor.
No retries are ever done for isochronous endpoints.
High bandwidth isochronous endpoints (IN and OUT) must support data PID sequencing. Data PID
sequencing provides the required support for the data receiver to detect one or more lost/damaged packets
per microframe.
Data PID sequencing for a high-speed, high bandwidth isochronous IN endpoint uses a repeating sequence
of DATA2, DATA1, DATA0 PIDs for the data packet of each transaction in a microframe. If there is only
a single transaction in the microframe, only a DATA0 data packet PID is used. If there are two transactions
per microframe, DATA1 is used for the first transaction data packet and DATA0 is used for the second
transaction data packet. If there are three transactions per microframe, DATA2 is used for the first
transaction data packet, DATA1 is used for the second, and DATA0 is used for the third. In all cases, the
data PID sequence starts over again the next microframe. Figure 5-11 shows the order of data packet PIDs
that are used in subsequent transactions within a microframe for high-bandwidth isochronous IN endpoints.
1 transaction, <1024 bytes:
DATA0
2 transactions, 513-1024 bytes ea.:
DATA1
DATA0
3 transactions, 683-1024 bytes ea.:
DATA2
DATA1
DATA0
Figure 5-11. Data Phase PID Sequence for Isochronous IN High Bandwidth Endpoints
An endpoint must respond to an IN token for the first transaction with a DATA2 when it requires three
transactions of data to be moved. It must respond with a DATA1 for the first transaction when it requires
two transactions and with a DATA0 when it requires only a single transaction. After the first transaction,
the endpoint follows the data PID sequence as described above.
The host knows the maximum number of allowed transactions per microframe for the IN endpoint. The
host expects the response to the first transaction to encode (via the data packet PID) how many transactions
are required by the endpoint for this microframe. If the host doesn’t receive an error-free, appropriate
response to any transaction, the host must not issue any further transactions to the endpoint for that
microframe. When the host receives a DATA0 data packet from the endpoint, it must not issue any further
transactions to the endpoint for that microframe.
Data PID sequencing for a high-speed, high bandwidth isochronous OUT endpoint uses a different sequence
than that used for an IN endpoint. The host must issue a DATA0 data packet when there is a single
transaction. The host must issue an MDATA for the first transaction and a DATA1 for the second
transaction when there are two transactions per microframe. The host must issue two MDATA transactions
and a DATA2 for the third transaction when there are three transactions per microframe. These sequences
allow the endpoint to detect if there was a lost/damaged transaction during a microframe. Figure 5-12
shows the order of data packet PIDs that are used in subsequent transactions within a microframe for highbandwidth isochronous OUT.
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Universal Serial Bus Specification Revision 2.0
1 transaction, <1024 bytes:
DATA0
2 transactions, 513-1024 bytes ea.:
MDATA
DATA1
3 transactions, 683-1024 bytes ea.:
MDATA
MDATA
DATA2
Figure 5-12. Data Phase PID Sequence for Isochronous OUT High Bandwidth Endpoints
If the wrong OUT transactions are detected by the endpoint, all of the data transferred during the
microframe must be treated as if it had encountered an error. Note that for the three transactions per
microframe case with a missing MDATA transaction, USB provides no way for the endpoint to determine
which of the two MDATA transactions was lost. There may be application specific methods to more
precisely determine which data was lost, but USB provides no method to do so at the bus level.
5.10 Split Transactions
Host controllers and hubs support one additional transaction type called split transactions. This transaction
type allows full- and low-speed devices to be attached to hubs operating at high-speed. These transactions
involve only host controllers and hubs and are not visible to devices. High-speed split transactions for
interrupt and isochronous transfers must be allocated by the host from the 80% periodic portion of a
microframe. More information on split transactions can be found in Chapter 8 and Chapter 11.
5.11 Bus Access for Transfers
Accomplishing any data transfer between the host and a USB device requires some use of the USB
bandwidth. Supporting a wide variety of isochronous and asynchronous devices requires that each device’s
transfer requirements are accommodated. The process of assigning bus bandwidth to devices is called
transfer management. There are several entities on the host that coordinate the information flowing over the
USB: client software, the USB Driver (USBD), and the Host Controller Driver (HCD). Implementers of
these entities need to know the key concepts related to bus access:
•
Transfer Management: The entities and the objects that support communication flow over the USB
•
Transaction Tracking: The USB mechanisms that are used to track transactions as they move through
the USB system
•
Bus Time: The time it takes to move a packet of information over the bus
•
Device/Software Buffer Size: The space required to support a bus transaction
•
Bus Bandwidth Reclamation: Conditions where bandwidth that was allocated to other transfers but was
not used and can now be possibly reused by control and bulk transfers
The previous sections focused on how client software relates to a function and what the logical flows are
over a pipe between the two entities. This section focuses on the different parts of the host and how they
must interact to support moving data over the USB. This information may also be of interest to device
implementers so they understand aspects of what the host is doing when a client requests a transfer and how
that transfer is presented to the device.
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Universal Serial Bus Specification Revision 2.0
5.11.1 Transfer Management
Transfer management involves several entities that operate on different objects in order to move
transactions over the bus:
•
Client Software: Consumes/generates function-specific data to/from a function endpoint via calls and
callbacks requesting IRPs with the USBD interface.
•
USB Driver (USBD): Converts data in client IRPs to/from device endpoint via calls/callbacks with the
appropriate HCD. A single client IRP may involve one or more transfers.
•
Host Controller Driver (HCD): Converts IRPs to/from transactions (as required by a Host Controller
implementation) and organizes them for manipulation by the Host Controller. Interactions between the
HCD and its hardware is implementation-dependent and is outside the scope of the USB Specification.
•
Host Controller: Takes transactions and generates bus activity via packets to move function-specific
data across the bus for each transaction.
Figure 5-13 shows how the entities are organized as information flows between client software and the
USB. The objects of primary interest to each entity are shown at the interfaces between entities.
Client Software
Data
USBD
Interface
IRPs
USBD
HCD
Interface
HCD
Transfers
Transaction
Transaction List
Transactions
Transaction
HW/SW
Interface
Host Controller
Packets
USB
Figure 5-13. USB Information Conversion From Client Software to Bus
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Universal Serial Bus Specification Revision 2.0
5.11.1.1 Client Software
Client software determines what transfers need to be made with a function. It uses appropriate operating
system-specific interfaces to request IRPs. Client software is aware only of the set of pipes (i.e., the
interface) it needs to manipulate its function. The client is aware of and adheres to all bus access and
bandwidth constraints as described previously for each transfer type. The requests made by the client
software are presented via the USBD interface.
Some clients may manipulate USB functions via other device class interfaces defined by the operating
system and may themselves not make direct USBD calls. However, there is always some lowest level client
that makes USBD calls to pass IRPs to the USBD. All IRPs presented are required to adhere to the
prenegotiated bandwidth constraints set when the pipe was established. If a function is moved from a nonUSB environment to the USB, the driver that would have directly manipulated the function hardware via
memory or I/O accesses is the lowest client software in the USB environment that now interacts with the
USBD to manipulate the driver’s USB function.
After client software has requested a transfer of its function and the request has been serviced, the client
software receives notification of the completion status of the IRP. If the transfer involved function-to-host
data transfer, the client software can access the data in the data buffer associated with the completed IRP.
The USBD interface is defined in Chapter 10.
5.11.1.2 USB Driver
The Universal Serial Bus Driver (USBD) is involved in mediating bus access at two general times:
•
While a device is attached to the bus during configuration
•
During normal transfers
When a device is attached and configured, the USBD is involved to ensure that the desired device
configuration can be accommodated on the bus. The USBD receives configuration requests from the
configuring software that describe the desired device configuration: endpoint(s), transfer type(s), transfer
period(s), data size(s), etc. The USBD either accepts or rejects a configuration request based on bandwidth
availability and the ability to accommodate that request type on the bus. If it accepts the request, the USBD
creates a pipe for the requester of the desired type and with appropriate constraints as defined for the
transfer type. Bandwidth allocation for periodic endpoints does not have to be made when the device is
configured and, once made, a bandwidth allocation can be released without changing the device
configuration.
The configuration aspects of the USBD are typically operating system-specific and heavily leverage the
configuration features of the operating system to avoid defining additional (redundant) interfaces.
Once a device is configured, the software client can request IRPs to move data between it and its function
endpoints.
5.11.1.3 Host Controller Driver
The Host Controller Driver (HCD) is responsible for tracking the IRPs in progress and ensuring that USB
bandwidth and (micro)frame time maximums are never exceeded. When IRPs are made for a pipe, the
HCD adds them to the transaction list. When an IRP is complete, the HCD notifies the requesting software
client of the completion status for the IRP. If the IRP involved data transfer from the function to the
software client, the data was placed in the client-indicated data buffer.
IRPs are defined in an operating system-dependent manner.
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Universal Serial Bus Specification Revision 2.0
5.11.1.4 Transaction List
The transaction list is a Host Controller implementation-dependent description of the current outstanding set
of bus transactions that need to be run on the bus. Only the HCD and its Host Controller have access to the
specific representation. Each description contains transaction descriptions in which parameters, such as
data size in bytes, the device address and endpoint number, and the memory area to which data is to be sent
or received, are identified.
A transaction list and the interface between the HCD and its Host Controller is typically represented in an
implementation-dependent fashion and is not defined explicitly as part of the USB Specification.
5.11.1.5 Host Controller
The Host Controller has access to the transaction list and translates it into bus activity. In addition, the Host
Controller provides a reporting mechanism whereby the status of a transaction (done, pending, halted, etc.)
can be obtained. The Host Controller converts transactions into appropriate implementation-dependent
activities that result in USB packets moving over the bus topology rooted in the root hub.
The Host Controller ensures that the bus access rules defined by the protocol are obeyed, such as
inter-packet timings, timeouts, babble, etc. The HCD interface provides a way for the Host Controller to
participate in deciding whether a new pipe is allowed access to the bus. This is done because Host
Controller implementations can have restrictions/constraints on the minimum inter-transaction times they
may support for combinations of bus transactions.
The interface between the transaction list and the Host Controller is hidden within an HCD and Host
Controller implementation.
5.11.2 Transaction Tracking
A USB function sees data flowing across the bus in packets as described in Chapter 8. The Host Controller
uses some implementation-dependent representation to track what packets to transfer to/from what
endpoints at what time or in what order. Most client software does not want to deal with packetized
communication flows because this involves a degree of complexity and interconnect dependency that limits
the implementation. The USB System Software (USBD and HCD) provides support for matching data
movement requirements of a client to packets on the bus. The Host Controller hardware and software uses
IRPs to track information about one or more transactions that combine to deliver a transfer of information
between the client software and the function. Figure 5-14 summarizes how transactions are organized into
IRPs for the four transfer types. Detailed protocol information for each transfer type can be found in
Chapter 8. More information about client software views of IRPs can be found in Chapter 10 and in the
operating system specific-information for a particular operating system.
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Universal Serial Bus Specification Revision 2.0
Data Flow Types
IRP
Transaction
Transaction
Transaction
Control Transfer
IRP
Setup
Transaction
Data
Transaction
Status
Transaction
Additional
Control Transfers
Interrupt Transfer
All transfers are
composed of one or more
transactions. An IRP
corresponds to one or
more transfers.
A control transfer is an OUT
Setup transaction followed
by multiple IN or OUT Data
transactions followed by one
“opposite of data direction”
Status transaction.
An interrupt transfer is one
or more IN / OUT Data
transactions.
IRP
Transaction
Transaction
Isochronous Transfer
An isochronous transfer
is one or more IN / OUT
Data transactions.
IRP
Transaction
Transaction
Transaction
Bulk Transfer
A bulk transfer is one
or more IN / OUT Data
transactions.
IRP
Transaction
Transaction
Transaction
Figure 5-14. Transfers for Communication Flows
Even though IRPs track the bus transactions that need to occur to move a specific data flow over the USB,
Host Controllers are free to choose how the particular bus transactions are moved over the bus subject to the
USB-defined constraints (e.g., exactly one transaction per (micro)frame for isochronous transfers). In any
case, an endpoint will see transactions in the order they appear within an IRP unless errors occur. For
example, Figure 5-15 shows two IRPs, one each for two pipes where each IRP contains three transactions.
For any transfer type, a Host Controller is free to move the first transaction of the first IRP followed by the
first transaction of the second IRP somewhere in (micro)Frame 1, while moving the second transaction of
each IRP in opposite order somewhere in (micro)Frame 2. If these are isochronous transfer types, that is the
only degree of freedom a Host Controller has. If these are control or bulk transfers, a Host Controller could
further move more or less transactions from either IRP within either (micro)frame. Functions cannot
depend on seeing transactions within an IRP back-to-back within a (micro)frame nor should they depend on
not seeing transactions back-to-back within a (micro)frame.
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Universal Serial Bus Specification Revision 2.0
Pipe
Pipe
IRP 1
IRP 2
Transaction
1-0
Transaction
1-1
Frame 1
Transaction
1-2
Transaction
2-0
Transaction
2-1
Transaction
2-2
Frame 2
Token Data,
Handshake
(1-0)
Token, Data,
Handshake
(2-0)
Token, Data,
Handshake
(2-1)
Token, Data,
Handshake
(1-1)
Figure 5-15. Arrangement of IRPs to Transactions/(Micro)frames
5.11.3 Calculating Bus Transaction Times
When the USB System Software allows a new pipe to be created for the bus, it must calculate how much
bus time is required for a given transaction. That bus time is based on the maximum packet size
information reported for an endpoint, the protocol overhead for the specific transaction type request, the
overhead due to signaling imposed bit stuffing, inter-packet timings required by the protocol,
inter-transaction timings, etc. These calculations are required to ensure that the time available in a
(micro)frame is not exceeded. The equations used to determine transaction bus time are:
KEY:
Data_bc
The byte count of data payload
Host_Delay
The time required for the host or transaction
translator to prepare for or recover from the
transmission; Host Controller implementation-specific
Floor()
The integer portion of argument
Hub_LS_Setup
The time provided by the Host Controller for hubs to
enable low-speed ports; measured as the delay from the
end of the PRE PID to the start of the low-speed SYNC;
minimum of four full-speed bit times
BitStuffTime
Function that calculates theoretical additional time
required due to bit stuffing in signaling; worst case
is (1.1667*8*Data_bc)
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Universal Serial Bus Specification Revision 2.0
High-speed (Input)
Non-Isochronous Transfer (Handshake Included)
= (55 * 8 * 2.083) + (2.083 * Floor(3.167 + BitStuffTime(Data_bc))) +
Host_Delay
Isochronous Transfer (No Handshake)
= (38 * 8 * 2.083) + (2.083 * Floor(3.167
Host_Delay
+ BitStuffTime(Data_bc))) +
High-speed (Output)
Non-Isochronous Transfer (Handshake Included)
= (55 * 8 * 2.083) + (2.083 * Floor(3.167 + BitStuffTime(Data_bc))) +
Host_Delay
Isochronous Transfer (No Handshake)
= (38 * 8 * 2.083) + (2.083 * Floor(3.167
Host_Delay
+ BitStuffTime(Data_bc))) +
Full-speed (Input)
Non-Isochronous Transfer (Handshake Included)
= 9107 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Isochronous Transfer (No Handshake)
= 7268 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Full-speed (Output)
Non-Isochronous Transfer (Handshake Included)
= 9107 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Isochronous Transfer (No Handshake)
= 6265 + (83.54 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Low-speed (Input)
= 64060 + (2 * Hub_LS_Setup) +
(676.67 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
Low-speed (Output)
= 64107 + (2 * Hub_LS_Setup) +
(667.0 * Floor(3.167 + BitStuffTime(Data_bc))) + Host_Delay
The bus times in the above equations are in nanoseconds and take into account propagation delays due to the
distance the device is from the host. These are typical equations that can be used to calculate bus time;
however, different implementations may choose to use coarser approximations of these times.
The actual bus time taken for a given transaction will almost always be less than that calculated because bit
stuffing overhead is data-dependent. Worst case bit stuffing is calculated as 1.1667 (7/6) times the raw time
(i.e., the BitStuffTime function multiplies the Data_bc by 8*1.1667 in the equations). This means that there
will almost always be time unused on the bus (subject to data pattern specifics) after all regularly scheduled
transactions have completed. The bus time made available due to less bit stuffing can be reused as
discussed in Section 5.11.5.
The Host_Delay term in the equations is Host Controller-, Transaction Translator(TT)-, and systemdependent and allows for additional time a Host Controller (or TT) may require due to delays in gaining
access to memory or other implementation dependencies. This term is incorporated into an implementation
of these equations by using the transfer management functions provided by the HCD interface. These
equations are typically implemented by a combination of USBD and HCD software working in cooperation.
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Universal Serial Bus Specification Revision 2.0
The results of these calculations are used to determine whether a transfer or pipe creation can be supported
in a given USB configuration.
5.11.4 Calculating Buffer Sizes in Functions and Software
Client software and functions both need to provide buffer space for pending data transactions awaiting their
turn on the bus. For non-isochronous pipes, this buffer space needs to be just large enough to hold the next
data packet. If more than one transaction request is pending for a given endpoint, the buffering for each
transaction must be supplied. Methods to calculate the precise absolute minimum buffering a function may
require because of specific interactions defined between its client software and the function are outside the
scope of this specification.
The Host Controller is expected to be able to support an unlimited number of transactions pending for the
bus subject to available system memory for buffer and descriptor space, etc. Host Controllers are allowed
to limit how many (micro)frames into the future they allow a transaction to be requested.
For isochronous pipes, Section 5.12.4 describes details affecting host side and device side buffering
requirements. In general, buffers need to be provided to hold approximately twice the amount of data that
can be transferred in 1ms for full-speed endpoints or 125 µs for high-speed endpoints.
5.11.5 Bus Bandwidth Reclamation
The USB bandwidth and bus access are granted based on a calculation of worst-case bus transmission time
and required latencies. However, due to the constraints placed on different transfer types and the fact that
the bit stuffing bus time contribution is calculated as a constant but is data-dependent, there will frequently
be bus time remaining in each (micro)frame time versus what the (micro)frame transmission time was
calculated to be. In order to support the most efficient use of the bus bandwidth, control and bulk transfers
are candidates to be moved over the bus as bus time becomes available. Exactly how a Host Controller
supports this is implementation-dependent. A Host Controller can take into account the transfer types of
pending IRPs and implementation-specific knowledge of remaining (micro)frame time to reuse reclaimed
bandwidth.
5.12 Special Considerations for Isochronous Transfers
Support for isochronous data movement between the host and a device is one of the system capabilities
supported by the USB. Delivering isochronous data reliably over the USB requires careful attention to
detail. The responsibility for reliable delivery is shared by several USB entities:
•
The device/function
•
The bus
•
The Host Controller
•
One or more software agents
Because time is a key part of an isochronous transfer, it is important for USB designers to understand how
time is dealt with within the USB by these different entities.
Note: The examples in this section describe USB for an example involving full-speed endpoints. The
general example details are also appropriate for high-speed endpoints when corresponding changes are
made; for example, frame replaced with microframe, 1 ms replaced with 125 µs, rate adjustments made
between full-speed and high-speed, etc.
All isochronous devices must report their capabilities in the form of device-specific descriptors. The
capabilities should also be provided in a form that the potential customer can use to decide whether the
device offers a solution to his problem(s). The specific capabilities of a device can justify price differences.
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In any communication system, the transmitter and receiver must be synchronized enough to deliver data
robustly. In an asynchronous communication system, data can be delivered robustly by allowing the
transmitter to detect that the receiver has not received a data item correctly and simply retrying transmission
of the data.
In an isochronous communication system, the transmitter and receiver must remain time- and datasynchronized to deliver data robustly. The USB does not support transmission retry of isochronous data so
that minimal bandwidth can be allocated to isochronous transfers and time synchronization is not lost due to
a retry delay. However, it is critical that a USB isochronous transmitter/receiver pair still remain
synchronized both in normal data transmission cases and in cases where errors occur on the bus.
In many systems that deal with isochronous data, a single global clock is used to which all entities in the
system synchronize. An example of such a system is the PSTN (Public Switched Telephone Network).
Given that a broad variety of devices with different natural frequencies may be attached to the USB, no
single clock can provide all the features required to satisfy the synchronization requirements of all devices
and software while still supporting the cost targets of mass-market PC products. The USB defines a clock
model that allows a broad range of devices to coexist on the bus and have reasonable cost implementations.
This section presents options or features that can be used by isochronous endpoints to minimize behavior
differences between a non-USB implemented function and a USB version of the function. An example is
included to illustrate the similarities and differences between the non-USB and USB versions of a function.
The remainder of the section presents the following key concepts:
•
USB Clock Model: What clocks are present in a USB system that have impact on isochronous data
transfers
•
USB (micro)frame Clock-to-function Clock Synchronization Options: How the USB (micro)frame
clock can relate to a function clock
•
SOF Tracking: Responsibilities and opportunities of isochronous endpoints with respect to the SOF
token and USB (micro)frames
•
Data Prebuffering: Requirements for accumulating data before generation, transmission, and
consumption
•
Error Handling: Isochronous-specific details for error handling
•
Buffering for Rate Matching: Equations that can be used to calculate buffer space required for
isochronous endpoints
5.12.1 Example Non-USB Isochronous Application
The example used is a reasonably generalized example. Other simpler or more complex cases are possible
and the relevant USB features identified can be used or not as appropriate.
The example consists of an 8 kHz mono microphone connected through a mixer driver that sends the input
data stream to 44 kHz stereo speakers. The mixer expects the data to be received and transmitted at some
sample rate and encoding. A rate matcher driver on input and output converts the sample rate and encoding
from the natural rate and encoding of the device to the rate and encoding expected by the mixer.
Figure 5-16 illustrates this example.
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Universal Serial Bus Specification Revision 2.0
Each DD has
independent
service rate
Mixer Device
Driver
Rate
Matcher
Rate
Matcher
1 speaker DD
service period
(n sample)
slop buffer
Speaker
Device Driver
20ms service
period
Master Clock
20ms service
period
Microphone
Device Driver
2x160 Byte Buffer
(2 Services,
160 samples per service)
Transfer
Complete
Interrupt
Transfer
Complete
Interrupt
2x3528 Byte Buffer
(2 Services,
882 samples per service)
DMA
controller
Software
Hardware
Traditional Bus
(e.g. PCI, ISA, ...)
Single sample
transfers
1 sample at a time
Mono
Microphone
8MHz Bus Clock
1 sample at a time
CD Stereo
Speakers
2x1 Byte Buffer
(2 Samples)
8kHz Sample Clock
(1 byte/sample)
2x4 Byte Buffer
(2 Samples)
44.1KHz Sample Clock
(4 bytes/sample)
Figure 5-16. Non-USB Isochronous Example
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A master clock (which can be provided by software driven from the real time clock) in the PC is used to
awaken the mixer to ask the input source for input data and to provide output data to the output sink. In this
example, assume it awakens every 20 ms. The microphone and speakers each have their own sample clocks
that are unsynchronized with respect to each other or the master mixer clock. The microphone produces
data at its natural rate (one-byte samples, 8,000 times a second) and the speakers consume data at their
natural rate (four-byte samples, 44,100 times a second). The three clocks in the system can drift and jitter
with respect to each other. Each rate matcher may also be running at a different natural rate than either the
mixer driver, the input source/driver, or output sink/driver.
The rate matchers also monitor the long-term data rate of their device compared to the master mixer clock
and interpolate an additional sample or merge two samples to adjust the data rate of their device to the data
rate of the mixer. This adjustment may be required every couple of seconds, but typically occurs
infrequently. The rate matchers provide some additional buffering to carry through a rate match.
Note: Some other application might not be able to tolerate sample adjustment and would need some other
means of accommodating master clock-to-device clock drift or else would require some means of
synchronizing the clocks to ensure that no drift could occur.
The mixer always expects to receive exactly a service period of data (20 ms service period) from its input
device and produce exactly a service period of data for its output device. The mixer can be delayed up to
less than a service period if data or space is not available from its input/output device. The mixer assumes
that such delays do not accumulate.
The input and output devices and their drivers expect to be able to put/get data in response to a hardware
interrupt from the DMA controller when their transducer has processed one service period of data. They
expect to get/put exactly one service period of data. The input device produces 160 bytes (ten samples)
every service period of 20 ms. The output device consumes 3,528 bytes (882 samples) every 20 ms service
period. The DMA controller can move a single sample between the device and the host buffer at a rate
much faster than the sample rate of either device.
The input and output device drivers provide two service periods of system buffering. One buffer is always
being processed by the DMA controller. The other buffer is guaranteed to be ready before the current
buffer is exhausted. When the current buffer is emptied, the hardware interrupt awakens the device driver
and it calls the rate matcher to give it the buffer. The device driver requests a new IRP with the buffer
before the current buffer is exhausted.
The devices can provide two samples of data buffering to ensure that they always have a sample to process
for the next sample period while the system is reacting to the previous/next sample.
The service periods of the drivers are chosen to survive interrupt latency variabilities that may be present in
the operating system environment. Different operating system environments will require different service
periods for reliable operation. The service periods are also selected to place a minimum interrupt load on
the system, because there may be other software in the system that requires processing time.
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5.12.2 USB Clock Model
Time is present in the USB system via clocks. In fact, there are multiple clocks in a USB system that must
be understood:
•
Sample Clock: This clock determines the natural data rate of samples moving between client software
on the host and the function. This clock does not need to be different between non-USB and USB
implementations.
•
Bus Clock: This clock runs at a 1.000 ms period (1 kHz frequency) on full-speed segments and
125.000 µs (8 kHz frequency) on high-speed segments of the bus and is indicated by the rate of SOF
packets on the bus. This clock is somewhat equivalent to the 8 MHz clock in the non-USB example.
In the USB case, the bus clock is often a lower-frequency clock than the sample clock, whereas the bus
clock is almost always a higher-frequency clock than the sample clock in a non-USB case.
•
Service Clock: This clock is determined by the rate at which client software runs to service IRPs that
may have accumulated between executions. This clock also can be the same in the USB and non-USB
cases.
In most existing operating systems, it is not possible to support a broad range of isochronous communication
flows if each device driver must be interrupted for each sample for fast sample rates. Therefore, multiple
samples, if not multiple packets, will be processed by client software and then given to the Host Controller
to sequence over the bus according to the prenegotiated bus access requirements. Figure 5-17 presents an
example for a reasonable USB clock environment equivalent to the non-USB example in Figure 5-16.
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Each DD has
independent
service rate
Mixer Device
Driver
Rate
Matcher
1 speaker DD
service period
(n sample)
slop buffer
Rate
Matcher
Master Clock
20ms service
period
1 sample
slop buffer
Microphone
Device Driver
Transfer
Complete
Interrupt
QueueBuffer
2x161 Byte Buffer
(2 Services,
159-161 samples per
service,
20 packets/service)
Transfer
Complete
Interrupt
USB SW
Host
Controller
1KHz Bus Clock
7-9 Byte Packets
7-9 samples per packet
Speaker
Device Driver
20ms service
period
QueueBuffer
2x3532 Byte Buffer
(2 Services,
881-883 samples per
service
20 packets/service)
1x3 Byte Buffer
(1 Services,
1 feedback per service
1 packets/service)
172-184 Byte Packets
43-46 samples per packet
3 byte packets
Feedback Info
Hub
Mono
Microphone
8+9 Byte Buffer
(2 Packets)
8kHz Sample Clock
(1 byte/sample)
CD Stereo
Speakers
(44+45+1+1)x4
Byte Buffer
(2 Packets)
1x3 Byte
Buffer
(1 Packets)
44.1KHz Sample Clock
(4 bytes/sample)
Figure 5-17. USB Full-speed Isochronous Application
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Hardware
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Figure 5-17 shows a typical round trip path of information from a microphone as an input device to a
speaker as an output device. The clocks, packets, and buffering involved are also shown. Figure 5-17 will
be explored in more detail in the following sections.
The focus of this example is to identify the differences introduced by the USB compared to the previous
non-USB example. The differences are in the areas of buffering, synchronization given the existence of a
USB bus clock, and delay. The client software above the device drivers can be unaffected in most cases.
5.12.3 Clock Synchronization
In order for isochronous data to be manipulated reliably, the three clocks identified above must be
synchronized in some fashion. If the clocks are not synchronized, several clock-to-clock attributes can be
present that can be undesirable:
•
Clock Drift: Two clocks that are nominally running at the same rate can, in fact, have implementation
differences that result in one clock running faster or slower than the other over long periods of time. If
uncorrected, this variation of one clock compared to the other can lead to having too much or too little
data when data is expected to always be present at the time required.
•
Clock Jitter: A clock may vary its frequency over time due to changes in temperature, etc. This may
also alter when data is actually delivered compared to when it is expected to be delivered.
•
Clock-to-clock Phase Differences: If two clocks are not phase locked, different amounts of data may
be available at different points in time as the beat frequency of the clocks cycle out over time. This can
lead to quantization/sampling related artifacts.
The bus clock provides a central clock with which USB hardware devices and software can synchronize to
one degree or another. However, the software will, in general, not be able to phase- or frequency-lock
precisely to the bus clock given the current support for “real time-like” operating system scheduling support
in most PC operating systems. Software running in the host can, however, know that data moved over the
USB is packetized. For isochronous transfer types, a unit of data is moved exactly once per (micro)frame
and the (micro)frame clock is reasonably precise. Providing the software with this information allows it to
adjust the amount of data it processes to the actual (micro)frame time that has passed.
Note: For high-speed high-bandwidth endpoints, the data exchanged in the two or three transactions per
microframe is still considered to belong to the same “single packet.” The large amount of data per packet is
split into two or three transactions only for bus efficiency reasons.
5.12.4 Isochronous Devices
The USB includes a framework for isochronous devices that defines synchronization types, how
isochronous endpoints provide data rate feedback, and how they can be connected together. Isochronous
devices include sampled analog devices (for example, audio and telephony devices) and synchronous data
devices. Synchronization type classifies an endpoint according to its capability to synchronize its data rate
to the data rate of the endpoint to which it is connected. Feedback is provided by indicating accurately
what the required data rate is, relative to the SOF frequency. The ability to make connections depends on
the quality of connection that is required, the endpoint synchronization type, and the capabilities of the host
application that is making the connection. Additional device class-specific information may be required,
depending on the application.
Note: The term “data” is used very generally, and may refer to data that represents sampled analog
information (like audio), or it may be more abstract information. “Data rate” refers to the rate at which
analog information is sampled, or the rate at which data is clocked.
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The following information is required in order to determine how to connect isochronous endpoints:
•
Synchronization type:
−
Asynchronous: Unsynchronized, although sinks provide data rate feedback
−
Synchronous: Synchronized to the USB’s SOF
−
Adaptive: Synchronized using feedback or feedforward data rate information
•
Available data rates
•
Available data formats
Synchronization type and data rate information are needed to determine if an exact data rate match exists
between source and sink, or if an acceptable conversion process exists that would allow the source to be
connected to the sink. It is the responsibility of the application to determine whether the connection can be
supported within available processing resources and other constraints (like delay). Specific USB device
classes define how to describe synchronization type and data rate information.
Data format matching and conversion is also required for a connection, but it is not a unique requirement for
isochronous connections. Details about format conversion can be found in other documents related to
specific formats.
5.12.4.1 Synchronization Type
Three distinct synchronization types are defined. Table 5-12 presents an overview of endpoint
synchronization characteristics for both source and sink endpoints. The types are presented in order of
increasing capability.
Table 5-12. Synchronization Characteristics
Asynchronous
Synchronous
Adaptive
Source
Sink
Free running Fs
Free running Fs
Provides implicit feedforward (data
stream)
Provides explicit feedback (isochronous
pipe)
Fs locked to SOF
Fs locked to SOF
Uses implicit feedback (SOF)
Uses implicit feedback (SOF)
Fs locked to sink
Fs locked to data flow
Uses explicit feedback (isochronous pipe)
Uses implicit feedforward (data stream)
5.12.4.1.1 Asynchronous
Asynchronous endpoints cannot synchronize to SOF or any other clock in the USB domain. They source or
sink an isochronous data stream at either a fixed data rate (single-frequency endpoints), a limited number of
data rates (32 kHz, 44.1 kHz, 48 kHz, …), or a continuously programmable data rate. If the data rate is
programmable, it is set during initialization of the isochronous endpoint. Asynchronous devices must report
their programming capabilities in the class-specific endpoint descriptor as described in their device class
specification. The data rate is locked to a clock external to the USB or to a free-running internal clock.
These devices place the burden of data rate matching elsewhere in the USB environment. Asynchronous
source endpoints carry their data rate information implicitly in the number of samples they produce per
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(micro)frame. Asynchronous sink endpoints must provide explicit feedback information to an adaptive
driver (refer to Section 5.12.4.2).
An example of an asynchronous source is a CD-audio player that provides its data based on an internal
clock or resonator. Another example is a Digital Audio Broadcast (DAB) receiver or a Digital Satellite
Receiver (DSR). Here, too, the sample rate is fixed at the broadcasting side and is beyond USB control.
Asynchronous sink endpoints could be low-cost speakers running off of their internal sample clock.
5.12.4.1.2 Synchronous
Synchronous endpoints can have their clock system (their notion of time) controlled externally through SOF
synchronization. These endpoints must slave their sample clock to the 1 ms SOF tick (by means of a
programmable PLL). For high-speed endpoints, the presence of the microframe SOF can be used for tighter
frame clock tracking.
Synchronous endpoints may source or sink isochronous data streams at either a fixed data rate (singlefrequency endpoints), a limited number of data rates (32 kHz, 44.1 kHz, 48 kHz, …), or a continuously
programmable data rate. If programmable, the operating data rate is set during initialization of the
isochronous endpoint. The number of samples or data units generated in a series of USB (micro)frames is
deterministic and periodic. Synchronous devices must report their programming capabilities in the classspecific endpoint descriptor as described in their device class specification.
An example of a synchronous source is a digital microphone that synthesizes its sample clock from SOF and
produces a fixed number of audio samples every USB (micro)frame. Likewise, a synchronous sink derives
its sample clock from SOF and consumes a fixed number of samples every USB (micro)frame.
5.12.4.1.3 Adaptive
Adaptive endpoints are the most capable endpoints possible. They are able to source or sink data at any rate
within their operating range. Adaptive source endpoints produce data at a rate that is controlled by the data
sink. The sink provides feedback (refer to Section 5.12.4.2) to the source, which allows the source to know
the desired data rate of the sink. For adaptive sink endpoints, the data rate information is embedded in the
data stream. The average number of samples received during a certain averaging time determines the
instantaneous data rate. If this number changes during operation, the data rate is adjusted accordingly.
The data rate operating range may center around one rate (e.g., 8 kHz), select between several
programmable or auto-detecting data rates (32 kHz, 44.1 kHz, 48 kHz, …), or may be within one or more
ranges (e.g., 5 kHz to 12 kHz or 44 kHz to 49 kHz). Adaptive devices must report their programming
capabilities in the class-specific endpoint descriptor as described in their device class specification.
An example of an adaptive source is a CD player that contains a fully adaptive sample rate converter (SRC)
so that the output sample frequency no longer needs to be 44.1 kHz but can be anything within the operating
range of the SRC. Adaptive sinks include such endpoints as high-end digital speakers, headsets, etc.
5.12.4.2 Feedback
An asynchronous sink must provide explicit feedback to the host by indicating accurately what its desired
data rate (Ff) is, relative to the USB (micro)frame frequency. This allows the host to continuously adjust the
number of samples sent to the sink so that neither underflow or overflow of the data buffer occurs.
Likewise, an adaptive source must receive explicit feedback from the host so that it can accurately generate
the number of samples required by the host. Feedback endpoints can be specified as described in
Section 9.6.6 for the bmAttributes field of the endpoint descriptor.
To generate the desired data rate Ff, the device must measure its actual sampling rate Fs, referenced to the
USB notion of time, i.e., the USB (micro)frame frequency. This specification requires the data rate Ff to be
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resolved to better than one sample per second (1Hz) in order to allow a high-quality source rate to be
created and to tolerate delays and errors in the feedback loop. To achieve this accuracy, the measurement
time Tmeas must be at least 1 second. Therefore:
Tmeas = 2 K
where Tmeas is now expressed in USB (micro)frames and K=10 for full-speed devices (1 ms frames) and
K=13 for high-speed devices (125 µs microframes). However, in most devices, the actual sampling rate Fs
is derived from a master clock Fm through a binary divider. Therefore:
Fm = Fs ∗2 P
where P is a positive integer (including 0 if no higher-frequency master clock is available). The
measurement time Tmeas can now be decreased by measuring Fm instead of Fs and:
Tmeas
2K
= P = 2( K − P )
2
(K-P)
In this way, a new estimate for Ff becomes available every 2
(micro)frames. P is practically bound to
be in the range [0,K] because there is no point in using a clock slower than Fs (P=0), and no point in trying
to update Ff more than once per (micro)frame (P=K). A sink can determine Ff by counting cycles of the
(K-P)
master clock Fm for a period of 2
(K-P)
2
(micro)frames. The counter is read into Ff and reset every
(micro)frames. As long as no clock cycles are skipped, the count will be accurate over the long term.
Each (micro)frame, an adaptive source adds Ff to any remaining fractional sample count from the previous
(micro)frame, sources the number of samples in the integer part of the sum, and retains the fractional
sample count for the next (micro)frame. The source can look at the behavior of Ff over many
(micro)frames to determine an even more accurate rate, if it needs to.
Ff is expressed in number of samples per (micro)frame. The Ff value consists of an integer part that
represents the (integer) number of samples per (micro)frame and a fractional part that represents the
“fraction” of a sample that would be needed to match the sampling frequency Fs to a resolution of 1 Hz or
better. The fractional part requires at least K bits to represent the “fraction” of a sample to a resolution of
1 Hz or better. The integer part must have enough bits to represent the maximum number of samples that
can ever occur in a single (micro)frame. Assuming that the minimum sample size is one byte, then this
number is limited to 1,023 for full-speed endpoints. Ten bits are therefore sufficient to encode this value.
For high-speed endpoints, this number is limited to 3*1,024=3,072 and twelve bits are needed.
In summary, for full-speed endpoints, the Ff value shall be encoded in an unsigned 10.10 (K=10) format
which fits into three bytes. Because the maximum integer value is fixed to 1,023, the 10.10 number will be
left-justified in the 24 bits, so that it has a 10.14 format. Only the first ten bits behind the binary point are
required. The lower four bits may be optionally used to extend the precision of Ff, otherwise, they shall be
reported as zero. For high-speed endpoints, the Ff value shall be encoded in an unsigned 12.13 (K=13)
format which fits into four bytes. The value shall be aligned into these four bytes so that the binary point is
located between the second and the third byte so that it has a 16.16 format. The most significant four bits
shall be reported zero. Only the first 13 bits behind the binary point are required. The lower three bits may
be optionally used to extend the precision of Ff, otherwise, they shall be reported as zero.
An endpoint needs to implement only the number of bits that it effectively requires for its maximum Ff.
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The choice of P is endpoint-specific. Use the following guidelines when choosing P:
•
P must be in the range [0,K].
•
Larger values of P are preferred, because they reduce the size of the frame counter and increase the rate
at which Ff is updated. More frequent updates result in a tighter control of the source data rate, which
reduces the buffer space required to handle Ff changes.
•
P should be less than K so that Ff is averaged across at least two frames in order to reduce SOF jitter
effects.
•
P should not be zero in order to keep the deviation in the number of samples sourced to less than 1 in
the event of a lost Ff value.
Isochronous transfers are used to read Ff from the feedback register. The desired reporting rate for the
(K-P)
feedback should be 2
frames. Ff will be reported at most once per update period. There is nothing to be
gained by reporting the same Ff value more than once per update period. The endpoint may choose to report
Ff only if the updated value has changed from the previous Ff value. If the value has not changed, the
endpoint may report the current Ff value or a zero length data payload. It is strongly recommended that an
endpoint always report the current Ff value any time it is polled.
It is possible that the source will deliver one too many or one too few samples over a long period due to
errors or accumulated inaccuracies in measuring Ff. The sink must have sufficient buffer capability to
accommodate this. When the sink recognizes this condition, it should adjust the reported Ff value to correct
it. This may also be necessary to compensate for relative clock drifts. The implementation of this
correction process is endpoint-specific and is not specified.
5.12.4.3 Implicit Feedback
In some cases, implementing a separate explicit feedback endpoint can be avoided. If a device implements
a group of isochronous data endpoints that are closely related and if:
•
All the endpoints in the group are synchronized (i.e. use sample clocks that are derived from a common
master clock)
•
The group contains one or more isochronous data endpoints in one direction that normally would need
explicit feedback
•
The group contains at least one isochronous data endpoint in the opposite direction
Under these circumstances, the device may elect not to implement a separate isochronous explicit feedback
endpoint. Instead, feedback information can be derived from the data endpoint in the opposite direction by
observing its data rate.
Two cases can arise:
•
One or more asynchronous sink endpoints are accompanied by an asynchronous source endpoint. The
data rate on the source endpoint can be used as implicit feedback information to adjust the data rate on
the sink endpoint(s).
•
One or more adaptive source endpoints are accompanied by an adaptive sink endpoint. The source
endpoint can adjust its data rate based on the data rate received by the sink endpoint.
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This specification provides the necessary framework to implement synchronization as described above (see
Chapter 9). However, exactly how the desired data rate Ff is derived from the data rate of the implied
feedback endpoint is implementation-dependent.
5.12.4.4 Connectivity
In order to fully describe the source-to-sink connectivity process, an interconnect model is presented. The
model indicates the different components involved and how they interact to establish the connection.
The model provides for multi-source/multi-sink situations. Figure 5-18 illustrates a typical situation (highly
condensed and incomplete). A physical device is connected to the host application software through
different hardware and software layers as described in this specification. At the client interface level, a
virtual device is presented to the application. From the application standpoint, only virtual devices exist. It
is up to the device driver and client software to decide what the exact relation is between physical and
virtual device.
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Host Environment
CD-ROM
Device
Driver
Client
Isoc. Pipe
Device
Driver
Client
Isoc. Pipe
Device
Driver
Client
Physical Sources
Source
Source
Virtual Sources
Application
USB Environment
Virtual Sinks
Sink
Sink
Isoc. Pipe
Device
Driver
Client
Isoc. Pipe
Device
Driver
Client
Device
Driver
Client
Physical Sinks
Hard Disk
Figure 5-18. Example Source/Sink Connectivity
Device manufacturers (or operating system vendors) must provide the necessary device driver software and
client interface software to convert their device from the physical implementation to a USB-compliant
software implementation (the virtual device). As stated before, depending on the capabilities built into this
software, the virtual device can exhibit different synchronization behavior from the physical device.
However, the synchronization classification applies equally to both physical and virtual devices. All
physical devices belong to one of the three possible synchronization types. Therefore, the capabilities that
have to be built into the device driver and/or client software are the same as the capabilities of a physical
device. The word “application” must be replaced by “device driver/client software.” In the case of a
physical source to virtual source connection, “virtual source device” must be replaced by “physical source
device” and “virtual sink device” must be replaced by “virtual source device.” In the case of a virtual sink
to physical sink connection, “virtual source device” must be replaced by “virtual sink device” and “virtual
sink device” must be replaced by “physical sink device.”
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Placing the rate adaptation (RA) functionality into the device driver/client software layer has the distinct
advantage of isolating all applications, relieving the device from the specifics and problems associated with
rate adaptation. Applications that would otherwise be multi-rate degenerate to simpler mono-rate systems.
Note: The model is not limited to only USB devices. For example, a CD-ROM drive containing 44.1 kHz
audio can appear as either an asynchronous, synchronous, or adaptive source. Asynchronous operation
means that the CD-ROM fills its buffer at the rate that it reads data from the disk, and the driver empties the
buffer according to its USB service interval. Synchronous operation means that the driver uses the USB
service interval (e.g., 10 ms) and nominal sample rate of the data (44.1 kHz) to determine to put out
441 samples every USB service interval. Adaptive operation would build in a sample rate converter to
match the CD-ROM output rate to different sink sampling rates.
Using this reference model, it is possible to define what operations are necessary to establish connections
between various sources and sinks. Furthermore, the model indicates at what level these operations must or
can take place. First, there is the stage where physical devices are mapped onto virtual devices and vice
versa. This is accomplished by the driver and/or client software. Depending on the capabilities included in
this software, a physical device can be transformed into a virtual device of an entirely different
synchronization type. The second stage is the application that uses the virtual devices. Placing rate
matching capabilities at the driver/client level of the software stack relieves applications communicating
with virtual devices from the burden of performing rate matching for every device that is attached to them.
Once the virtual device characteristics are decided, the actual device characteristics are not any more
interesting than the actual physical device characteristics of another driver.
As an example, consider a mixer application that connects at the source side to different sources, each
running at their own frequencies and clocks. Before mixing can take place, all streams must be converted to
a common frequency and locked to a common clock reference. This action can be performed in the
physical-to-virtual mapping layer or it can be handled by the application itself for each source device
independently. Similar actions must be performed at the sink side. If the application sends the mixed data
stream out to different sink devices, it can either do the rate matching for each device itself or it can rely on
the driver/client software to do that, if possible.
Table 5-13 indicates at the intersections what actions the application must perform to connect a source
endpoint to a sink endpoint.
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Universal Serial Bus Specification Revision 2.0
Table 5-13. Connection Requirements
Source Endpoint
Sink Endpoint
Asynchronous
Synchronous
Adaptive
Asynchronous
Async Source/Sink RA
See Note 1.
Async SOF/Sink RA
See Note 2.
Data + Feedback
Feedthrough
See Note 3.
Synchronous
Async Source/SOF RA
See Note 4.
Sync RA
See Note 5.
Data Feedthrough +
Application Feedback
See Note 6.
Adaptive
Data Feedthrough
See Note 7.
Data Feedthrough
See Note 8.
Data Feedthrough
See Note 9.
Notes:
1. Asynchronous RA in the application. Fsi is determined by the source, using the feedforward information
embedded in the data stream. Fso is determined by the sink, based on feedback information from the
sink. If nominally Fsi = Fso, the process degenerates to a feedthrough connection if slips/stuffs due to
lack of synchronization are tolerable. Such slips/stuffs will cause audible degradation in audio
applications.
2. Asynchronous RA in the application. Fsi is determined by the source but locked to SOF. Fso is
determined by the sink, based on feedback information from the sink. If nominally Fsi = Fso, the
process degenerates to a feedthrough connection if slips/stuffs due to lack of synchronization are
tolerable. Such slips/stuffs will cause audible degradation in audio applications.
3. If Fso falls within the locking range of the adaptive source, a feedthrough connection can be established.
Fsi = Fso and both are determined by the asynchronous sink, based on feedback information from the
sink. If Fso falls outside the locking range of the adaptive source, the adaptive source is switched to
synchronous mode and Note 2 applies.
4. Asynchronous RA in the application. Fsi is determined by the source. Fso is determined by the sink
and locked to SOF. If nominally Fsi = Fso, the process degenerates to a feedthrough connection if
slips/stuffs due to lack of synchronization are tolerable. Such slips/stuffs will cause audible degradation
in audio applications.
5. Synchronous RA in the application. Fsi is determined by the source and locked to SOF. Fso is
determined by the sink and locked to SOF. If Fsi = Fso, the process degenerates to a loss-free
feedthrough connection.
6. The application will provide feedback to synchronize the source to SOF. The adaptive source appears
to be a synchronous endpoint and Note 5 applies.
7. If Fsi falls within the locking range of the adaptive sink, a feedthrough connection can be established.
Fsi = Fso and both are determined by and locked to the source.
If Fsi falls outside the locking range of the adaptive sink, synchronous RA is done in the host to provide
an Fso that is within the locking range of the adaptive sink.
8. If Fsi falls within the locking range of the adaptive sink, a feedthrough connection can be established.
Fso = Fsi and both are determined by the source and locked to SOF.
If Fsi falls outside the locking range of the adaptive sink, synchronous RA is done in the host to provide
an Fso that is within the locking range of the adaptive sink.
9. The application will use feedback control to set Fso of the adaptive source when the connection is set
up. The adaptive source operates as an asynchronous source in the absence of ongoing feedback
information and Note 7 applies.
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In cases where RA is needed but not available, the rate adaptation process could be mimicked by sample
dropping/stuffing. The connection could then still be made, possibly with a warning about poor quality,
otherwise, the connection cannot be made.
5.12.4.4.1 Audio Connectivity
When the above is applied to audio data streams, the RA process is replaced by sample rate conversion,
which is a specialized form of rate adaptation. Instead of error control, some form of sample interpolation
is used to match incoming and outgoing sample rates. Depending on the interpolation techniques used, the
audio quality (distortion, signal to noise ratio, etc.) of the conversion can vary significantly. In general,
higher quality requires more processing power.
5.12.4.4.2 Synchronous Data Connectivity
For the synchronous data case, RA is used. Occasional slips/stuffs may be acceptable to many applications
that implement some form of error control. Error control includes error detection and discard, error
detection and retransmit, or forward error correction. The rate of slips/stuffs will depend on the clock
mismatch between the source and sink and may be the dominant error source of the channel. If the error
control is sufficient, then the connection can still be made.
5.12.5 Data Prebuffering
The USB requires that devices prebuffer data before processing/transmission to allow the host more
flexibility in managing when each pipe’s transaction is moved over the bus from (micro)frame to
(micro)frame.
For transfers from function to host, the endpoint must accumulate samples during (micro)frame X until it
receives the SOF token for (micro)frame X+1. It “latches” the data from (micro)frame X into its packet
buffer and is now ready to send the packet containing those samples during (micro)frame X+1. When it
will send that data during the (micro)frame is determined solely by the Host Controller and can vary from
(micro)frame to (micro)frame.
For transfers from host to function, the endpoint will accept a packet from the host sometime during
(micro)frame Y. When it receives the SOF for (micro)frame Y+1, it can then start processing the data
received in (micro)frame Y.
This approach allows an endpoint to use the SOF token as a stable clock with very little jitter and/or drift
when the Host Controller moves the packet over the bus. This approach also allows the Host Controller to
vary within a (micro)frame precisely when the packet is actually moved over the bus. This prebuffering
introduces some additional delay between when a sample is available at an endpoint and when it moves over
the bus compared to an environment where the bus access is at exactly the same time offset from SOF from
(micro)frame to (micro)frame.
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Universal Serial Bus Specification Revision 2.0
Figure 5-19 shows the time sequence for a function-to-host transfer (IN process). Data D0 is accumulated
during (micro)frame Fi at time Ti and transmitted to the host during (micro)frame Fi+1. Similarly, for a
host-to-function transfer (OUT process), data D0 is received by the endpoint during (micro)frame Fi+1 and
processed during (micro)frame Fi+2.
Time:
Ti
Ti+1 Ti+2 Ti+3
Tm
Tm+1
(Micro)Frame:
Fi
Fi+1 Fi+2 Mi+3
Fm
Fm+1
D0
D0
D1
Data on Bus:
OUT Process:
IN Process:
D0
D1
D1
D2
D0
D1
D0
D0
Figure 5-19. Data Prebuffering
5.12.6 SOF Tracking
Functions supporting isochronous pipes must receive and comprehend the SOF token to support
prebuffering as previously described. Given that SOFs can be corrupted, a device must be prepared to
recover from a corrupted SOF. These requirements limit isochronous transfers to full-speed and high-speed
devices only, because low-speed devices do not see SOFs on the bus. Also, because SOF packets can be
damaged in transmission, devices that support isochronous transfers need to be able to synthesize the
existence of an SOF that they may not see due to a bus error.
Isochronous transfers require the appropriate data to be transmitted in the corresponding (micro)frame. The
USB requires that when an isochronous transfer is presented to the Host Controller, it identifies the
(micro)frame number for the first (micro)frame. The Host Controller must not transmit the first transaction
before the indicated (micro)frame number. Each subsequent transaction in the IRP must be transmitted in
succeeding (micro)frames (except for high-speed high-bandwidth transfers where up to three transactions
may occur in the same microframe). If there are no transactions pending for the current (micro)frame, then
the Host Controller must not transmit anything for an isochronous pipe. If the indicated (micro)frame
number has passed, the Host Controller must skip (i.e., not transmit) all transactions until the one
corresponding to the current (micro)frame is reached.
5.12.7 Error Handling
Isochronous transfers provide no data packet retries (i.e., no handshakes are returned to a transmitter by a
receiver) so that timeliness of data delivery is not perturbed. However, it is still important for the agents
responsible for data transport to know when an error occurs and how the error affects the communication
flow. In particular, for a sequence of data packets (A, B, C, D), the USB allows sufficient information such
that a missing packet (A, _, C, D) can be detected and will not unknowingly be turned into an incorrect data
or time sequence (A, C, D or A, _, B, C, D). The protocol provides four mechanisms that support this: a
strictly defined periodicity for the transmission of packets and data PID sequencing mechanisms for highspeed high-bandwidth endpoints, SOF, CRC, and bus transaction timeout.
•
Isochronous transfers require periodic occurrence of data transactions for normal operation. The period
must be an exact power of two (micro)frames. The USB does not dictate what data is transmitted in
each frame. The data transmitter/source determines specifically what data to provide. This regular
periodic data delivery provides a framework that is fundamental to detecting missing data errors. For
high-speed high-bandwidth endpoints, data PID sequencing allows the detection of missing or damaged
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Universal Serial Bus Specification Revision 2.0
transactions during a microframe. Any phase of a transaction can be damaged during transmission on
the bus. Chapter 8 describes how each error case affects the protocol.
•
Because every (micro)frame is preceded by an SOF and a receiver can see SOFs on the bus, a receiver
can determine that its expected transaction for that (micro)frame did not occur between two SOFs.
Additionally, because even an SOF can be damaged, a device must be able to reconstruct the existence
of a missed SOF as described in Section 5.12.6.
•
A data packet may be corrupted on the bus; therefore, CRC protection allows a receiver to determine
that the data packet it received was corrupted.
•
The protocol defines the details that allow a receiver to determine via bus transaction timeout that it is
not going to receive its data packet after it has successfully seen its token packet.
Once a receiver has determined that a data packet was not received, it may need to know the size of the data
that was missed in order to recover from the error with regard to its functional behavior. If the
communication flow is always the same data size per (micro)frame, then the size is always a known
constant. However, in some cases, the data size can vary from (micro)frame to (micro)frame. In this case,
the receiver and transmitter have an implementation-dependent mechanism to determine the size of the lost
packet.
In summary, whether a transaction is actually moved successfully over the bus or not, the transmitter and
receiver always advance their data/buffer streams as indicated by the bus access period to keep data-pertime synchronization. The detailed mechanisms described above allow detection, tracking, and reporting of
damaged transactions so that a function or its client software can react to the damage in a functionappropriate fashion. The details of that function- or application-specific reaction are outside the scope of
the USB Specification.
5.12.8 Buffering for Rate Matching
Given that there are multiple clocks that affect isochronous communication flows in the USB, buffering is
required to rate match the communication flow across the USB. There must be buffer space available both
in the device per endpoint and on the host side on behalf of the client software. These buffers provide space
for data to accumulate until it is time for a transfer to move over the USB. Given the natural data rates of
the device, the maximum size of the data packets that move over the bus can also be calculated.
Figure 5-20 shows the equations used to determine buffer size on the device and host and maximum packet
size that must be requested to support a desired data rate. These equations are a function of the service
clock rate (FX), bus clock rate (FSOF), sample clock rate (FS), bus access period (I), and sample size (S).
These equations should provide design information for selecting the appropriate packet size that an endpoint
will report in its characteristic information and the appropriate buffer requirements for the device/endpoint
and its client software. Figure 5-17 shows actual buffer, packet, and clock values for a typical full-speed
isochronous example.
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Universal Serial Bus Specification Revision 2.0
FS Sample Clock
#Bytes/Sample:
S
#Bytes/Packet:
P = Ceil (
#Bytes/Buffer (2 Packets):
B = 2× P
FSOF Bus Clock
Fs
)× S
FSOF
I
FX Service Clock
FSOF
)
I
FX
Ceil (
#Packets/Service:
N=
Byte Buffer for 2 Services:
M = 2× N × P
Figure 5-20. Packet and Buffer Size Formulas for Rate-matched Isochronous Transfers
The USB data model assumes that devices have some natural sample size and rate. The USB supports the
transmission of packets that are multiples of sample size to make error recovery handling easier when
isochronous transactions are damaged on the bus. If a device has no natural sample size or if its samples are
larger than a packet, it should describe its sample size as being one byte. If a sample is split across a data
packet, the error recovery can be harder when an arbitrary transaction is lost. In some cases, data
synchronization can be lost unless the receiver knows in what (micro)frame number each partial sample is
transmitted. Furthermore, if the number of samples can vary due to clock correction (e.g., for a non-derived
device clock), it may be difficult or inefficient to know when a partial sample is transmitted. Therefore, the
USB does not split samples across packets.
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Universal Serial Bus Specification Revision 2.0
Chapter 6
Mechanical
This chapter provides the mechanical and electrical specifications for the cables, connectors, and cable
assemblies used to interconnect USB devices. The specification includes the dimensions, materials,
electrical, and reliability requirements. This chapter documents minimum requirements for the external
USB interconnect. Substitute material may be used as long as it meets these minimums.
6.1 Architectural Overview
The USB physical topology consists of connecting the downstream hub port to the upstream port of another
hub or to a device. The USB can operate at three speeds. High-speed (480 Mb/s) and full-speed (12 Mb/s)
require the use of a shielded cable with two power conductors and twisted pair signal conductors. Lowspeed (1.5 Mb/s) recommends, but does not require the use of a cable with twisted pair signal conductors.
The connectors are designed to be hot plugged. The USB Icon on the plugs provides tactile feedback
making it easy to obtain proper orientation.
6.2 Keyed Connector Prot ocol
To minimize end user termination problems, USB uses a “keyed connector” protocol. The physical
difference in the Series “A” and “B” connectors insures proper end user connectivity. The “A” connector
is the principle means of connecting USB devices directly to a host or to the downstream port of a hub. All
USB devices must have the standard Series “A” connector specified in this chapter. The “B” connector
allows device vendors to provide a standard detachable cable. This facilitates end user cable replacement.
Figure 6-1 illustrates the keyed connector protocol.
Series "A" Connectors
♦ Series "A" plugs are
always oriented upstream
towards the Host System
Series "B" Connectors
♦ Series "B" plugs are
always oriented
downstream towards the
USB Device
"A" Plugs
(From the
USB Device)
"B" Plugs
(From the
Host System)
"A" Receptacles
(Downstream Output
from the USB Host or
Hub)
"B" Receptacles
(Upstream Input to the
USB Device or Hub)
Figure 6-1. Keyed Connector Protocol
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Universal Serial Bus Specification Revision 2.0
The following list explains how the plugs and receptacles can be mated:
•
Series “A” receptacle mates with a Series “A” plug. Electrically, Series “A” receptacles function as
outputs from host systems and/or hubs.
•
Series “A” plug mates with a Series “A” receptacle. The Series “A” plug always is oriented towards
the host system.
•
Series “B” receptacle mates with a Series “B” plug (male). Electrically, Series “B” receptacles
function as inputs to hubs or devices.
•
Series “B” plug mates with a Series “B” receptacle. The Series “B” plug is always oriented towards
the USB hub or device.
6.3 Cable
USB cable consists of four conductors, two power conductors, and two signal conductors.
High-/full-speed cable consists of a signaling twisted pair, VBUS, GND, and an overall shield. High-/fullspeed cable must be marked to indicate suitability for USB usage (see Section 6.6.2). High-/full-speed
cable may be used with either low-speed, full-speed, or high-speed devices. When high-/full-speed cable is
used with low-speed devices, the cable must meet all low-speed requirements.
Low-speed recommends, but does not require the use of a cable with twisted signaling conductors.
6.4 Cable Assembly
This specification describes three USB cable assemblies: standard detachable cable, high-/full-speed
captive cable, and low-speed captive cable.
A standard detachable cable is a high-/full-speed cable that is terminated on one end with a Series “A” plug
and terminated on the opposite end with a series “B” plug. A high-/full-speed captive cable is terminated
on one end with a Series “A” plug and has a vendor-specific connect means (hardwired or custom
detachable) on the opposite end for the high-/full-speed peripheral. The low-speed captive cable is
terminated on one end with a Series “A” plug and has a vendor-specific connect means (hardwired or
custom detachable) on the opposite end for the low-speed peripheral. Any other cable assemblies are
prohibited.
The color used for the cable assembly is vendor specific; recommended colors are white, grey, or black.
6.4.1 Standard Detachable Cable Assemblies
High-speed and full-speed devices can utilize the “B” connector. This allows the device to have a standard
detachable USB cable. This eliminates the need to build the device with a hardwired cable and minimizes
end user problems if cable replacement is necessary.
Devices utilizing the “B” connector must be designed to work with worst case maximum length detachable
cable. Standard detachable cable assemblies may be used only on high-speed and full-speed devices.
Using a high-/full-speed standard detachable cable on a low-speed device may exceed the maximum lowspeed cable length.
Figure 6-2 illustrates a standard detachable cable assembly.
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Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
3
2
1
IMPORTANT NOTICE: All standard detachable cable assemblies must be
high-/full-speed.
H
H
G
A
B
A
B
G
Overmolded Series "A" Plug
Overmolded Series "B" Plug
(Always upstream towards the "host" system.)
(Always downstream towards the USB Device.)
C
C
F
F
C
E
Detail A - A
(Series "A" Plug)
C
Detail C - C
(Typical USB Shielded Cable)
Detail B - B
(Series "B" Plug)
E
Polyvinyl Chloride (PVC) Jacket
> 65% Tinned Copper Braided Shield
11.5
10.5
15.7
Aluminum Metallized Polyester Inner Shield
7.5
1
2
3
28 AWG STC Drain Wire
2
1
3
4
4
D
D
Green (D +)
Red (VBUS)
12.0
27.0
C
12.0
Black (Ground)
White (D -)
C
32.0
All dimensions are in millimeters (mm)
unless otherwise noted.
Dimensions are TYPICAL and are for
general reference purposes only.
9.0
B
B
9.0
Optional Molded
Strain Relief
Series "A" Plug to Series "B" Plug
USB Standard Detachable
Cable Assembly
A
SIZE
DATE
N/A
A
2/98
SCALE: N/A
8
7
6
5
4
3
C
SHEET
2
A
REV
DRAWING NUMBER
1 of 1
1
Figure 6-2. USB Standard Detachable Cable Assembly
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Universal Serial Bus Specification Revision 2.0
Standard detachable cable assemblies must meet the following electrical requirements:
•
The cable must be terminated on one end with an overmolded Series “A” plug and the opposite end is
terminated with an overmolded Series “B” plug.
•
The cable must be rated for high-speed and full-speed.
•
The cable impedance must match the impedance of the high-speed and full-speed drivers. The drivers
are characterized to drive specific cable impedance. Refer to Section 7.1.1 for details.
•
The maximum allowable cable length is determined by signal pair attenuation and propagation delay.
Refer to Sections 7.1.14 and 7.1.17 for details.
•
Differences in propagation delay between the two signal conductors must be minimized. Refer to
Section 7.1.3 for details.
•
The GND lead provides a common ground reference between the upstream and downstream ports.
The maximum cable length is limited by the voltage drop across the GND lead. Refer to Section 7.2.2
for details. The minimum acceptable wire gauge is calculated assuming the attached device is high
power.
•
The VBUS lead provides power to the connected device. For standard detachable cables, the VBUS
requirement is the same as the GND lead.
6.4.2
High-/full-speed Captive Cable Assemblies
Assemblies are considered captive if they are provided with a vendor-specific connect means (hardwired or
custom detachable) to the peripheral. High-/full-speed hardwired cable assemblies may be used with either
high-speed, full-speed, or low-speed devices. When using a high-/full-speed hardwired cable on a lowspeed device, the cable must meet all low-speed requirements.
Figure 6-3 illustrates a high-/full-speed hardwired cable assembly.
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Universal Serial Bus Specification Revision 2.0
8
7
6
H
5
4
3
2
1
A
B
A
B
H
G
G
Overmolded Series "A" Plug
(Always upstream towards the "host" system.)
Detail A - A
(Series "A" Plug)
F
F
15.7
Cut End
(Always downstream towards the USB Device.)
7.5
1
2
3
4
Detail B - B (Typical Terminations)
E
12.0
Prepared Termination
Blunt Cut Termination
Polyvinyl Chloride (PVC) Jacket
Polyvinyl Chloride (PVC) Jacket
27.0
E
> 65% Tinned Copper Braided
Shield
Metallized
Mylar Inner Shield
Blunt Cut Termination
(Length Dimension Point)
D
D
28 AWG STC Drain Wire
Red (VBUS)
Black (Ground)
Green (D +)
White (D -)
9.0
User Specified
Length Dimension Point
C
B
C
Optional Molded
Strain Relief
All dimensions are in millimeters (mm)
unless otherwise note.
B
Dimensions are TYPICAL and are for
general reference purposes only.
Series "A" Plug to Cut End
USB High-/full-speed
Hardwired Cable Assembly
A
SIZE
DATE
DRAWING NUMBER
N/A
A
2/98
SCALE: N/A
8
7
6
5
4
3
REV
C
SHEET
2
1 of 1
1
Figure 6-3. USB High-/full-speed Hardwired Cable Assembly
89
A
Universal Serial Bus Specification Revision 2.0
High-/full-speed captive cable assemblies must meet the following electrical requirements:
•
The cable must be terminated on one end with an overmolded Series “A” plug and the opposite end is
vendor specific. If the vendor specific interconnect is to be hot plugged, it must meet the same
performance requirements as the USB “B” connector.
•
The cable must be rated for high-speed and full-speed.
•
The cable impedance must match the impedance of the high-speed and full-speed drivers. The drivers
are characterized to drive specific cable impedance. Refer to Section 7.1.1 for details.
•
The maximum allowable cable length is determined by signal pair attenuation and propagation delay.
Refer to Sections 7.1.14 and 7.1.17 for details.
•
Differences in propagation delay between the two signal conductors must be minimized. Refer to
Section 7.1.3 for details.
•
The GND lead provides a common reference between the upstream and downstream ports. The
maximum cable length is determined by the voltage drop across the GND lead. Refer to Section 7.2.2
for details. The minimum wire gauge is calculated using the worst case current consumption.
•
The VBUS lead provides power to the connected device. The minimum wire gauge is vendor specific.
6.4.3 Low-speed Captive Cable Assemblies
Assemblies are considered captive if they are provided with a vendor-specific connect means (hardwired or
custom detachable) to the peripheral. Low-speed cables may only be used on low-speed devices.
Figure 6-4 illustrates a low-speed hardwired cable assembly.
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Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
3
2
1
IMPORTANT NOTICE: For use in low-speed applications only.
H
H
G
A
B
A
B
G
Overmolded Series "A" Plug
(Always upstream towards the "host" system.)
F
F
Detail A - A
(Series "A" Plug)
Cut End
(Always downstream towards the USB Device.)
15.7
7.5
1
E
2
3
4
E
Detail B - B (Typical Terminations)
12.0
Blunt Cut Termination
Prepared Termination
Polyvinyl Chloride (PVC) Jacket
Polyvinyl Chloride (PVC) Jacket
Red (VBUS)
Black (Ground)
Blunt Cut Termination
(Length Dimension Point)
D
D
27.0
Green (D +)
White (D -)
User Specified
Length Dimension Point
C
C
9.0
Optional Molded
Strain Relief
B
B
All dimensions are in millimeters (mm)
unless otherwise noted.
Dimensions are TYPICAL and are for
general reference purposes only.
Series "A" Plug to Cut End
USB Low-speed
Hardwired Cable Assembly
A
SIZE
DATE
DRAWING NUMBER
N/A
A
2/98
SCALE: N/A
8
7
6
5
4
3
C
SHEET
2
A
REV
1 of 1
1
Figure 6-4. USB Low-speed Hardwired Cable Assembly
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Universal Serial Bus Specification Revision 2.0
Low-speed captive cable assemblies must meet the following electrical requirements:
•
The cable must be terminated on one end with an overmolded Series “A” plug and the opposite end is
vendor specific. If the vendor specific interconnect is to be hot plugged, it must meet the same
performance requirements as the USB “B” connector.
•
Low-speed drivers are characterized for operation over a range of capacitive loads. This value
includes all sources of capacitance on the D+ and D-lines, not just the cable. Cable selection must
insure that total load capacitance falls between specified minimum and maximum values. If the
desired implementation does not meet the minimum requirement, additional capacitance needs to be
added to the device. Refer to Section 7.1.1.2 for details.
•
The maximum low-speed cable length is determined by the rise and fall times of low-speed signaling.
This forces low-speed cable to be significantly shorter than high-/full-speed. Refer to Section 7.1.1.2
for details.
•
Differences in propagation delay between the two signal conductors must be minimized. Refer to
Section 7.1.3 for details.
•
The GND lead provides a common reference between the upstream and downstream ports. The
maximum cable length is determined by the voltage drop across the GND lead. Refer to Section 7.2.2
for details. The minimum wire gauge is calculated using the worst case current consumption.
•
The VBUS lead provides power to the connected device. The minimum wire gauge is vendor specific.
6.4.4 Prohibited Cable Assemblies
USB is optimized for ease of use. The expectation is that if the device can be plugged in, it will work.
By specification, the only conditions that prevent a USB device from being successfully utilized are
lack of power, lack of bandwidth, and excessive topology depth. These conditions are well understood
by the system software.
Prohibited cable assemblies may work in some situations, but they cannot be guaranteed to work in all
instances.
•
Extension cable assembly
A cable assembly that provides a Series “A” plug with a series “A” receptacle or a Series “B” plug
with a Series “B” receptacle. This allows multiple cable segments to be connected together,
possibly exceeding the maximum permissible cable length.
•
Cable assembly that violates USB topology rules
A cable assembly with both ends terminated in either Series “A” plugs or Series “B” receptacles.
This allows two downstream ports to be directly connected.
Note: This prohibition does not prevent using a USB device to provide a bridge between two USB
buses.
•
92
Standard detachable cables for low-speed devices
Low-speed devices are prohibited from using standard detachable cables. A standard detachable
cable assembly must be high-/full-speed. Since a standard detachable cable assembly is high-/fullspeed rated, using a long high-/full-speed cable exceeds the capacitive load of low-speed.
Universal Serial Bus Specification Revision 2.0
6.5 Connector Mechanica l Configuration and Material Requirements
The USB Icon is used to identify USB plugs and the receptacles. Figure 6-5 illustrates the USB Icon.
All dimensions are ± 5%
L
Dia:1.67 L
0.33 L
Dia:1.33 L
L
Dia:L
1.50 L
Dia:L
0.33 L
L
0.5 L
1.50 L
Dia:L
Dia:L
L
L
Dia:1.33 L
1.67 L
0.33 L
L
2.33 L
3.75 L
5.00 L
5.17 L
6.25 L
8.00 L
Figure 6-5. USB Icon
6.5.1 USB Icon Location
The USB Icon is embossed, in a recessed area, on the topside of the USB plug. This provides easy user
recognition and facilitates alignment during the mating process. The USB Icon and Manufacturer’s logo
should not project beyond the overmold surface. The USB Icon is required, while the Manufacturer’s logo
is recommended, for both Series “A” and “B” plug assemblies. The USB Icon is also located adjacent to
each receptacle. Receptacles should be oriented to allow the Icon on the plug to be visible during the
mating process. Figure 6-6 illustrates the typical plug orientation.
Top View
Optional Top
"Locator Detail"
A
A
Locator
Height
Approximately
0.6mm
(0.024")
Manufacturer’s
Engraved Logo
Engraved USB
Icon
Locator Width
Approximately
0.5mm
(0.020")
0.6mm (0.024") Max
Manufacturer’s
Logo
Engraving Recess
Overmolding
0.6mm (0.024")
Max
USB Icon
Engraving Recess
1
2
3
4
Optional Top
"Locator Detail"
Section A - A
(Plug Cross-Section)
Figure 6-6. Typical USB Plug Orientation
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Universal Serial Bus Specification Revision 2.0
6.5.2 USB Connector Termination Data
Table 6-1 provides the standardized contact terminating assignments by number and electrical value for
Series “A” and Series “B” connectors.
Table 6-1. USB Connector Termination Assignment
Contact
Number
Signal Name
Typical Wiring
Assignment
1
VBUS
Red
2
D-
White
3
D+
Green
4
GND
Black
Shell
Shield
Drain Wire
6.5.3 Series “A” and Series “B” Receptacles
Electrical and mechanical interface configuration data for Series "A" and Series "B" receptacles are shown
in Figure 6-7 and Figure 6-8. Also, refer to Figure 6-12, Figure 6-13, and Figure 6-14 at the end of this
chapter for typical PCB receptacle layouts.
94
Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
3
2
1
USB Series "A" Receptacle Interface
12.50 ± 0.10
H
C
A
8.88 ± 0.20
8.38 ± 0.08
R 0.64 ± 0.13 (Typical)
1.84 ± 0.05
B Center Line
H
11.10 ± 0.10
R 0.32 ± 0.13 (Typical)
B
0.50 ± 0.10
G
G
2
1
300 ± 20 (2)
3
4
5.12 ± 0.10
0.38 ± 0.13
F
F
Center Line of 5.12
4.98 ± 0.25
0.64 ± 0.13 (8)
Receptacle Contact
1.00 ± 0.05 (2)
Contact Point
3.50 ± 0.05 (2)
1.00 ± 0.05 (4)
Printed Circuit Board
C Center Line
4.13 REF
E
E
All dimensions are in millimeters (mm) unless
otherwise noted.
USB Series "A" Receptacle and Plug
Mating Features
D
D
Fully Mated Series "A"
Receptacle and Plug
0.50 ± 0.10 (2)
Overmold Boot
300
C
±
20
(2)
C
8.0 MAX
B
B
Receptacle Flange
1
2.67 MIN
Interface and Mating Drawing
Allow a minimum spacing of 2.67mm between
the face of the receptacle and the plug
overmold boot.
1
A
Series "A" Receptacle
SIZE
DATE
DRAWING NUMBER
A
2/98
N/A
SHEET
SCALE: N/A
8
7
6
5
4
3
2
A
REV
C
1 of 1
1
Figure 6-7. USB Series "A" Receptacle Interface and Mating Drawing
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Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
3
2
1
USB Series "B" Receptacle Interface
A
H
5.60 + 0.10
C
8.38 + 0.08
Receptacle Contact
H
8.45 + 0.10
8.88 + 0.20
1.63 + 0.05 (2)
450 + 0.50 (2)
B
300 + 20 (4)
G
2
7.78 + 0.10
1
B Center Line
G
3.18 + 0.05
3
4
0.80 + 0.08
3.67 + 0.08
0.38 + 0.13 (4)
F
R 0.38 (6)
Contact Point
1.00 + 0.05 (4)
4.98 + 0.25
F
3.67 Center Line
1.25 + 0.10 (4)
C Center Line
Receptacle Housing
300 + 20 (2)
E
E
B Center Line
All dimensions are in millimeters (mm)
unless otherwise noted.
0.50 + 0.10 (2)
D
D
Receptacle Shell
USB Series "B" Receptacle and Plug Mating Features
Overmold Boot
C
Overmold Boot
C
10.5 MAX
11.5 MAX
B
B
2.67 MIN
1
Interface and Mating Drawing
Receptacle Shell
Fully Mated Plug and Receptacle
USB Series "B" Receptacle
A
Allow a minimum spacing of 2.67mm between the
face of the receptacle and the plug overmold boot.
1
SIZE
DATE
DRAWING NUMBER
A
2/98
N/A
SHEET
SCALE: N/A
8
7
6
5
4
3
2
Figure 6-8. USB Series "B" Receptacle Interface and Mating Drawing
96
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Universal Serial Bus Specification Revision 2.0
6.5.3.1 Receptacle Injection M olded Thermoplastic Insulator Material
Minimum UL 94-V0 rated, thirty percent (30%) glass-filled polybutylene terephthalate (PBT) or
polyethylene terephthalate (PET) or better.
Typical Colors: Black, gray, and natural.
Flammability Characteristics: UL 94-V0 rated.
Flame Retardant Package must meet or exceed the requirements for UL, CSA, VDE, etc.
Oxygen Index (LOI): Greater than 21%. ASTM D 2863.
6.5.3.2 Receptacle Shell Mate rials
Substrate Material: 0.30 + 0.05 mm phosphor bronze, nickel silver, or other copper based high strength
materials.
Plating:
1.
Underplate: Optional. Minimum 1.00 micrometers (40 microinches) nickel. In addition,
manufacturer may use a copper underplate beneath the nickel.
2.
Outside: Minimum 2.5 micrometers (100 microinches) bright tin or bright tin-lead.
6.5.3.3 Receptacle Contact M aterials
Substrate Material: 0.30 + 0.05 mm minimum half-hard phosphor bronze or other high strength copper
based material.
Plating: Contacts are to be selectively plated.
A. Option I
1.
Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.
2.
Mating Area: Minimum 0.05 micrometers (2 microinches) gold over a minimum of
0.70 micrometers (28 microinches) palladium.
3.
Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over the
underplate.
B. Option II
1.
Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.
2.
Mating Area: Minimum 0.05 micrometers (2 microinches) gold over a minimum of
0.75 micrometers (30 microinches) palladium-nickel.
3.
Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over the
underplate.
C. Option III
1.
Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.
2.
Mating Area: Minimum 0.75 micrometers (30 microinches) gold.
3.
Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over the
underplate.
97
Universal Serial Bus Specification Revision 2.0
6.5.4 Series “A” and Series “B” Plugs
Electrical and mechanical interface configuration data for Series "A" and Series "B" plugs are shown in
Figure 6-9 and Figure 6-10.
98
Universal Serial Bus Specification Revision 2.0
8
7
8.0 MAX
5
12.00 ± 0.10
B
H
6
4
3
1
11.75 MIN
R 0.64 + 0.13 Typical
H
300 ± 20 Typical
0.15 ± 0.10 Typical
0.315 ± 0.03 Typical
2
4.50 ± 0.10
300 ± 20
4
3
2
1
G
Plug Contact
8.0 MAX
A
A
UL 94-V0 Plug Housing
11.75 MIN
A
1
16.0 MAX
4.0 MAX
G
0.38 ± 0.13
1.95 ± 0.05
2.00 ± 0.13 (4)
5.16 ± 0.10
B Center Line
1.00 ± 0.05 (4)
F
2.50 ± 0.05 (2)
B Center Line
E
2.00 ± 0.05 (2)
B
Section A - A
Overmold Boot
F
E
2.50 ± 0.13 (4)
B
Overmold Boot
1
D
8.65 ± 0.19
7.41 ± 0.31
6.41 ± 0.31
4.2 MIN
GOLD PLATE AREA
Overall connector and cable assembly
length is measured from Datum ’A’ of
the Series "A" Plug to Datum ’A’ of the
Series "B" Plug or to the blunt end
termination.
D
All dimensions are in millimeters (mm)
unless otherwise noted.
C
C
1.0 ± 0.05 (2)
3.5 ± 0.05 (2)
9.70 ± 0.13
Section B - B
B
B
Interface Drawing
A
USB Series "A" Plug
0.16 ± 0.15
0.13 ± 0.13
SIZE
DATE
DRAWING NUMBER
A
2/98
N/A
SHEET
SCALE: N/A
8
7
6
5
4
3
2
A
REV
C
1 of 1
1
Figure 6-9. USB Series "A" Plug Interface Drawing
99
Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
3
2
1
8.00 ± 0.10
H
H
5.83 ± 0.10
C
10.5 MAX
0.38 MAX
G
450 ± 0.50 (2)
1
4
F
300 ± 20 Typical
B
Overmold Boot
1.46 ± 0.10
2
3.29 ± 0.05
7.26 ± 0.10
3
0.80 ± 0.05
Center Line
of 2.85
G
300 ± 20 (2)
A
F
A
2.85 ± 0.13 (2)
C Center Line
B Center Line
11.75 MIN
11.5 MAX
3.70 ± 0.13
A
1
B
E
0.25 ± 0.05
6.41 ± 0.31
D
E
Overmold Boot
Section A - A
D
4.20 MIN
Gold Plate Area
B
8.65 ± 0.19
1
C
1.16 MAX
7.41 ± 0.31
1.25 ± 0.10 (4)
Overall connector and cable assembly length
is measured from Datum 'A' of the Series "B"
Plug to Datum 'A' of the Series "A" Plug or
the blunt end termination.
C
4.67 ± 0.10
All dimensions are in millimeters (mm)
unless otherwise noted.
C Center Line
Section B - B
B
B
0.13 ± 0.13
Typical
0.16 ± 0.15
Typical
Interface Drawing
USB Series "B" Plug
A
SIZE
DATE
DRAWING NUMBER
A
2/98
N/A
SHEET
SCALE: N/A
8
7
6
5
4
3
Figure 6-10. USB Series “B” Plug Interface Drawing
100
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6.5.4.1 Plug Injection Molded Thermoplastic Insulator Material
Minimum UL 94-V0 rated, thirty percent (30%) glass-filled polybutylene terephthalate (PBT) or
polyethylene terephthalate (PET) or better.
Typical Colors: Black, gray, and natural.
Flammability Characteristics: UL 94-V0 rated.
Flame Retardant Package must meet or exceed the requirements for UL, CSA, and VDE.
Oxygen Index (LOI): 21%. ASTM D 2863.
6.5.4.2 Plug Shell Materials
Substrate Material: 0.30 + 0.05 mm phosphor bronze, nickel silver, or other suitable material.
Plating:
A. Underplate: Optional. Minimum 1.00 micrometers (40 microinches) nickel. In addition,
manufacturer may use a copper underplate beneath the nickel.
B. Outside: Minimum 2.5 micrometers (100 microinches) bright tin or bright tin-lead.
6.5.4.3 Plug (Male) Contact M aterials
Substrate Material: 0.30 + 0.05 mm half-hard phosphor bronze.
Plating: Contacts are to be selectively plated.
A. Option I
1.
Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.
2.
Mating Area: Minimum 0.05 micrometers (2 microinches) gold over a minimum of
0.70 micrometers (28 microinches) palladium.
3.
Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over the
underplate.
B. Option II
1.
Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.
2.
Mating Area: Minimum 0.05 micrometers (2 microinches) gold over a minimum of
0.75 micrometers (30 microinches) palladium-nickel.
3.
Wire Crimp/Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over
the underplate.
C. Option III
1.
Underplate: Minimum 1.25 micrometers (50 microinches) nickel. Copper over base material
is optional.
2.
Mating Area: Minimum 0.75 micrometers (30 microinches) gold.
3.
Solder Tails: Minimum 3.8 micrometers (150 microinches) bright tin-lead over the
underplate.
101
Universal Serial Bus Specification Revision 2.0
6.6 Cable Mechanical Con figuration and Material Requirements
High-/full-speed and low-speed cables differ in data conductor arrangement and shielding. Low-speed
recommends, but does not require, use of a cable with twisted data conductors. Low speed recommends,
but does not require, use of a cable with a braided outer shield. Figure 6-11 shows the typical high-/fullspeed cable construction.
Polyvinyl Chloride (PVC) Jacket
on-Twisted Power Pair:
Red:
VBUS
Black: Power Ground
Outer Shield > 65% Interwoven
Tinned Copper Braid
W
R
B
Inner Shield Aluminum
Metallized Polyester
G
28 AWG Tinned
Copper Drain Wire
Twisted Signaling Pair:
White:
DGreen: D+
Figure 6-11. Typical High-/full-speed Cable Construction
6.6.1 Description
High-/full-speed cable consists of one 28 to 20 AWG non-twisted power pair and one 28 AWG twisted data
pair with an aluminum metallized polyester inner shield, 28 AWG stranded tinned copper drain wire,
> 65% tinned copper wire interwoven (braided) outer shield, and PVC outer jacket.
Low-speed cable consists of one 28 to 20 AWG non-twisted power pair and one 28 AWG data pair (a twist
is recommended) with an aluminum metallized polyester inner shield, 28 AWG stranded tinned copper
drain wire and PVC outer jacket. A > 65% tinned copper wire interwoven (braided) outer shield is
recommended.
102
Universal Serial Bus Specification Revision 2.0
6.6.2 Construction
Raw materials used in the fabrication of this cable must be of such quality that the fabricated cable is
capable of meeting or exceeding the mechanical and electrical performance criteria of the most current
USB Specification revision and all applicable domestic and international safety/testing agency
requirements; e.g., UL, CSA, BSA, NEC, etc., for electronic signaling and power distribution cables in its
category.
Table 6-2. Power Pair
American Wire
Gauge (AWG)
Nominal Conductor
Outer Diameter
Stranded Tinned
Conductors
0.381 mm (0.015”)
7 x 36
0.406 mm (0.016”)
19 x 40
0.483 mm (0.019”)
7 x 34
0.508 mm (0.020”)
19 x 38
0.610 mm (0.024”)
7 x 32
0.610 mm (0.024”)
19 x 36
0.762 mm (0.030”)
7 x 30
0.787 mm (0.031”)
19 x 34
0.890 mm (0.035”)
7 x 28
0.931 mm (0.037”)
19 x 32
28
26
24
22
20
Note: Minimum conductor construction must be stranded tinned copper.
Non-Twisted Power Pair:
A. Wire Gauge: Minimum 28 AWG or as specified by the user contingent upon the specified cable
length. Refer to Table 6-2.
B. Wire Insulation: Semirigid polyvinyl chloride (PVC).
1.
Nominal Insulation Wall Thickness: 0.25 mm (0.010”)
2.
Typical Power (VBUS) Conductor: Red Insulation
3.
Typical Ground Conductor: Black Insulation
Signal Pair:
A. Wire Gauge: 28 AWG minimum. Refer to Table 6-3.
103
Universal Serial Bus Specification Revision 2.0
Table 6-3. Signal Pair
American Wire
Gauge (AWG)
Nominal Conductor
Outer Diameter
Stranded Tinned
Conductors
0.381 mm (0.015”)
7 x 36
0.406 mm (0.016”)
19 x 40
28
Note: Minimum conductor construction must be stranded tinned copper.
B. Wire Insulation: High-density polyethylene (HDPE), alternately foamed polyethylene or foamed
polypropylene
1.
Nominal Insulation Wall Thickness: 0.31 mm (0.012”)
2.
Typical Data Plus (+) Conductor: Green Insulation
3.
Typical Data Minus (-) Conductor: White Insulation
C. Nominal Twist Ratio (not required for low-speed): One full twist every 60 mm (2.36”) to 80 mm
(3.15”)
Aluminum Metallized Polyester Inner Shield (required for low-speed):
A. Substrate Material: Polyethylene terephthalate (PET) or equivalent material
B. Metallizing: Vacuum deposited aluminum
C. Assembly:
1.
The aluminum metallized side of the inner shield must be positioned facing out to ensure
direct contact with the drain wire.
2.
The aluminum metallized inner shield must overlap by approximately one-quarter turn.
Drain Wire (required for low-speed):
A. Wire Gauge: Minimum 28 AWG stranded tinned copper (STC) non-insulated. Refer to
Table 6-4.
Table 6-4. Drain Wire Signal Pair
American Wire
Gauge (AWG)
Nominal Conductor
Outer Diameter
Stranded Tinned
Conductors
0.381 mm (0.015”)
7 x 36
0.406 mm (0.016”)
19 x 40
28
Interwoven (Braided) Tinned Copper Wire (ITCW) Outer Shield (recommended but not required for lowspeed):
A. Coverage Area: Minimum 65%.
B. Assembly: The interwoven (braided) tinned copper wire outer shield must encase the aluminum
metallized PET shielded power and signal pairs and must be in direct contact with the drain wire.
Outer Polyvinyl Chloride (PVC) Jacket:
A. Assembly: The outer PVC jacket must encase the fully shielded power and signal pairs and must
be in direct contact with the tinned copper outer shield.
104
Universal Serial Bus Specification Revision 2.0
B. Nominal Wall Thickness: 0.64 mm (0.025”).
Marking: The cable must be legibly marked using contrasting color permanent ink.
A. Minimum marking information for high-/full-speed cable must include:
USB SHIELDED <Gauge/2C + Gauge/2C> UL CM 75 oC — UL Vendor ID.
B. Minimum marking information for low-speed cable shall include:
USB specific marking is not required for low-speed cable.
Nominal Fabricated Cable Outer Diameter:
This is a nominal value and may vary slightly from manufacturer to manufacturer as a function of the
conductor insulating materials and conductor specified. Refer to Table 6-5.
Table 6-5. Nominal Cable Diameter
Shielded USB
Nominal Outer
Cable Configuration
Cable Diameter
28/28
4.06 mm (0.160”)
28/26
4.32 mm (0.170”)
28/24
4.57 mm (0.180”)
28/22
4.83 mm (0.190”)
28/20
5.21 mm (0.205”)
6.6.3 Electrical Characteristics
All electrical characteristics must be measured at or referenced to +20 oC (68 oF).
Voltage Rating: 30 V rms maximum.
Conductor Resistance: Conductor resistance must be measured in accordance with ASTM-D-4566
Section 13. Refer to Table 6-6.
Conductor Resistance Unbalance (Pairs): Conductor resistance unbalance between two (2) conductors of
any pair must not exceed five percent (5%) when measured in accordance with ASTM-D-4566 Section 15.
The DC resistance from plug shell to plug shell (or end of integrated cable) must be less than 0.6 ohms.
Table 6-6. Conductor Resistance
American
Wire Gauge (AWG)
Ohms (Ω) / 100 Meters
Maximum
28
23.20
26
14.60
24
9.09
22
5.74
20
3.58
105
Universal Serial Bus Specification Revision 2.0
6.6.4 Cable Environmental Characteristics
Temperature Range:
A. Operating Temperature Range: 0 oC to +50 oC
B. Storage Temperature Range: -20 oC to +60 oC
C. Nominal Temperature Rating: +20 oC
Flammability: All plastic materials used in the fabrication of this product shall meet or exceed the
requirements of NEC Article 800 for communications cables Type CM (Commercial).
6.6.5 Listing
The product shall be UL listed per UL Subject 444, Class 2, Type CM for Communications Cable
Requirements.
6.7 Electrical, Mechanical , and Environmental Compliance Standards
Table 6-7 lists the minimum test criteria for all USB cable, cable assemblies, and connectors.
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards
Test Description
Test Procedure
EIA 364-18
Visual and Dimensional
Inspection
Visual, dimensional, and functional
inspection in accordance with the
USB quality inspection plans.
EIA 364-21
Insulation Resistance
106
Must meet or exceed the
requirements specified by the
most current version of Chapter 6
of the USB Specification.
1,000 MΩ minimum.
The object of this test procedure is
to detail a standard method to
assess the insulation resistance of
USB connectors. This test
procedure is used to determine the
resistance offered by the insulation
materials and the various seals of a
connector to a DC potential tending
to produce a leakage of current
through or on the surface of these
members.
EIA 364-20
Dielectric
Withstanding Voltage
Performance Requirement
The object of this test procedure is
to detail a test method to prove that
a USB connector can operate
safely at its rated voltage and
withstand momentary
over-potentials due to switching,
surges, and/or other similar
phenomena.
The dielectric must withstand
500 V AC for one minute at sea
level.
Universal Serial Bus Specification Revision 2.0
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards (Continued)
Test Description
Test Procedure
EIA 364-23
Low Level
Contact Resistance
The object of this test is to detail a
standard method to measure the
electrical resistance across a pair
of mated contacts such that the
insulating films, if present, will not
be broken or asperity melting will
not occur.
EIA 364-70 — Method B
Contact Current
Rating
The object of this test procedure is
to detail a standard method to
assess the current carrying
capacity of mated USB connector
contacts.
EIA 364-30
Contact Capacitance
The object of this test is to detail a
standard method for determining
the mechanical forces required for
inserting a USB connector.
EIA 364-13
Extraction Force
30 mΩ maximum when measured
at 20 mV maximum open circuit at
100 mA. Mated test contacts
must be in a connector housing.
1.5 A at 250 V AC minimum when
measured at an ambient
°
temperature of 25 C. With power
applied to the contacts, the ∆ T
°
must not exceed +30 C at any
point in the USB connector under
test.
2 pF maximum unmated per
contact.
The object of this test is to detail a
standard method to determine the
capacitance between conductive
elements of a USB connector.
EIA 364-13
Insertion Force
Performance Requirement
The object of this test is to detail a
standard method for determining
the mechanical forces required for
extracting a USB connector.
35 Newtons maximum at a
maximum rate of 12.5 mm
(0.492”) per minute.
10 Newtons minimum at a
maximum rate of 12.5 mm
(0.492”) per minute.
107
Universal Serial Bus Specification Revision 2.0
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards (Continued)
Test Description
Test Procedure
EIA 364-09
Durability
The object of this test procedure is
to detail a uniform test method for
determining the effects caused by
subjecting a USB connector to the
conditioning action of insertion and
extraction, simulating the expected
life of the connectors. Durability
cycling with a gauge is intended
only to produce mechanical stress.
Durability performed with mating
components is intended to produce
both mechanical and wear stress.
EIA 364-38
Test Condition A
Cable Pull-Out
Test Condition H
108
1,500 insertion/extraction cycles
at a maximum rate of 200 cycles
per hour.
After the application of a steady
state axial load of 40 Newtons for
one minute.
The object of this test procedure is
to detail a standard method for
determining the holding effect of a
USB plug cable clamp without
causing any detrimental effects
upon the cable or connector
components when the cable is
subjected to inadvertent axial
tensile loads.
EIA 364-27
Physical Shock
Performance Requirement
The object of this test procedure is
to detail a standard method to
assess the ability of a USB
connector to withstand specified
severity of mechanical shock.
No discontinuities of 1 µs or
longer duration when mated USB
connectors are subjected to 11 ms
duration 30 Gs half-sine shock
pulses. Three shocks in each
direction applied along three
mutually perpendicular planes for
a total of 18 shocks.
Universal Serial Bus Specification Revision 2.0
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards (Continued)
Test Description
Test Procedure
EIA 364-28
Test Condition V Test Letter A
Random Vibration
This test procedure is applicable to
USB connectors that may, in
service, be subjected to conditions
involving vibration. Whether a USB
connector has to function during
vibration or merely to survive
conditions of vibration should be
clearly stated by the detailed
product specification. In either
case, the relevant specification
should always prescribe the
acceptable performance
tolerances.
EIA 364-32
Test Condition I
Thermal Shock
Test Condition A Method III
The object of this test procedure is
to detail a standard test method for
the evaluation of the properties of
materials used in USB connectors
as they are influenced by the
effects of high humidity and heat.
EIA 364-52
Solderability
No discontinuities of 1 µs or
longer duration when mated USB
connectors are subjected to
5.35 Gs RMS. 15 minutes in each
of three mutually perpendicular
planes.
10 cycles –55 °C and +85 °C. The
USB connectors under test must
be mated.
The object of this test is to
determine the resistance of a USB
connector to exposure at extremes
of high and low temperatures and
to the shock of alternate exposures
to these extremes, simulating the
worst case conditions for storage,
transportation, and application.
EIA 364-31
Humidity Life
Performance Requirement
The object of this test procedure is
to detail a uniform test method for
determining USB connector
solderability. The test procedure
contained herein utilizes the solder
dip technique. It is not intended to
test or evaluate solder cup, solder
eyelet, other hand-soldered type, or
SMT type terminations.
168 hours minimum (seven
complete cycles). The USB
connectors under test must be
tested in accordance with
EIA 364-31.
USB contact solder tails must
pass 95% coverage after one
hour steam aging as specified in
Category 2.
109
Universal Serial Bus Specification Revision 2.0
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards (Continued)
Test Description
Test Procedure
UL 94 V-0
Flammability
This procedure is to ensure
thermoplastic resin compliance to
UL flammability standards.
UL 94 V-0
Flammability
This procedure is to ensure
thermoplastic resin compliance to
UL flammability standards.
The object of this test is to insure
the signal conductors have the
proper impedance.
Cable Impedance
(Only required for high-/full-speed)
110
1.
Connect the Time Domain
Reflectometer (TDR) outputs
to the impedance/delay/skew
test fixture (Note 1). Use
separate 50 Ω cables for the
plus (or true) and minus (or
complement) outputs. Set the
TDR head to differential TDR
mode.
2.
Connect the Series "A" plug of
the cable to be tested to the
text fixture, leaving the other
end open-circuited.
3.
Define a waveform composed
of the difference between the
true and complement
waveforms, to allow
measurement of differential
impedance.
4.
Measure the minimum and
maximum impedances found
between the connector and the
open circuited far end of the
cable.
Performance Requirement
The manufacturer will require its
thermoplastic resin vendor to
supply a detailed C of C with each
resin shipment. The C of C shall
clearly show the resin’s UL listing
number, lot number, date code,
etc.
The manufacturer will require its
thermoplastic resin vendor to
supply a detailed C of C with each
resin shipment. The C of C shall
clearly show the resin’s UL listing
number, lot number, date code,
etc.
Impedance must be in the range
specified in Table 7-9 (ZO).
Universal Serial Bus Specification Revision 2.0
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards (Continued)
Test Description
Test Procedure
The object of this test is to insure
that adequate signal strength is
presented to the receiver to
maintain a low error rate.
1.
Connect the Network Analyzer
output port (port 1) to the input
connector on the attenuation
test fixture (Note 2).
2.
Connect the Series “A” plug of
the cable to be tested to the
test fixture, leaving the other
end open-circuited.
3.
Calibrate the network analyzer
and fixture using the
appropriate calibration
standards over the desired
frequency range.
4.
Follow the method listed in
Hewlett Packard Application
Note 380-2 to measure the
open-ended response of the
cable.
5.
Short circuit the Series “B” end
(or bare leads end, if a captive
cable) and measure the shortcircuit response.
6.
Using the software in H-P App.
Note 380-2 or equivalent,
calculate the cable attenuation
accounting for resonance
effects in the cable as needed.
Signal Pair Attenuation
(Only required for high-/full-speed)
Performance Requirement
Refer to Section 7.1.17 for
frequency range and allowable
attenuation.
111
Universal Serial Bus Specification Revision 2.0
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards (Continued)
Test Description
Test Procedure
The purpose of the test is to verify
the end to end propagation of the
cable.
1.
2.
Connect the cable to be tested
to the test fixture. If
detachable, plug both
connectors in to the matching
fixture connectors. If captive,
plug the series “A” plug into
the matching fixture connector
and solder the stripped leads
on the other end to the test
fixture.
3.
Measure the propagation delay
of the test fixture by
connecting a short piece of
wire across the fixture from
input to output and recording
the delay.
4.
Remove the short piece of wire
and remeasure the
propagation delay. Subtract
from it the delay of the test
fixture measured in the
previous step.
Propagation Delay
112
Connect one output of the
TDR sampling head to the D+
and D- inputs of the
impedance/delay/skew test
fixture (Note 1). Use one 50 Ω
cable for each signal and set
the TDR head to differential
TDR mode.
Performance Requirement
High-/full-speed.
See Section 7.1.1.1,
Section 7.1.4, Section 7.1.16, and
Table 7-9 (TFSCBL).
Low-speed.
See Section 7.1.1.2,
Section 7.1.16, and Table 7-9
(TLSCBL).
Universal Serial Bus Specification Revision 2.0
Table 6-7. USB Electrical, Mechanical, and Environmental Compliance Standards (Continued)
Test Description
Test Procedure
This test insures that the signal on
both the D+ and D- lines arrive at
the receiver at the same time.
Propagation Delay Skew
1.
Connect the TDR to the fixture
with test sample cable, as in
the previous section.
2.
Measure the difference in
delay for the two conductors in
the test cable. Use the TDR
cursors to find the opencircuited end of each
conductor (where the
impedance goes infinite) and
subtract the time difference
between the two values.
The purpose of this test is to insure
the distributed inter-wire
capacitance is less than the
lumped capacitance specified by
the low-speed transmit driver.
1.
Connect the one lead of the
Impedance Analyzer to the D+
pin on the
impedance/delay/skew fixture
(Note 1) and the other lead to
the D- pin.
2.
Connect the series "A" plug to
the fixture, with the series “B”
end leads open-circuited.
3.
Set the Impedance Analyzer to
a frequency of 100 kHz, to
measure the capacitance.
Capacitive Load
Only required for low-speed
Performance Requirement
Propagation skew must meet the
requirements as listed in
Section 7.1.3.
See Section 7.1.1.2 and Table 7-7
(CLINUA).
Note1:
Impedance, propagation delay, and skew test fixture
This fixture will be used with the TDR for measuring the time domain performance of the cable under test. The
fixture impedance should be matched to the equipment, typically 50 Ω. Coaxial connectors should be provided
on the fixture for connection from the TDR.
Note 2:
Attenuation text fixture
This fixture provides a means of connection from the network analyzer to the Series "A" plug. Since USB
signals are differential in nature and operate over balanced cable, a transformer or balun (North Hills NH13734
or equivalent) is ideally used. The transformer converts the unbalanced (also known as single-ended) signal
from the signal generator which is typically a 50 Ω output to the balanced (also known as differential) and likely
different impedance loaded presented by the cable. A second transformer or balun should be used on the other
end of the cable under test to convert the signal back to unbalanced form of the correct impedance to match the
network analyzer.
113
Universal Serial Bus Specification Revision 2.0
6.7.1 Applicable Documents
American National Standard/Electronic Industries Association
ANSI/EIA-364-C (12/94)
Electrical Connector/Socket Test Procedures
Including Environmental Classifications
American Standard Test Materials
ASTM-D-4565
Physical and Environmental Performance Properties
of Insulation and Jacket for Telecommunication
Wire and Cable, Test Standard Method
ASTM-D-4566
Electrical Performance Properties of Insulation and
Jacket for Telecommunication Wire and Cable, Test
Standard Method
Underwriters’ Laboratory, Inc.
UL STD-94
Test for Flammability of Plastic materials for Parts
in Devices and Appliances
UL Subject-444
Communication Cables
6.8 USB Grounding
The shield must be terminated to the connector plug for completed assemblies. The shield and chassis are
bonded together. The user selected grounding scheme for USB devices, and cables must be consistent with
accepted industry practices and regulatory agency standards for safety and EMI/ESD/RFI.
6.9 PCB Reference Drawin gs
The drawings in Figure 6-12, Figure 6-13, and Figure 6-14 describe typical receptacle PCB interfaces.
These drawings are included for informational purposes only.
114
Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
3
2
1
5HIHUHQFH'UDZLQJ2QO\
H
H
16.0 REF
6.5 REF
13.1 REF
13.9 REF
2.80 + 0.10
2.0 REF
G
G
14.3 REF
10.3 REF
3.8 REF
F
F
15.0 REF
12.5 + 0.10
R 0.64 + 0.13 Typical (2)
11.1 + 0.10
E
1
2
3
6.00 + 0.10
7.6 REF
1.84 + 0.05
4
E
5.12 + 0.10
9.0 REF
10.7 REF
2.56 + 0.05
Thermoplastic Insulator UL 94-V0
D
2.50 + 0.05
1.0 + 0.05 Wide - Selectively Plated Contact (4)
2.50 + 0.05
D
2.00 + 0.05
NOTES:
Ø 0.92 + 0.10 (4)
7.00 + 0.10
C
1. Critical Dimensions are TOLERANCED
and should not be deviated.
2.00 + 0.10
C
2. Dimensions that are labeled REF are
typical dimensions and may vary from
manufacturer to manufacturer.
2.71 + 0.10
B
3. All dimensions are in millimeters (mm) unless
otherwise noted.
13.14 + 0.10
B
Ø 2.30 + 0.10 (2)
Printed Circuit Board (PCB) Layout
Single Pin-Type
Series "A" Receptacle
A
SIZE
DATE
DRAWING NUMBER
A
2/98
N/A
SCALE: N/A
8
7
6
5
4
3
SHEET
2
A
REV
C
1 of 1
1
Figure 6-12. Single Pin-type Series "A" Receptacle
115
Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
H
3
2
1
1
2
3
4
1
2
3
4
H
15.60 REF
14.70 ± 0.10
G
G
3.70 REF
13.78 + 0.10
12.30 REF
F
2.62 ± 0.05
F
11.01 ± 0.10
5.70 REF
2.00 REF
12.50 + 0.10
3.07 ± 0.10 (2)
7.00 ± 0.10
2.00 ± 0.10
2.50 ± 0.10
2.50 ± 0.10
E
E
2.62 ± 0.05
5.68 ± 0.10
D
D
Ø 0.92 ± 0.10 (8)
10.28 ± 0.20
Ø 2.3 ± 0.10 (4)
11.10 REF
10.30 REF
C
Connector Front Edge
16.95 REF
C
Printed Circuit Board (PCB) Layout
NOTES:
B
1. Critical Dimensions are TOLERANCED
and should not be deviated.
5HIHUHQFH'UDZLQJ2QO\
2. Dimensions that are labeled REF are
typical dimensions and may vary from
manufacturer to manufacturer.
A
Dual Pin-Type
Series "A" Receptacle
3. All dimensions are in millimeters (mm)
unless otherwise noted.
8
7
6
5
SIZE
DATE
4
3
REV
DRAWING NUMBER
N/A
A
2/98
SCALE: N/A
Figure 6-13. Dual Pin-type Series "A" Receptacle
116
B
SHEET
2
C
1 of 1
1
A
Universal Serial Bus Specification Revision 2.0
8
7
6
5
4
3
2
1
1.0 + 0.05 Wide - Selectively Plated Contacts (4) ....
H
H
Thermoplastic Insulator UL 94-V0 ....
2
7.78 + 0.10
1
11.50 REF
G
G
5.60 + 0.10
3
4
F
F
3.01 + 0.10
2.71 + 0.10
3.50 REF
8.45 + 0.10
2.50 + 0.10
2.00 + 0.10
4.71 + 0.10
E
E
12.04 + 0.10
2.50 + 0.10
12.00 REF
4.77 + 0.10
2.00 + 0.10
D
D
2.71 + 0.10
10.30 REF
C
C
16.00 REF
Ø 0.92 + 0.1 (4)
NOTES:
B
Ø 2.30 + 0.1 (2)
1. Critical Dimensions are TOLERANCED
and should not be deviated.
B
5HIHUHQFH'UDZLQJ2QO\
2. Dimensions that are labeled REF are
typical dimensions and may vary from
manufacturer to manufacturer.
A
Printed Circuit Board (PCB) Layout
Single Pin-Type
Series "B" Receptacle
3. All dimensions are in millimeters (mm)
unless otherwise noted.
SIZE
DATE
DRAWING NUMBER
A
2/98
N/A
SHEET
SCALE: N/A
8
7
6
5
4
3
2
A
REV
C
1 of 1
1
Figure 6-14. Single Pin-type Series "B" Receptacle
117
Universal Serial Bus Specification Revision 2.0
118
Universal Serial Bus Specification Revision 2.0
Chapter 7
Electrical
This chapter describes the electrical specification for the USB. It contains signaling, power distribution, and
physical layer specifications. This specification does not address regulatory compliance. It is the responsibility
of product designers to make sure that their designs comply with all applicable regulatory requirements.
The USB 2.0 specification requires hubs to support high-speed mode. USB 2.0 devices are not required to
support high-speed mode. A high-speed capable upstream facing transceiver must not support low-speed
signaling mode. A USB 2.0 downstream facing transceiver must support high-speed, full-speed, and low-speed
modes.
To assure reliable operation at high-speed data rates, this specification requires the use of cables that conform to
all current cable specifications.
In this chapter, there are numerous references to strings of J’s and K’s, or to strings of 1’s and 0’s. In each of
these instances, the leftmost symbol is transmitted/received first, and the rightmost is transmitted/received last.
7.1 Signaling
The signaling specification for the USB is described in the following subsections.
Overview of High-speed Signaling
A high-speed USB connection is made through a shielded, twisted pair cable that conforms to all current USB
cable specifications.
119
Universal Serial Bus Specification Revision 2.0
+3.3V
Rpu_Enable
HS_Current_Source_Enable
HS_Drive_Enable
HS_Data_Driver_Input
High Speed Current Driver
LS/FS Driver
LS/FS_Data_Driver_Input
Note: The Rpu pull-up resistor, and
the circuitry required to enable and
disable it, are only required in
upstream facing transceivers
Rs
Rpu
Data Input
Assert_Single_Ended_Zero
Assert SE0
Rs
FS_Edge_Mode_Sel
LS/FS_Driver_Output_Enable
Data+
HS_Differential_Receiver_Output
Data-
HS Differential Data Receiver
Squelch
Transmission Envelope Detector
LS/FS_Differential_Receiver_Output
LS/FS Differential Data Receiver
HS_Disconnect
Disconnection Envelope Detector
SE_Data+_Receiver_Output
SE_Data-_Receiver_Output
Single Ended Receivers
Rpd
Note: The Rpd resistors to ground
are only required in downstream
facing transceivers
Rpd
Figure 7-1. Example High-speed Capable Transceiver Circuit
Figure 7-1 depicts an example implementation which largely utilizes USB 1.1 transceiver elements and adds the
new elements required for high-speed operation.
High-speed operation supports signaling at 480 Mb/s. To achieve reliable signaling at this rate, the cable is
terminated at each end with a resistance from each wire to ground. The value of this resistance (on each wire) is
nominally set to 1/2 the specified differential impedance of the cable, or 45 Ω. This presents a differential
termination of 90 Ω.
For a link operating in high-speed mode, the high-speed idle state occurs when the transceivers at both ends of
the cable present high-speed terminations to ground, and when neither transceiver drives signaling current into
the D+ or D- lines. This state is achieved by using the low-/full-speed driver to assert a single ended zero, and to
closely control the combined total of the intrinsic driver output impedance and the RS resistance (to 45 Ω,
nominal). The recommended practice is to make the intrinsic driver impedance as low as possible, and to let RS
contribute as much of the 45 Ω as possible. This will generally lead to the best termination accuracy with the
least parasitic loading.
In order to transmit in high-speed mode, a transceiver activates an internal current source which is derived from
its positive supply voltage and directs this current into one of the two data lines via a high speed current steering
switch. In this way, the transceiver generates the high-speed J or K state on the cable.
The dynamic switching of this current into the D+ or D- line follows the same NRZI data encoding scheme used
in low-speed or full-speed operation and also in the bit stuffing behavior. To signal a J, the current is directed
into the D+ line, and to signal a K, the current is directed into the D- line. The SYNC field and the EOP
delimiters have been modified for high-speed mode.
120
Universal Serial Bus Specification Revision 2.0
The magnitude of the current source and the value of the termination resistors are controlled to specified
tolerances, and together they determine the actual voltage drive levels. The DC resistance from D+ or D- to the
device ground is required to be 45 Ω ±10% when measured without a load, and the differential output voltage
measured across the lines (in either the J or K state) must be ±400 mV ±10% when D+ and D- are terminated
with precision 45 Ω resistors to ground.
The differential voltage developed across the lines is used for three purposes:
•
A differential receiver at the receiving end of the cable receives the differential data signal.
•
A differential envelope detector at the receiving end of the cable determines when the link is in the Squelch
state. A receiver uses squelch detection as indication that the signal at its connector is not valid.
•
In the case of a downstream facing hub transceiver, a differential envelope detector monitors whether the
signal at its connector is in the high-speed state. A downstream facing transceiver operating in high-speed
mode is required to test for this state at a particular point in time when it is transmitting a SOF packet, as
described in Section 7.1.7.3. This is used to detect device disconnection. In the absence of the far end
terminations, the differential voltage will nominally double (as compared to when a high-speed device is
present) when a high-speed J or K are continuously driven for a period exceeding the round-trip delay for
the cable and board-traces between the two transceivers.
USB 2.0 requires that a downstream facing transceiver must be able to operate in low-speed, full-speed, and
high-speed signaling modes. An upstream facing high-speed capable transceiver must not operate in low-speed
signaling mode, but must be able to operate in full-speed signaling mode. Therefore, a 1.5 kΩ pull-up on the Dline is not allowed for a high-speed capable device, since a high-speed capable transceiver must never signal
low-speed operation to the hub port to which it is attached.
Table 7-1 describes the required functional elements of a high-speed capable transceiver, using the diagram
shown in Figure 7-1 as an example.
121
Universal Serial Bus Specification Revision 2.0
Table 7-1. Description of Functional Elements in the Example Shown in Figure 7-1
Element
Low-/full-speed Driver
Description
The low-/full-speed driver is used for low-speed and full-speed transmission. It
is required to meet all specifications called out in USB 1.1 for low-speed and fullspeed operation, with one exception. The exception is that in high-speed
capable transceivers, the impedance of each output, including the contribution of
RS, must be 45 Ω ±10%.
The line terminations for high-speed operation are created by having this driver
drive D+ and D- to ground. (This is equivalent to driving SE0 in the full-speed or
low-speed mode.) Because of the output impedance requirement described
above, this provides a well-controlled high-speed termination on each data line
to ground. This is equivalent to a 90 Ω differential termination.
Low-/full-speed Differential
Receiver
The low-/full-speed differential receiver is used for receiving low-speed and fullspeed data.
Single Ended Receivers
The single ended receivers are used for low-speed and full-speed signaling.
High-speed Current Driver
The high-speed current driver is used for high-speed data transmission. A
current source derived from a positive supply is switched into either the D+ or Dlines to signal a J or a K, respectively. The nominal value of the current source
is 17.78 mA. When this current is applied to a data line with a 45 Ω termination
to ground at each end, the nominal high level voltage (VHSOH) is +400 mV. The
nominal differential high-speed voltage (D+ - D-) is thus 400 mV for a J and
-400 mV for a K.
The current source must comply with the Transmit Eye Pattern Templates
specified in Section 7.1.2.2, starting with the first symbol of a packet. One
means of achieving this is to leave the current source on continuously when a
transceiver is operating in high-speed mode. If this approach is used, the
current can be directed to the port ground when the transceiver is not
transmitting (the example design in Figure 7-1 shows a control line called
HS_Current_Source_Enable to turn the current on, and another called
HS_Drive_Enable to direct the current into the data lines.) The penalty of this
approach is the 17.78 mA of standing current for every such enabled transceiver
in the system.
The preferred design is to fully turn the current source off when the transceiver
is not transmitting.
High-speed Differential Data
Receiver
122
The high-speed differential data receiver is used to receive high-speed data. It
is left to transceiver designers to choose between incorporating separate highspeed and low-/full-speed receivers, as shown in Figure 7-1, or combining both
functions into a single receiver.
Universal Serial Bus Specification Revision 2.0
Table 7-1. Description of Functional Elements in the Example Shown in Figure 7-1 (Continued)
Transmission Envelope
Detector
This envelope detector is used to indicate that data is invalid when the
amplitude of the differential signal at a receiver’s inputs falls below the squelch
threshold (VHSSQ). It must indicate Squelch when the signal drops below
100 mV differential amplitude, and it must indicate that the line is not in the
Squelch state when the signal exceeds 150 mV differential amplitude. The
response time of the detector must be fast enough to allow a receiver to detect
data transmission, to achieve DLL lock, and to detect the end of the SYNC field
within 12 bit times, the minimum number of SYNC bits that a receiver is
guaranteed to see. This envelope detector must incorporate a filtering
mechanism that prevents indication of squelch during the longest differential
data transitions allowed by the receiver eye pattern specifications.
Disconnection Envelope
Detector
This envelope detector is required in downstream facing ports to detect the highspeed Disconnect state on the line (VHSDSC). Disconnection must be indicated
when the amplitude of the differential signal at the downstream facing driver’s
connector ≥625 mV, and it must not be indicated when the signal amplitude is
≤525 mV. The output of this detector is sampled at a specific time during the
transmission of the high-speed SOF EOP, as described in Section 7.1.7.3.
Pull-up Resistor (RPU)
This resistor is required only in upstream facing transceivers and is used to
indicate signaling speed capability. A high-speed capable device is required to
initially attach as a full-speed device and must transition to high-speed as
described in this specification. Once operating in high-speed, the 1.5 kΩ
resistor must be electrically removed from the circuit. In Figure 7-1, a control
line called RPU_Enable is indicated for this purpose. The preferred embodiment
is to attach matched switching devices to both the D+ and D- lines so as to keep
the lines' parasitic loading balanced, even though a pull-up resistor must never
be used on the D- line of an upstream facing high-speed capable transceiver.
When connected, this pull-up must meet all the specifications called out for fullspeed operation.
Pull-down Resistors (RPD)
These resistors are required only in downstream facing transceivers and must
conform to the same specifications called out for low-speed and full-speed
operation.
7.1.1 USB Driver Characteristics
The USB uses a differential output driver to drive the USB data signal onto the USB cable.
For low-speed and full-speed operation, the static output swing of the driver in its low state must be below VOL
(max) of 0.3 V with a 1.5 kΩ load to 3.6 V, and in its high state must be above the VOH (min) of 2.8 V with a
15 kΩ load to ground as listed in Table 7-7. Full-speed drivers have more stringent requirements, as described
in Section 7.1.1.1. The output swings between the differential high and low state must be well-balanced to
minimize signal skew. Slew rate control on the driver is required to minimize the radiated noise and cross talk.
The driver’s outputs must support three-state operation to achieve bi-directional half-duplex operation.
Low-speed and full-speed USB drivers must never “intentionally” generate an SE1 on the bus. SE1 is a state in
which both the D+ and D- lines are at a voltage above VOSE1 (min), which is 0.8 V.
High-speed drivers use substantially different signaling levels, as described in Section 7.1.1.3.
USB ports must be capable of withstanding continuous exposure to the waveforms shown in Figure 7-2 while in
any drive state. These waveforms are applied directly into each USB data pin from a voltage source with an
123
Universal Serial Bus Specification Revision 2.0
output impedance of 39 Ω. The open-circuit voltage of the source shown in Figure 7-2 is based on the expected
worst-case overshoot and undershoot.
AC Stress Evaluation Setup
D+ or D- pin
on USB connector
nearest device
USB
Device
60nS
(min)
4.6V
4-20ns
RSRC
V
RSRC = 39Ω ±2%
-1.0V
166.7ns
(6MHz)
The signal produced by the voltage generator may be
distorted when observed at the data pin due to input
protection devices possibly incorporated in the USB
device.
Figure 7-2. Maximum Input Waveforms for USB Signaling
Short Circuit Withstand
A USB transceiver is required to withstand a continuous short circuit of D+ and/or D- to VBUS, GND, other data
line, or the cable shield at the connector, for a minimum of 24 hours without degradation. It is recommended
that transceivers be designed so as to withstand such short circuits indefinitely. The device must not be damaged
under this short circuit condition when transmitting 50% of the time and receiving 50% of the time (in all
supported speeds). The transmit phase consists of a symmetrical signal that toggles between drive high and
drive low. This requirement must be met for max value of VBUS (5.25 V).
It is recommended that these AC and short circuit stresses be used as qualification criteria against which the
long-term reliability of each device is evaluated.
7.1.1.1 Full-speed (12 Mb/s) Driver Characteristics
A full-speed USB connection is made through a shielded, twisted pair cable with a differential characteristic
impedance (Z0) of 90 Ω ±15%, a common mode impedance (ZCM) of 30 Ω ±30%, and a maximum one-way
delay (TFSCBL) of 26 ns. When the full-speed driver is not part of a high-speed capable transceiver, the
impedance of each of the drivers (ZDRV) must be between 28 Ω and 44 Ω, i.e., within the gray area in Figure 7-4.
When the full-speed driver is part of a high-speed capable transceiver, the impedance of each of the drivers
(ZHSDRV) must be between 40.5 Ω and 49.5 Ω, i.e., within the gray area in Figure 7-5.
For a CMOS implementation, the driver impedance will typically be realized by a CMOS driver with an
impedance significantly less than this resistance with a discrete series resistor making up the balance as shown in
Figure 7-3. The series resistor RS is included in the buffer impedance requirement shown in Figure 7-4 and
Figure 7-5. In the rest of the chapter, references to the buffer assume a buffer with the series impedance unless
stated otherwise.
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Universal Serial Bus Specification Revision 2.0
Buffer Output Imped. (ZBUF)
TxD+
RS
OE
Identical
CMOS
Buffers
TxD-
D+ (28Ω to 44Ω Equiv. Imped.)
D- (28Ω to 44Ω Equiv. Imped.)
RS
Figure 7-3. Example Full-speed CMOS Driver Circuit (non High-speed capable)
Full-speed Buffers in Transceivers Which are Not High-speed Capable
The buffer impedance must be measured for driving high as well as driving low. Figure 7-4 shows the
composite V/I characteristics for the full-speed drivers with included series damping resistor (RS). The
characteristics are normalized to the steady-state, unloaded output swing of the driver. The normalized driver
characteristics are found by dividing the measured voltages and currents by the actual swing of the driver under
test. The normalized V/I curve for the driver must fall entirely inside the shaded region. The V/I region is
bounded by the minimum driver impedance above and the maximum driver impedance below. The minimum
drive region is intersected by a constant current region of |6.1VOH| mA when driving low and -|6.1VOH| mA
when driving high. In the special case of a full-speed driver which is driving low, and which is part of a highspeed capable transceiver, the low drive region is intersected by a constant current region of 22.0 mA. This is
the minimum current drive level necessary to ensure that the waveform at the receiver crosses the opposite
single-ended switching level on the first reflection.
When testing, the current into or out of the device need not exceed ±10.71*VOH mA and the voltage applied to
D+/D- need not exceed 0.3*VOH for the drive low case and need not drop below 0.7*VOH for the drive high
case.
Full-speed Buffers in High-speed Capable Transceivers
Figure 7-5 shows the V/I characteristics for a Full-speed buffer which is part of a high-speed capable
transceiver. The output impedance, ZHSDRV (including the contribution of RS), is required to be between 40.5
and 49.5 . Additionally, the output voltage must be within 10mV of ground when no current is flowing in or
out of the pin (VHSTERM).
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Universal Serial Bus Specification Revision 2.0
drive low
IOUT
(mA)
Slope = 1/28Ω
Test Limit
10.71 * |VOH|
6.1 * |VOH|
Slope = 1/44Ω
2.32
0
0
0.3V
0.27*VOH
0.3*VOH
VOH
VOUT (Volts)
0
drive high
Slope = 1/44Ω
-6.1*|VOH|
Test Limit
-10.71 * |VOH|
Slope = 1/28Ω
IOUT
(mA)
0
VOUT (Volts)
0.7*VOH 0.73*VOH
Figure 7-4. Full-speed Buffer V/I Characteristics
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VOH
Universal Serial Bus Specification Revision 2.0
drive low
IOUT
(mA)
Slope = 1/40.5Ω
Test Limit
10.71 * |VOH|
22.0
Slope = 1/49.5Ω
0
0
1.09V 0.434*VOH
VOH
VOUT (Volts)
0
drive high
Slope = 1/49.5Ω
-6.1*|VOH|
Test Limit
-10.71 * |VOH|
Slope = 1/40.5Ω
IOUT
(mA)
0
VOUT (Volts)
0.566*VOH 0.698*VOH
VOH
Figure 7-5. Full-speed Buffer V/I Characteristics for High-speed Capable Transceiver
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Figure 7-6 shows the full-speed driver signal waveforms.
One Bit
Time
(12Mb/s)
Driver
Signal Pins
VSS
One-Way
Trip Cable
Delay
VIH (min)
Signal pins pass
input spec levels
after one cable
delay
Receiver
Signal Pins
VIL (max)
VSS
Figure 7-6. Full-speed Signal Waveforms
7.1.1.2 Low-speed (1.5 Mb/s) Driver Characteristics
A low-speed device must have a captive cable with the Series A connector on the plug end. The combination of
the cable and the device must have a single-ended capacitance of no less than 200 pF and no more than 450 pF
on the D+ or D- lines.
The propagation delay (TLSCBL) of a low-speed cable must be less than 18 ns. This is to ensure that the
reflection occurs during the first half of the signal rise/fall, which allows the cable to be approximated by a
lumped capacitance.
Figure 7-7 shows the low-speed driver signal waveforms.
One Bit
Time
(1.5Mb/s)
VIH (min)
Driver
Signal Pins
Signal pins
pass output
spec levels
with minimal
reflections and
ringing
VIL (max)
VSS
Figure 7-7. Low-speed Driver Signal Waveforms
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7.1.1.3 High-speed (480 Mb/s) Driver Characteristics
A high-speed USB connection is made through a shielded, twisted pair cable with a differential characteristic
impedance (Z0) of 90 Ω ±15%, a common mode impedance (ZCM) of 30 Ω ±30%, and a maximum one-way
delay of 26 ns (TFSCBL). The D+ and D- circuit board traces which run between a transceiver and its associated
connector should also have a nominal differential impedance of 90 Ω, and together they may add an additional
4 ns of delay between the transceivers. (See Section 7.1.6 for details on impedance specifications of boards and
transceivers.) The differential output impedance of a high-speed capable driver is required to be 90 Ω ±10%.
When either the D+ or D- lines are driven high, VHSOH (the high-speed mode high-level output voltage driven on
a data line with a precision 45 Ω load to GND) must be 400 mV ±10%. On a line which is not driven, either
because the transceiver is not transmitting or because the opposite line is being driven high, VHSOL (the highspeed mode low-level output voltage driven on a data line with a 45 Ω load to GND) must be 0 V ± 10 mV.
Note: Unless indicated otherwise, all voltage measurements are to be made with respect to the local circuit
ground.
Note: This specification requires that a high-speed capable transceiver operating in full-speed or low-speed
mode must have a driver impedance (ZHSDRV) of 45 Ω ±10%. It is recommended that the driver impedances be
matched to within 5 Ω within a transceiver. For upstream facing transceivers which do not support high-speed
mode, the driver output impedance (ZDRV) must fall within the range of 28 Ω to 44 Ω.
On downstream facing ports, RPD resistors (15 kΩ ±5%) must be connected from D+ and D- to ground.
When a high-speed capable transceiver transitions to high-speed mode, the high-speed idle state is achieved by
driving SE0 with the low-/full-speed drivers at each end of the link (so as to provide the required terminations),
and by disconnecting the D+ pull-up resistor in the upstream facing transceiver.
In the preferred embodiment, a transceiver activates its high-speed current driver only when transmitting highspeed signals. This is a potential design challenge, however, since the signal amplitude and timing specifications
must be met even on the first symbol within a packet. As a less efficient alternative, a transceiver may cause its
high-speed current source to be continually active while in high-speed mode. When the transceiver is not
transmitting, the current may be directed into the device ground rather than through the current steering switch
which is used for data signaling. In the example circuit, steering the current to ground is accomplished by
setting HS_Drive_Enable low.
In CMOS implementations, the driver impedance will typically be realized by the combination of the driver’s
intrinsic output impedance and RS. To optimally control ZHSDRV and to minimize parasitics, it is preferred the
driver impedance be minimized (under 5 Ω) and the balance of the 45 Ω should be contributed by the RS
component.
When a transceiver operating in high-speed mode transmits, the transmit current is directed into either the D+ or
D- data line. A J is asserted by directing the current to the D+ line, a K by directing it to the D- line.
When each of the data lines is terminated with a 45 Ω resistor to the device ground, the effective load resistance
on each side is 22.5 Ω. Therefore, the line into which the drive current is being directed rises to 17.78 ma *
22.5 Ω or 400 mV (nominal). The other line remains at the device ground voltage. When the current is directed
to the opposite line, these voltages are reversed.
7.1.2 Data Signal Rise and Fall, Eye Patterns
The following sections specify the data signal rise and fall times for full-speed and low-speed signaling, and the
rise time and eye patterns for high-speed signaling.
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7.1.2.1 Low-speed and Full-speed Data Signal Rise and Fall
For low-speed and full-speed, the output rise time and fall times are measured between 10% and 90% of the
signal (Figure 7-8). Rise and fall time requirements apply to differential transitions as well as to transitions
between differential and single-ended signaling.
The rise and fall times for full-speed buffers are measured with the load shown in Figure 7-9. The rise and fall
times must be between 4 ns and 20 ns and matched to within ±10% to minimize RFI emissions and signal skew.
The transitions must be monotonic.
The rise and fall times for low-speed buffers are measured with the load shown in Figure 7-10. The capacitive
load shown in Figure 7-10 is representative of the worst-case load allowed by the specification. A downstream
facing transceiver is allowed 150 pF of input/output capacitance (CIND). A low-speed device (including cable)
may have a capacitance of as little as 200 pF and as much as 450 pF. This gives a range of 200 pF to 600 pF as
the capacitive load that a downstream facing low-speed buffer might encounter. Upstream facing buffers on
low-speed devices must be designed to drive the capacitance of the attached cable plus an additional 150 pF. If
a low-speed buffer is designed for an application where the load capacitance is known to fall in a different range,
the test load can be adjusted to match the actual application. Low-speed buffers on hosts and hubs that are
attached to USB receptacles must be designed for the 200 pF to 600 pF range. The rise and fall time must be
between 75 ns and 300 ns for any balanced, capacitive test load. In all cases, the edges must be matched to
within ±20% to minimize RFI emissions and signal skew. The transitions must be monotonic.
For both full-speed and low-speed signaling, the crossover voltage (VCRS) must be between 1.3 V and 2.0 V.
For low-speed and full-speed, this specification does not require matching signal swing matching to any greater
degree than described above. However, when signaling, it is preferred that the average voltage on the D+ and
D- lines should be constant. This means that the amplitude of the signal swing on both D+ and D- should be the
same; the low and high going transition should begin at the same time and change at the same rate; and the
crossover voltage should be the same when switching to a J or K. Deviations from signal matching will result in
common-mode noise that will radiate and affect the ability of devices and systems to pass tests that are
mandated by government agencies.
Rise Time
Fall Time
90%
90%
VCRS
10%
Differential
Data Lines
10%
tF
tR
Figure 7-8. Data Signal Rise and Fall Time
Full-speed
Buffer
RS
TxD+
CL
RS
TxDCL
CL = 50pF
Figure 7-9. Full-speed Load
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Universal Serial Bus Specification Revision 2.0
Low-speed
Buffer
Low-speed
Buffer
RS
RS
TxD+
TxD+
RS
CL 3.6V
1.5KΩ
TxD-
15KΩ
CL
RS
TxD-
CL
15KΩ
CL
CL = 200pF to 600pF
CL = 50pF to 150pF
Low-speed downstream port load
Low-speed upstream port load
Figure 7-10. Low-speed Port Loads
Note: The CL for low-speed port load only represents the range of loading that might be added when the lowspeed device is attached to a hub. The low-speed buffer must be designed to drive the load of its attached cable
plus CL. A low-speed buffer design that can drive the downstream test load would be capable of driving any
legitimate upstream load.
7.1.2.2 High-speed Signaling Eye Patterns and Rise and Fall Time
The following specifications apply to high-speed mode signaling. All bits, including the first and last bit of a
packet, must meet the following eye pattern requirements for timing and amplitude.
TP1
TP2
T ra ce s
T ra n sc e ive r
TP3
T ra c e s
U S B C a b le
A
C o n n e c to r
TP4
B
C o n n e cto r
T ra n s ce ive r
D e v ic e C ircu it B o a rd
H u b C ircu it B o a rd
Figure 7-11. Measurement Planes
Figure 7-11 defines four test planes which will be referenced in this section. TP1 and TP4 are the points where
the transceiver IC pins are soldered to the hub and device circuit boards, respectively. TP2 is at the mated pins
of the A connector, and TP3 is at the mated pins of the B connector (or, in the case of a captive cable, where the
cable is attached to the circuit board). The following differential eye pattern templates specify transmit
waveform and receive sensitivity requirements at various points and under various conditions.
When testing high-speed transmitters and receivers, measurements are made with the Transmitter/Receiver Test
Fixture shown in Figure 7-12. In either case, the fixture is attached to the USB connector closest to the
transceiver being tested.
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Universal Serial Bus Specification Revision 2.0
Transmitter Test Attenuation: Voltage at Scope Inputs = 0.760 * Voltage at Transmitter Outputs
Receiver Test Attenuation: Voltage at Receiver Inputs = 0.684 * Voltage at Data Generator Outputs
Test Supply Voltage
15.8 Ohms
USB
Connector
Nearest
Device
Under Test
Vbus
D+
DGnd
15.8 Ohms
+
50 Ohm
Coax
50 Ohm
Coax
To 50 Ohm Inputs of a
High Speed Differential
Oscilloscope, or 50 Ohm
Outputs of a High Speed
Differential Data
Generator
143
Ohms
143
Ohms
Figure 7-12. Transmitter/Receiver Test Fixture
Note: When testing the upstream facing port of a device, VBUS must be provided from the time the device is
placed in the appropriate test mode until the test is completed. This requirement will likely necessitate
additional switching functionality in the test fixture (for example, to switch the D+ and D- lines between the host
controller and the test instrument). Such additions must have minimal impact on the high frequency
measurement results.
Transmit eye patterns specify the minimum and maximum limits, as well as limits on timing jitter, within which
a driver must drive signals at each of the specified test planes. Receive eye patterns specify the minimum and
maximum limits, as well as limits on timing jitter, within which a receiver must recover data.
Conformance to Templates 1, 2, 3, and 4 is required for USB 2.0 hubs and devices:
Template 1: Transmit waveform requirements for hub measured at TP2, and for device (without a captive
cable) measured at TP3
Template 2: Transmit waveform requirements for device (with a captive cable) measured at TP2
Template 3: Receiver sensitivity requirements for device (with a captive cable) when signal is applied at TP2
Template 4: Receiver sensitivity requirements for device (without a captive cable) when signal is applied at
TP3, and for hub when signal is applied at TP2
Templates 5 and 6 are recommended guidelines for designers:
Template 5: Transmit waveform requirements for hub transceiver measured at TP1, and for device transceiver
measured at TP4
Template 6: Receiver sensitivity requirements for device transceiver when signal is applied at TP4, and for hub
transceiver at when signal is applied at TP1
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Universal Serial Bus Specification Revision 2.0
Template 1
Figure 7-13 shows the transmit waveform requirements for a hub measured at TP2, and for a device (without a
captive cable) measured at TP3.
Level 1
Point 3
+ 400mV
Differential
Point 4
Point 1
0 Volts
Differential
Point 2
Point 5
Point 6
- 400mV
Differential
Level 2
Unit Interval
0%
100%
Voltage Level (D+ - D-)
Time (% of Unit Interval)
Level 1
525 mV in UI following a transition,
475 mV in all others
N/A
Level 2
-525 mV in UI following a transition,
-475 in all others
N/A
Point 1
0V
7.5% UI
Point 2
0V
92.5% UI
Point 3
300 mV
37.5% UI
Point 4
300 mV
62.5% UI
Point 5
-300 mV
37.5% UI
Point 6
-300 mV
62.5% UI
Figure 7-13. Template 1
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Universal Serial Bus Specification Revision 2.0
Template 2
Figure 7-14 shows transmit waveform requirements for a device (with a captive cable) measured at TP2.
Level 1
+ 400mV
Differential
Point 3
Point 1
Point 4
0 Volts
Differential
Point 2
Point 5
Point 6
- 400mV
Differential
Level 2
Unit Interval
0%
100%
Voltage Level (D+ - D-)
Time (% of Unit Interval)
Level 1
525 mV in UI following a transition,
475 mV in all others
N/A
Level 2
-525 mV in UI following a transition,
-475 in all others
N/A
Point 1
0V
12.5% UI
Point 2
0V
87.5% UI
Point 3
175 mV
35% UI
Point 4
175 mV
65% UI
Point 5
-175 mV
35% UI
Point 6
-175 mV
65% UI
Figure 7-14. Template 2
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Universal Serial Bus Specification Revision 2.0
Template 3
Figure 7-15 shows receiver sensitivity requirements for a device (with a captive cable) when a signal is applied
at TP2.
Level 1
+ 400mV
Differential
Point 3
Point 4
Point 1
0 Volts
Differential
Point 2
Point 6
Point 5
- 400mV
Differential
Level 2
0%
Unit Interval
Voltage Level (D+ - D-)
100%
Time (% of Unit Interval)
Level 1
575 mV
N/A
Level 2
-575 mV
N/A
Point 1
0V
10% UI
Point 2
0V
90% UI
Point 3
275 mV
40% UI
Point 4
275 mV
60% UI
Point 5
-275 mV
40% UI
Point 6
-275 mV
60% UI
Figure 7-15. Template 3
Note: This eye is intended to specify differential data receiver sensitivity requirements. Levels 1 and 2 are
outside the Disconnect Threshold values, but disconnection is detected at the source (after a minimum of 32 bit
times without any transitions), not at the target receiver.
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Universal Serial Bus Specification Revision 2.0
Template 4
Figure 7-16 shows receiver sensitivity requirements for a device (without a captive cable) when signal is applied
at TP3, and for a hub when a signal is applied at TP2.
Level 1
+ 400mV
Differential
Point 3
Point 4
Point 1
0 Volts
Differential
Point 2
Point 5
Point 6
- 400mV
Differential
Level 2
0%
100%
Unit Interval
Voltage Level (D+ - D-)
Time (% of Unit Interval)
Level 1
575 mV
N/A
Level 2
-575 mV
N/A
Point 1
0V
15% UI
Point 2
0V
85% UI
Point 3
150 mV
35% UI
Point 4
150 mV
65% UI
Point 5
-150 mV
35% UI
Point 6
-150 mV
65% UI
Figure 7-16. Template 4
Note: This eye is intended to specify differential data receiver sensitivity requirements. Levels 1 and 2 are
outside the Disconnect Threshold values, but disconnection is detected at the source (after a minimum of 32 bit
times without any transitions), not at the target receiver.
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Universal Serial Bus Specification Revision 2.0
Template 5
Figure 7-17 shows transmit waveform requirements for a hub transceiver measured at TP1 and for a device
transceiver measured at TP4.
Level 1
+ 400mV
Differential
Point 3
Point 4
Point 1
0 Volts
Differential
Point 2
Point 5
Point 6
- 400mV
Differential
Level 2
Unit Interval
0%
100%
Voltage Level (D+ - D-)
Time (% of Unit Interval)
Level 1
525 mV in UI following a transition,
475 mV in all others
N/A
Level 2
-525 mV in UI following a transition,
-475 in all others
N/A
Point 1
0V
5% UI
Point 2
0V
95% UI
Point 3
300 mV
35% UI
Point 4
300 mV
65% UI
Point 5
-300 mV
35% UI
Point 6
-300 mV
65% UI
Figure 7-17. Template 5
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Universal Serial Bus Specification Revision 2.0
Template 6
Figure 7-18 shows receiver sensitivity requirements for a device transceiver when a signal is applied at TP4 and
for a hub transceiver when a signal is applied at TP1.
Level 1
+ 400mV
Differential
Point 3
Point 1
Point 4
0 Volts
Differential
Point 2
Point 5
Point 6
- 400mV
Differential
Level 2
0%
Unit Interval
Voltage Level (D+ - D-)
100%
Time (% of Unit Interval)
Level 1
575 mV
N/A
Level 2
-575 mV
N/A
Point 1
0V
20% UI
Point 2
0V
80% UI
Point 3
150 mV
40% UI
Point 4
150 mV
60% UI
Point 5
-150 mV
40% UI
Point 6
-150 mV
60% UI
Figure 7-18. Template 6
Note: This eye is intended to specify differential data receiver sensitivity requirements. Levels 1 and 2 are
outside the Disconnect Threshold values, but disconnection is detected at the source (after a minimum of 32 bit
times without any transitions), not at the target receiver.
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Universal Serial Bus Specification Revision 2.0
High-speed Signaling Rise and Fall Times
The transition time of a high-speed driver must not be less than the specified minimum allowable differential
rise and fall time (THSR and THSF). Transition times are measured when driving a reference load of 45 Ω to
ground on D+ and D-. Figure 7-12 shows a recommended “Transmitter Test Fixture” for performing these
measurements.
For a hub, or for a device with detachable cable, the 10% to 90% high-speed differential rise and fall times must
be 500 ps or longer when measured at the A or B receptacles (respectively).
For a device with a captive cable assembly, it is a recommended design guideline that the 10% to 90% highspeed differential rise and fall times must be 500 ps or longer when measured at the point where the cable is
attached to the device circuit board.
It is required that high-speed data transitions be monotonic over the minimum vertical openings specified in the
preceding eye pattern templates.
7.1.2.3 Driver Usage
The upstream facing ports of functions must use one and only one of the following three driver configurations:
1.
Low-speed – Low-speed drivers only
2.
Full-speed – Full-speed drivers only
3.
Full-/high-speed – Combination full-speed and high-speed drivers
Upstream facing USB 2.0 hub ports must use full-/high-speed drivers. Such ports must be capable of
transmitting data at low-speed and full-speed rates with full-speed signaling, and at the high-speed rate using
high-speed signaling. Downstream facing ports (including the host) must support low-speed, full-speed, and
high-speed signaling, and must be able to transmit data at each of the three associated data rates.
In this section, there is reference to a situation in which high-speed operation is “disallowed.” This topic is
discussed in depth in Chapter 11 of this specification. In brief, a high-speed capable hub's downstream facing
ports are “high-speed disallowed” if the hub is unable to establish a high-speed connection on its upstream
facing port. For example, this would be the case for the downstream facing ports of a high-speed capable hub
when the hub is connected to a USB 1.1 host controller.
When a full-/high-speed device is attached to a pre-USB 2.0 hub, or to a hub port which is high-speed
disallowed, it is required to behave as a full-speed only device. When a full-/high-speed device is attached to a
USB 2.0 hub which is not high-speed disallowed, it must operate with high-speed signaling and data rate.
7.1.3 Cable Skew
The maximum skew introduced by the cable between the differential signaling pair (i.e., D+ and D- (TSKEW))
must be less than 100 ps and is measured as described in Section 6.7.
7.1.4 Receiver Characteristics
This section discusses the receiver characteristics for low-speed, full-speed, and full-/high-speed transceivers.
7.1.4.1 Low-speed and Full-speed Receiver Characteristics
A differential input receiver must be used to accept the USB data signal. The receiver must feature an input
sensitivity (VDI) of at least 200 mV when both differential data inputs are in the differential common mode range
(VCM) of 0.8 V to 2.5 V, as shown in Figure 7-19.
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Universal Serial Bus Specification Revision 2.0
In addition to the differential receiver, there must be a single-ended receiver for each of the two data lines. The
receivers must have a switching threshold between 0.8 V (VIL) and 2.0 V (VIH). It is recommended that the
single-ended receivers incorporate hysteresis to reduce their sensitivity to noise.
Both D+ and D- may temporarily be less than VIH (min) during differential signal transitions. This period can be
up to 14 ns (TFST) for full-speed transitions and up to 210 ns (TLST) for low-speed transitions. Logic in the
receiver must ensure that that this is not interpreted as an SE0.
Differential Input Voltage Range
Differential Output
Crossover
Voltage Range
-1.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
4.6
Input Voltage Range (volts)
Figure 7-19. Differential Input Sensitivity Range for Low-/full-speed
7.1.4.2 High-speed Receiver Characteristics
A high-speed capable transceiver receiver must conform to the receiver characteristics specifications called out
in Section 7.1.4.1 when receiving in low-speed or full-speed modes.
As shown in Figure 7-1, a high-speed capable transceiver which is operating in high-speed mode “listens” for an
incoming serial data stream with the high-speed differential data receiver and the transmission envelope
detector. Additionally, a downstream facing high-speed capable transceiver monitors the amplitude of the
differential voltage on the lines with the disconnection envelope detector.
When receiving in high-speed mode, the differential receiver must be able to reliably receive signals that
conform to the Receiver Eye Pattern templates shown in Section 7.1.2. Additionally, it is a strongly
recommended guideline that a high-speed receiver should be able to reliably receive such signals in the presence
of a common mode voltage component (VHSCM) over the range of –50 mV to 500 mV (the nominal common
mode component of high-speed signaling is 200 mV). Low frequency chirp J and K signaling, which occurs
during the Reset handshake, should be reliably received with a common mode voltage range of –50 mV to
600 mV.
Reception of data is qualified by the output of the transmission envelope detector. The receiver must disable data
recovery when the signal falls below the high-speed squelch level (VHSSQ) defined in Table 7-3. (Detector must
indicate squelch when the magnitude of the differential voltage envelope is ≤ 100 mV, and must not indicate
squelch if the amplitude of differential voltage envelope is ≥ 150 mV.) Squelch detection must be done with a
differential envelope detector, such as the one shown in Figure 7-1. The envelope detector used to detect the
squelch state must incorporate a filtering mechanism that prevents indication of squelch during differential data
crossovers.
The definition of a high-speed packet’s SYNC pattern, together with the requirements for high-speed hub
repeaters, guarantee that a receiver will see at least 12 bits of SYNC (KJKJKJKJKJKK) followed by the data
portion of the packet. This means that the combination of squelch response time, DLL lock time, and end of
SYNC detection must occur within 12 bit times. This is required to assure that the first bit of the packet payload
will be received correctly.
In the case of a downstream facing port, a high-speed capable transceiver must include a differential envelope
detector that indicates when the signal on the data exceeds the high-speed Disconnect level (VHSDSC) as defined
in Table 7-3. (The detector must not indicate that the disconnection threshold has been exceeded if the
differential signal amplitude is ≤525 mV, and must indicate that the threshold has been exceeded if the
differential signal amplitude is ≥625 mV.)
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Universal Serial Bus Specification Revision 2.0
When sampled at the appropriate time, this detector provides indication that the device has been disconnected.
The details of how the disconnection envelope detector is used are described in Section 7.1.7.3.
7.1.5 Device Speed Identification
The following sections specify the speed identification mechanisms for low-speed, full-speed, and high-speed.
7.1.5.1 Low-/Full-speed Device Speed Identification
The USB is terminated at the hub and function ends as shown in Figure 7-20 and Figure 7-21. Full-speed and
low-speed devices are differentiated by the position of the pull-up resistor on the downstream end of the cable:
•
Full-speed devices are terminated as shown in Figure 7-20 with the pull-up resistor on the D+ line.
•
Low-speed devices are terminated as shown in Figure 7-21 with the pull-up resistor on the D- line.
•
The pull-down terminators on downstream facing ports are resistors of 15 kΩ ±5% connected to ground.
The design of the pull-up resistor must ensure that the signal levels satisfy the requirements specified in
Table 7-2. In order to facilitate bus state evaluation that may be performed at the end of a reset, the design must
be able to pull-up D+ or D- from 0 V to VIH (min) within the minimum reset relaxation time of 2.5 µs. A device
that has a detachable cable must use a 1.5 kΩ ±5% resistor tied to a voltage source between 3.0 V and 3.6 V
(VTERM) to satisfy these requirements. Devices with captive cables may use alternative termination means.
However, the Thevenin resistance of any termination must be no less than 900 Ω.
Note: Thevenin resistance of termination does not include the 15 kΩ ±5% resistor on host/hub.
The voltage source on the pull-up resistor must be derived from or controlled by the power supplied on the USB
cable such that when VBUS is removed, the pull-up resistor does not supply current on the data line to which it is
attached.
R pu
D+
D+
Full-speed or
R pd
Low-speed USB
Transceiver
DR pd
Host or
Hub Port
Z 0=90Ω ±15%
R pd =15KΩ ±5%
R pu =1.5KΩ ±5%
Full-speed USB
Transceiver
DHub Upstream Port
or
Full-speed Function
Figure 7-20. Full-speed Device Cable and Resistor Connections
Rpu
D+ Low-speed USB
Transceiver
D+
Full-speed or
Low-speed USB
Rpd
Transceiver
DRpd
Host or
Hub Port
Rpd=15KΩ ±5%
D- Slow Slew Rate
Buffers
Rpu=1.5KΩ ±5%
Low-speed Function
Figure 7-21. Low-speed Device Cable and Resistor Connections
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7.1.5.2 High-speed Device Speed Identification
The high-speed Reset and Detection mechanisms follow the behavioral model for low-/full-speed. When reset is
complete, the link must be operating in its appropriate signaling mode (low-speed, full-speed, or high-speed as
governed by the preceding usage rules), and the speed indication bits in the port status register will correctly
report this mode. Software need only initiate the assertion of reset and read the port status register upon
notification of reset completion.
High-speed capable devices initially attach as full-speed devices. This means that for high-speed capable
upstream facing ports, RPU (1.5 kΩ ±5%) must be connected from D+ to the 3.3 V supply (as shown in
Figure 7-1) through a switch which can be opened under SW control.
After the initial attachment, high-speed capable transceivers engage in a low level protocol during reset to
establish a high-speed link and to indicate high-speed operation in the appropriate port status register. This
protocol is described in Section 7.1.7.5.
7.1.6 Input Characteristics
The following sections describe the input characteristics for transceivers operating in low-speed, full-speed, and
high-speed modes.
7.1.6.1 Low-speed and Full-speed Input Characteristics
The input impedance of D+ or D- without termination should be > 300 kΩ (ZINP). The input capacitance of a
port is measured at the connector pins. Upstream facing and downstream facing ports are allowed different
values of capacitance. The maximum capacitance (differential or single-ended) (CIND) allowed on a
downstream facing port of a hub or host is 150 pF on D+ or D- when operating in low-speed or full-speed. This
is comprised of up to 75 pF of lumped capacitance to ground on each line at the transceiver and in the connector,
and an additional 75 pF capacitance on each conductor in the transmission line between the receptacle and the
transceiver. The transmission line between the receptacle and RS must be 90 Ω ±15%.
The maximum capacitance on an upstream facing port of a full-speed device with a detachable cable (CINUB) is
100 pF on D+ or D-. This is comprised of up to 75 pF of lumped capacitance to ground on each line at the
transceiver and in the connector and an additional 25 pF capacitance on each conductor in the transmission line
between the receptacle and the transceiver. The difference in capacitance between D+ and D- must be less than
10%.
For full-speed devices with captive cables, the device itself may have up to 75 pF of lumped capacitance to
ground on D+ and D-. The cable accounts for the remainder of the input capacitance.
A low-speed device is required to have a captive cable. The input capacitance of the low-speed device will
include the cable. The maximum single-ended or differential input capacitance of a low-speed device is 450 pF
(CLINUA).
For devices with captive cables, the single-ended input capacitance must be consistent with the termination
scheme used. The termination must be able to charge the D+ or D- line from 0 V to VIH (min) within 2.5 µs.
The capacitance on D+/D- includes the single-ended input-capacitance of the device (measured from the pins on
the connector on the cable) and the 150 pF of input capacitance of the host/hub.
An implementation may use small capacitors at the transceiver for purposes of edge rate control. The sum of the
capacitance of the added capacitor (CEDGE), the transceiver, and the trace connecting capacitor and transceiver to
RS must not exceed 75 pF (either single-ended or differential) and the capacitance must be balanced to within
10%. The added capacitor, if present, must be placed between the transceiver pins and RS (see Figure 7-22).
Use of ferrite beads on the D+ or D- lines of full-speed devices is discouraged.
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RS
TxD+
CEDGE
RS
TxD-
CEDGE
Figure 7-22. Placement of Optional Edge Rate Control Capacitors for Low-/full-speed
7.1.6.2 High-speed Input Characteristics
Figure 7-23 shows the simple equivalent loading circuit of a USB device operating in high-speed receive mode.
Chip Boundary
If Terminations
Integrated On-die
Transceiver
Chip
Legacy Driver
(Output Impedance = ZDRV)
RS
Vbus
Vbus
USB Cable
Receivers,
RPU pull-up,
and HS
Driver
RS
CHSLOAD
CHSLOAD
Data+
Data-
USB
Connector
(if cable is
detachable)
Device Circuit Board
Figure 7-23. Diagram for High-speed Loading Equivalent Circuit
When operating in high-speed signaling mode, a transceiver must meet the following loading specifications:
1.
DC output voltage and resistance specifications
2.
TDR loading specification
Additionally, it is strongly recommended that a transceiver component operating in high-speed signaling mode
should meet the following lumped capacitance guideline.
The use of ferrites on high-speed data lines is strongly discouraged.
DC output voltage and resistance specifications – A transceiver that is in high-speed mode must
present a DC load on each of the data lines nominally equivalent to 45 Ω to ground. The actual resistance,
ZHSDRV, must be 40.5 Ω ≤ ZHSDRV ≤ 49.5 Ω. The output voltage in the high-speed idle state (VHSTERM) is
specified in Table 7-3
TDR loading specification – The AC loading specifications of a transceiver in the high-speed idle state are
specified in terms of differential TDR (Time Domain Reflectometer) measurements.
These measurements govern the maximum allowable transmission line discontinuities for the port connector, the
interconnect leading from the connector to the transceiver, the transceiver package, and the transceiver IC itself.
In the special case of a high-speed capable device with a captive cable, the transmission line discontinuities of
the cable assembly are also governed.
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The following specifications must be met with the incident rise time of the differential TDR set to 400 ps. It is
important to note that all times are “as displayed” on the TDR and are hence “round trip times.”
Termination Impedance (ZHSTERM) is measured on the TDR trace at a specific measurement time following the
connector reference time. The connector reference time is determined by disconnecting the TDR connection
from the port connector and noting the time of the open circuit step. For an A connector, the measurement time
is 8 ns after the connector reference location. For a B connector, the measurement time is 4 ns after the
connector reference location. The differential termination impedance must be:
80 Ω ≤ ZHSTERM ≤ 100 Ω
Through Impedance (ZHSTHRU) is the impedance measured from 500 ps before the connector reference location
until the time governed by the Termination impedance specification.
70 Ω ≤ ZHSTHRU ≤ 110 Ω
In the Exception Window (a sliding 1.4 ns window inside the Through Impedance time window), the differential
impedance may exceed the Through limits. No single excursion, however, may exceed the Through limits for
more than twice the TDR rise time (400 ps).
In the special case of a high-speed capable device with a captive cable, the same specifications must be met, but
the TDR measurements must be made through the captive cable assembly. Determination of the connector
reference time can be more difficult in this case, since the cable may not be readily removable from the port
being tested. It is left to the tester of a specific device to determine the connector reference location by whatever
means are available.
Lumped capacitance guideline for the transceiver component
When characterizing a transceiver chip as an isolated component, the measurement can be performed effectively
at the chip boundary shown in Figure 7-23 without USB connectors or cables. Parasitic capacitance of the test
fixture can be corrected by measuring the capacitance of the fixture itself and subtracting this reading from the
reading taken with the transceiver inserted. If the terminations are off-chip, discrete RS resistors should be in
place during the measurements, and measurements should be taken on the “connector side” of the resistors. The
transceiver should be in Test_SE0_NAK mode during testing.
Capacitance measurements are taken from each of the data lines to ground while the other line is left open. The
instrument used to perform this measurement must be able to determine the effective capacitance to ground in
the presence of the parallel effective resistance to ground.
Capacitance to Ground on each line: CHSLOAD ≤ 10 pF
Matching of Capacitances to Ground: ≤ 1.0 pF
The guideline is to allow no more than 5.0 pF for the transceiver die itself and no more than an additional 5 pF
for the package. The differential capacitance across the transceiver inputs should be no more than 5.0 pF
7.1.7 Signaling Levels
The following sections specify signaling levels for low-speed, full-speed, and high-speed operation.
7.1.7.1 Low-/Full-speed Signaling Levels
Table 7-2 summarizes the USB signaling levels. The source is required to drive the levels specified in the
second column, and the target is required to identify the correct bus state when it sees the levels in the third
column. (Target receivers can be more sensitive as long as they are within limits specified in the fourth
column.)
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Table 7-2. Low-/full-speed Signaling Levels
Bus State
Signaling Levels
At originating source
connector (at end of bit time)
At final target connector
Required
Acceptable
Differential “1”
D+ > VOH (min) and D- < VOL (max)
(D+) - (D-) > 200 mV
and D+ > VIH (min)
(D+) - (D-) > 200 mV
Differential “0”
D- > VOH (min) and D+ < VOL (max)
(D-) - (D+) > 200 mV
and D- > VIH (min)
(D-) - (D+) > 200 mV
Single-ended 0 (SE0)
D+ and D- < VOL (max)
D+ and D- < VIL (max)
D+ and D- < VIH (min)
Single-ended 1 (SE1)
D+ and D- > VOSE1(min)
D+ and D- > VIL (max)
Data J state:
Low-speed
Differential “0”
Differential “0”
Full-speed
Differential “1”
Differential “1”
Low-speed
Differential “1”
Differential “1”
Full-speed
Differential “0”
Differential “0”
Data K state:
Idle state:
NA
Low-speed
Full-speed
Resume state
Data K state
Start-of-Packet (SOP)
Data lines switch from Idle to K state
D- > VIHZ (min) and
D- > VIHZ (min) and
D+ < VIL (max)
D+ < VIH (min)
D+ > VIHZ (min) and
D+ > VIHZ (min) and
D- < VIL (max)
D- < VIH (min)
Data K state
SE0 for ≥ 1 bit time
followed by a J state
End-of-Packet (EOP)
SE0 for approximately 2 bit times
3
followed by a J for 1 bit time
SE0 for ≥ 1 bit time
followed by a J state
for 1 bit time
Disconnect
(at downstream port)
NA
SE0 for ≥2.5 µs
Connect
(at downstream port)
NA
Idle for ≥2 ms
Idle for ≥2.5 µs
Reset
D+ and D- < VOL (max) for ≥10ms
D+ and D- < VIL (max)
for ≥10 ms
D+ and D- < VIL (max)
for ≥2.5 µs
4
1
2
2
Note 1: The width of EOP is defined in bit times relative to the speed of transmission. (Specification EOP widths are given in
Table 7-7 and Table 7-8.)
Note 2: The width of EOP is defined in bit times relative to the device type receiving the EOP. The bit time is approximate.
Note 3: The width of the J state following the EOP is defined in bit times relative to the buffer edge rate. The J state from a
low-speed buffer must be a low-speed bit time wide and, from a full-speed buffer, a full-speed bit time wide.
Note 4: The keep-alive is a low-speed EOP.
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The J and K data states are the two logical levels used to communicate differential data in the system.
Differential signaling is measured from the point where the data line signals cross over. Differential data
signaling is not concerned with the level at which the signals cross, as long as the crossover voltage meets the
requirements in Section 7.1.2. Note that, at the receiver, the Idle and Resume states are logically equivalent to
the J and K states respectively.
As shown in Table 7-2, the J and K states for full-speed signaling are inverted from those for low-speed
signaling. The sense of data, idle, and resume signaling is set by the type of device that is being attached to a
port. If a full-speed device is attached to a port, that segment of the USB uses full-speed signaling conventions
(and fast rise and fall times), even if the data being sent across the data lines is at the low-speed data rate. The
low-speed signaling conventions shown in Table 7-2 (plus slow rise and fall times) are used only between a lowspeed device and the port to which it is attached.
3.0V≤V≤3.6V
1.5KΩ ±5%
or equivalent
+
RxD
Differential Receiver
RxD+
Single-ended Receivers
RxD-
TxD+
OE
Output Buffers
TxD-
Figure 7-24. Upstream Facing Full-speed Port Transceiver
D+
RxD
DDifferential Receiver
RxD+
Single-ended Receivers
RxD-
TxD+
Output Buffers
OE
Speed
TxD-
15KΩ ±5%
Note: Additional logic is required
to invert signal polarity on
data in/out when low-speed
devices are attached.
Figure 7-25. Downstream Facing Low-/full-speed Port Transceiver
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7.1.7.2 Full-/High-speed Signaling Levels
The high-speed signaling voltage specifications in Table 7-3 must be met when measuring at the connector
closest to the transceiver, using precision 45 Ω load resistors to the device ground as reference loads. All
voltage measurements are taken with respect to the local device ground.
Table 7-3. High-speed Signaling Levels
Bus State
High-speed Differential “1”
Required Signaling Level at
Source Connector
Required Signaling Level at
Target Connector
DC Levels:
VHSOH (min) ≤ D+ ≤ VHSOH (max)
VHSOL (min) ≤ D- ≤ VHSOL (max)
See Note 1.
High-speed Differential “0”
AC Differential Levels:
AC Differential Levels
A transmitter must conform to
the eye pattern templates called
out in Section 7.1.2.
The signal at the target connector
must be recoverable, as defined
by the eye pattern templates
called out in Section 7.1.2.
See Note 2.
See Note 2.
DC Levels:
VHSOH (min) ≤ D- ≤ VHSOH (max)
VHSOL (min) ≤ D+ ≤ VHSOL (max)
See Note 1.
AC Differential Levels:
AC Differential Levels:
A transmitter must conform to
the eye pattern templates called
out in Section 7.1.2.
The signal at the target connector
must be recoverable, as defined
by the eye pattern templates
called out in Section 7.1.2.
See Note 2.
See Note 2.
High-speed J State
High-speed Differential “1”
High-speed Differential “1”
High-speed K State
High-speed Differential “0”
High-speed Differential “0”
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Table 7-3. High-speed Signaling Levels (Continued)
Chirp J State
(differential voltage; applies only
during reset when both hub and
device are high-speed capable)
DC Levels:
AC Differential Levels
VCHIRPJ (min) ≤ (D+ - D-) ≤
VCHIRPJ (max)
The differential signal at the target
connector must be ≥ 300 mV
Chirp K State
(differential voltage; applies only
during reset when both hub and
device are high-speed capable)
DC Levels:
AC Differential Levels
VCHIRPK (min) ≤ (D+ - D-) ≤
VCHIRPK (max)
The differential signal at the target
connector must be ≤ -300 mV
High-speed Squelch State
NA
VHSSQ - Receiver must indicate
squelch when magnitude of
differential voltage is ≤100 mV;
receiver must not indicate squelch
if magnitude of differential voltage
is ≥150 mV.
See Note 3.
High-speed Idle State
DC Levels:
NA
VHSOI min ≤ (D+, D-) ≤ VHSOI max
See Note 1.
AC Differential Levels:
Magnitude of differential voltage is
≤ 100 mV
See Note 3.
Start of High-speed Packet
(HSSOP)
Data lines switch from high-speed Idle to high-speed J or high-speed
K state.
End of High-speed Packet
(HSEOP)
Data lines switch from high-speed J or K to high-speed Idle state.
High-speed Disconnect State
(at downstream facing port)
NA
VHSDSC - Downstream facing port
must not indicate device
disconnection if differential voltage
is ≤ 525 mV, and must indicate
device disconnection when
magnitude of differential voltage is
≥ 625 mV, at the sample time
discussed in Section 7.1.7.3.
Note 1: Measured with a 45 Ω resistor to ground at each data line, using test modes Test_J and Test_K
Note 2: Measured using test mode Test_Packet with fixture shown in Figure 7-12
Note 3: Measured with fixture shown in Figure 7-12, using test mode SE0_NACK
Note 4: A high-speed driver must never “intentionally” generate a signal in which both D+ and D- are driven to a level above
200 mV. The current-steering design of a high-speed driver should naturally preclude this possibility.
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Universal Serial Bus Specification Revision 2.0
7.1.7.3 Connect and Disconnect Signaling
When no function is attached to the downstream facing port of a host or hub in low-/full-speed, the pull-down
resistors present there will cause both D+ and D- to be pulled below the single-ended low threshold of the host
or hub transceiver when that port is not being driven by the hub. This creates an SE0 state on the downstream
facing port. A disconnect condition is indicated if the host or hub is not driving the data lines and an SE0
persists on a downstream facing port for more than TDDIS (see Figure 7-26). The specifications for TDDIS and
TDCNN are defined in Table 7-13.
A connect condition will be detected when the hub detects that one of the data lines is pulled above its VIH
threshold for more than TDCNN (see Figure 7-27 and Figure 7-28).
Hubs must determine the speed of the attached device by sampling the state of the bus immediately before
driving SE0 to indicate a reset condition to the device.
All signaling levels given in Table 7-2 are set for this bus segment (and this segment alone) once the speed of
the attached device is determined. The mechanics of speed detection are described in Section 11.8.2.
D+/DVIHZ (min)
VIL
D-/D+
VSS
TDDIS
Device
Disconnected
Disconnect
Detected
Figure 7-26. Low-/full-speed Disconnect Detection
D+
VIH
DVSS
TDCNN
Device
Connected
Connect
Detected
Figure 7-27. Full-/high-speed Device Connect Detection
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Universal Serial Bus Specification Revision 2.0
DVIH
D+
VSS
TDCNN
Device
Connected
Connect
Detected
Figure 7-28. Low-speed Device Connect Detection
Because USB components may be hot plugged, and hubs may implement power switching, it is necessary to
comprehend the delays between power switching and/or device attach and when the device’s internal power has
stabilized. Figure 7-29 shows all the events associated with both turning on port power with a device connected
and hot-plugging a device. There are six delays and a sequence of events that are defined by this specification.
Hub port
power OK
Reset Recovery
Time
Attach Detected
Hub port
power-on
≥4.01V
∆t4
USB System Software
reads device speed
∆t5
VBUS
VIH(min)
VIH
D+
or
D∆t1
100ms
∆t2
10ms
100ms
∆t3
∆t6
Figure 7-29. Power-on and Connection Events Timing
∆t1
This is the amount of time required for the hub port power switch to operate. This delay is a function of
the type of hub port switch. Hubs report this time in the hub descriptor (see Section 11.15.2.1), which can
be read via a request to the Hub Controller (see Section 11.16.2.4). If a device were plugged into a nonswitched or already-switched on port, ∆t1 is equal to zero.
∆t2 (TSIGATT) This is the maximum time from when VBUS is up to valid level (4.01 V) to when a device has
to signal attach. ∆t2 represents the time required for the device’s internal power rail to stabilize and for
D+ or D- to reach VIH (min) at the hub. ∆t2 must be less than 100 ms for all hub and device
implementations. (This requirement only applies if the device is drawing power from the bus.)
∆t3 (TATTDB) This is a debounce interval with a minimum duration of 100 ms that is provided by the USB
System Software. It ensures that the electrical and mechanical connection is stable before software
attempts to reset the attached device. The interval starts when the USB System Software is notified of a
connection detection. The interval restarts if there is a disconnect. The debounce interval ensures that
power is stable at the device for at least 100 ms before any requests will be sent to the device.
∆t4 (T2SUSP) Anytime a device observes no bus activity, it must obey the rules of going into suspend (see
Section 7.1.7.6).
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∆t5 (TDRST) This is the period of time hubs drive reset to a device. Refer to Section 7.1.7.5 and
Section 11.5.1.5 for details.
∆t6 (TRSTRCY) The USB System Software guarantees a minimum of 10 ms for reset recovery. Device
response to any bus transactions addressed to the default device address during the reset recovery time is
undefined.
High-speed capable devices must initially attach as full-speed devices and must comply with all full-speed
connection requirements. A high-speed capable downstream facing port must correctly detect the attachment of
low-speed and full-speed devices and must also comply with all low-speed and full-speed connection behaviors.
Transition to high-speed signaling is accomplished by means of a low level electrical protocol which occurs
during Reset. This protocol is specified in Section 7.1.7.5.
A downstream facing transceiver operating in high-speed mode detects disconnection of a high-speed device by
sensing the doubling in differential signal amplitude across the D+ and D- lines that can occur when the device
terminations are removed. The Disconnection Envelope Detector output goes high when the downstream facing
transceiver transmits and positive reflections from the open line return with a phase which is additive with the
transceiver driver signal. Signals with differential amplitudes ≥ 625 mV must reliably activate the Disconnection
Envelope Detector. Signals with differential amplitudes ≤ 525 mV must never activate the Disconnection
Envelope Detector.
To assure that this additive effect occurs and is of sufficient duration to be detected, the EOP at the end of a
high-speed SOF is lengthened to a continuous string of 40 bits without any transitions, as discussed in
Section 7.1.13.2. This length is sufficient to guarantee that the voltage at the downstream facing port’s
connector will double, since the maximum allowable round trip signal delay is 30 bit times.
When a downstream facing port is transmitting in high-speed mode and detects that it has sent 32 bits without a
transition, the disconnection envelope detector’s output must be sampled once during transmission of the next
8 bits at the transceiver output. (In the absence of bus errors, the next 8 bits will not include a transition.) If the
sample indicates that the disconnection detection threshold has been exceeded, the downstream facing port must
indicate that the high-speed device has been disconnected. See Section 11.12.4.
7.1.7.4 Data Signaling
Data transmission within a packet is done with differential signals.
7.1.7.4.1 Low-/Full-Speed Signaling
The start of a packet (SOP) is signaled by the originating port by driving the D+ and D- lines from the Idle state
to the opposite logic level (K state). This switch in levels represents the first bit of the SYNC field. Hubs must
limit the change in the width of the first bit of SOP when it is retransmitted to less than ± 5 ns. Distortion can be
minimized by matching the nominal data delay through the hub with the output enable delay of the hub.
The SE0 state is used to signal an end-of-packet (EOP). EOP will be signaled by driving D+ and D- to the SE0
state for two bit times followed by driving the lines to the J state for one bit time. The transition from the SE0 to
the J state defines the end of the packet at the receiver. The J state is asserted for one bit time and then both the
D+ and D- output drivers are placed in their high-impedance state. The bus termination resistors hold the bus in
the Idle state. Figure 7-30 shows the signaling for start and end of a packet.
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Universal Serial Bus Specification Revision 2.0
VOH(min)
VIH(min)
VIL(max)
VOL(max)
VSS Bus Idle
SOP
First Bit
of Packet
Last Bit
of Packet
Bus Driven to
J State at end
of EOP
Bus
SE0
Floats
portion
of EOP
Bus Idle
VOH(min)
VIH(min)
VIL(max)
VOL(max)
VSS
Figure 7-30. Low-/full-speed Packet Voltage Levels
7.1.7.4.2 High-speed Signaling
The high-speed Idle state is when both lines are nominally at GND.
The source of the packet signals the Start of Packet (SOP) in high-speed mode by driving the D+ and D- lines
from the high-speed Idle state to the K state. This K is the first symbol of the SYNC pattern (NRZI sequence
KJKJKJKJ KJKJKJKJ KJKJKJKJ KJKJKJKK) as described in Section 7.1.10.
The high-speed End of Packet (EOP) begins with a transition from the last symbol before the EOP to the
opposite symbol. This opposite symbol is the first symbol in the EOP pattern (NRZ 01111111 with bit stuffing
disabled) as described in Section 7.1.13.2. Upon completion of the EOP pattern, the driver ceases to inject
current into the D+ or D- lines, and the lines return to the high-speed Idle state. The high-speed SOF EOP is a
special case. This SOF EOP is 40 symbols without a transition (rather than 8 for a non-SOF packet).
The fact that the first symbol in the EOP pattern forces a transition simplifies the process of determining
precisely which is the last bit in the packet prior to the EOP delimiter.
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7.1.7.5 Reset Signaling
A hub signals reset to a downstream port by driving an extended SE0 at the port. After the reset is removed, the
device will be in the Default state (refer to Section 9.1).
The reset signaling can be generated on any Hub or Host Controller port by request from the USB System
Software. The reset signaling must be driven for a minimum of 10ms (TDRST). After the reset, the hub port will
transition to the Enabled state (refer to Section 11.5).
As an additional requirement, Host Controllers and the USB System Software must ensure that resets issued to
the root ports drive reset long enough to overwhelm any concurrent resume attempts by downstream devices. It
is required that resets from root ports have a duration of at least 50 ms (TDRSTR). It is not required that this be
50 ms of continuous Reset signaling. However, if the reset is not continuous, the interval(s) between reset
signaling must be less than 3 ms (TRHRSI), and the duration of each SE0 assertion must be at least 10 ms
(TDRST).
A device operating in low-/full-speed mode that sees an SE0 on its upstream facing port for more than 2.5 µs
(TDETRST) may treat that signal as a reset. The reset must have taken effect before the reset signaling ends.
Hubs will propagate traffic to a newly reset port after the port is in the Enabled state. The device attached to this
port must recognize this bus activity and keep from going into the Suspend state.
Hubs must be able to accept all hub requests and devices must be able to accept a SetAddress() request (refer to
Section 11.24.2 and Section 9.4 respectively) after the reset recovery time 10 ms (TRSTRCY) after the reset is
removed. Failure to accept this request may cause the device not to be recognized by the USB system software.
Hubs and devices must complete commands within the times specified in Chapter 9 and Chapter 11.
Reset must wake a device from the Suspend state.
It is required that a high-speed capable device can be reset while in the Powered, Default, Address, Configured,
or Suspended states shown in Figure 9-1. The reset signaling is compatible with low-/full-speed reset. This
means that a hub must successfully reset any device (even USB 1.X devices), and a device must be successfully
reset by any hub (even USB1.X hubs).
If, and only if, a high-speed capable device is reset by a high-speed capable hub which is not high-speed
disallowed, both hub and device must be operating in the default state in high-speed signaling mode at the end of
reset. The hub port status register must indicate that the port is in high-speed signaling mode. This requirement
is met by having such a device and such a hub engage in a low level protocol during the reset signaling time.
The protocol is defined in such a way that USB 1.X devices will not be disrupted from their normal reset
behaviors.
Note: Because the downstream facing port will not be in Transmit state during the Reset Protocol, high-speed
Chirp signaling levels will not provoke disconnect detection. (Refer to Section 7.1.7.3 and Section 11.5.1.7.)
Reset Protocol for high-speed capable hubs and devices
1.
The hub checks to make sure the attached device is not low-speed. (A low-speed device is not allowed to
support high-speed operation. If the hub determines that it is attached to a low-speed device, it does not
conduct the following high-speed detection protocol during reset.)
2.
The hub drives SE0. In this description of the Reset Protocol and High-speed Detection Handshake, the
start of SE0 is referred to as time T0.
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3.
The device detects assertion of SE0.
a)
If the device is being reset from suspend, then the device begins a high-speed detection handshake after
the detection of SE0 for no less than 2.5 µs (TFILTSE0). Since a suspended device will generally have its
clock oscillator disabled, the detection of SE0 will cause the oscillator to be restarted. The clock must
be useable (although not necessarily settled to 500 ppm accuracy) in time to detect the high-speed hub
chirp as described in Step 8.
b) If the device is being reset from a non-suspended full-speed state, then the device begins a high-speed
detection handshake after the detection of SE0 for no less than 2.5 µs and no more than 3.0 ms
(TWTRSTFS).
c) If the device is being reset from a non-suspended high-speed state, then the device must wait no less
than 3.0 ms and no more than 3.125 ms (TWTREV) before reverting to full-speed. Reversion to fullspeed is accomplished by removing the high-speed termination and reconnecting the D+ pull-up
resistor. The device samples the bus state, and checks for SE0 (reset as opposed to suspend), no less
than 100 µs and no more than 875 µs (TWTRSTHS) after starting reversion to full-speed. If SE0 (reset) is
detected, then the device begins a high-speed detection handshake.
High-speed Detection Handshake (not performed if low-speed device detected by hub):
Note: In the following handshake, both the hub and device are required to detect Chirp J’s and K’s of specified
minimum durations. It is strongly recommended that “gaps” in these Chirp signals as short as 16 high-speed bit
times should restart the duration timers.
4.
The high-speed device leaves the D+ pull-up resistor connected, leaves the high-speed terminations
disabled, and drives the high-speed signaling current into the D- line. This creates a Chirp K on the bus.
The device chirp must last no less than 1.0 ms (TUCH) and must end no more than 7.0 ms (TUCHEND) after
high-speed Reset time T0.
5.
The hub must detect the device chirp after it has seen assertion of the Chirp K for no less than 2.5 µs (TFILT).
If the hub does not detect a device chirp, it must continue the assertion of SE0 until the end of reset.
6.
No more than 100 µs (TWTDCH) after the bus leaves the Chirp K state, the hub must begin to send an
alternating sequence of Chirp K’s and Chirp J’s. There must be no Idle states on the bus between the J’s
and K’s. This sequence must continue until a time (TDCHSE0) no more than 500 µs before and no less than
100 µs before the end of Reset. (This will guarantee that the bus remains active, preventing the device from
entering the high-speed Suspend state.) Each individual Chirp K and Chirp J must last no less than 40 µs
and no more than 60 µs (TDCHBIT).
7.
After completing the hub chirp sequence, the hub asserts SE0 until end of Reset. At the end of reset, the
hub must transition to the high-speed Enabled state without causing any transitions on the data lines.
8.
After the device completes its chirp, it looks for the high-speed hub chirp. At a minimum, the device is
required to see the sequence Chirp K-J-K-J-K-J in order to detect a valid hub chirp. Each individual Chirp
K and Chirp J must be detected for no less than 2.5 µs (TFILT).
a)
If the device detects the sequence Chirp K-J-K-J-K-J, then no more than 500 µs (TWTHS) after detection,
the device is required to disconnect the D+ pull-up resistor, enable the high-speed terminations, and
enter the high-speed Default state.
b) If the device has not detected the sequence Chirp K-J-K-J-K-J by a time no less than 1.0 ms and no
more than 2.5 ms (TWTFS) after completing its own chirp, then the device is required to revert to the
full-speed Default state and wait for the end of Reset.
7.1.7.6 Suspending
All devices must support the Suspend state. Devices can go into the Suspend state from any powered state.
They begin the transition to the Suspend state after they see a constant Idle state on their upstream facing bus
lines for more than 3.0 ms. The device must actually be suspended, drawing only suspend current from the bus
after no more than 10 ms of bus inactivity on all its ports. Any bus activity on the upstream facing port will keep
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a device out of the Suspend state. In the absence of any other bus traffic, the SOF token (refer to Section 8.4.3)
will occur once per (micro)frame to keep full-/high-speed devices from suspending. In the absence of any lowspeed traffic, low-speed devices will see at least one keep-alive (defined in Table 7-2) in every frame in which
an SOF occurs, which keeps them from suspending. Hubs generate this keep-alive as described in
Section 11.8.4.1.
While in the Suspend state, a device must continue to provide power to its D+ (full-/high-speed) or D- (lowspeed) pull-up resistor to maintain an idle so that the upstream hub can maintain the correct connectivity status
for the device.
Additional Requirements for High-speed Capable Devices
From the perspective of a device operating in high-speed mode, a Reset and a Suspend are initially
indistinguishable, so the first part of the device response is the same as for a Reset. When a device operating in
high-speed mode detects that the data lines have been in the high-speed Idle state for at least 3.0 ms, it must
revert to the full-speed configuration no later than 3.125 ms (TWTREV) after the start of the idle state. Reversion
to full-speed is accomplished by disconnecting its termination resistors and reconnecting its D+ pull-up resistor.
No earlier than 100 µs and no later than 875 µs (TWTRSTHS) after reverting to full-speed, the device must sample
the state of the line. If the state is a full-speed J, the device continues with the Suspend process. (SE0 would
have indicated that the downstream facing port was driving reset, and the device would have gone into the
“High-speed Detection Handshake” as described in Section 7.1.7.5.)
A device or downstream facing port which is suspended from high-speed operation actually transitions to fullspeed signaling during the suspend process, but is required to remember that it was operating in high-speed
mode when suspended. When the resume occurs, the device or downstream facing transceiver must revert to
high-speed as discussed in Section 7.1.7.7 without the need for a reset.
7.1.7.6.1 Global Suspend
Global suspend is used when no communication is desired anywhere on the bus and the entire bus is placed in
the Suspend state. The host signals the start of global suspend by ceasing all its transmissions (including the
SOF token). As each device on the bus recognizes that the bus is in the Idle state for the appropriate length of
time, it goes into the Suspend state.
After 3.0 ms of continuous idle state, a downstream facing transceiver operating in high-speed must revert to the
full-speed idle configuration (high-speed terminations disabled), but it does not enable full-speed disconnect
detection until 1.0 ms later. This is to make sure that the device has returned to the full-speed Idle state prior to
the enabling of full-speed disconnect detection, thereby preventing an unintended disconnect detection. After reenabling the full-speed disconnect detection mechanism, the hub continues with the suspend process.
7.1.7.6.2 Selective Suspend
Segments of the bus can be selectively suspended by sending the command SetPortFeature(PORT_SUSPEND)
to the hub port to which that segment is attached. The suspended port will block activity to the suspended bus
segment, and devices on that segment will go into the Suspend state after the appropriate delay as described
above.
When a downstream facing port operating in high-speed mode receives the SetPortFeature(PORT_SUSPEND)
command, the port immediately reverts to the full-speed Idle state and blocks any activity to the suspend
segment. Full-speed disconnect detection is disabled until the port has been in full-speed idle for 4.0 ms. This
prevents an unintended disconnect detection. After re-enabling the full-speed disconnect detection mechanism,
the hub continues with the suspend process.
Section 11.5 describes the port Suspend state and its interaction with the port state machine. Suspend is further
described in Section 11.9.
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7.1.7.7 Resume
If a device is in the Suspend state, its operation is resumed when any non-idle signaling is received on its
upstream facing port. Additionally, the device can signal the system to resume operation if its remote wakeup
capability has been enabled by the USB System Software. Resume signaling is used by the host or a device to
bring a suspended bus segment back to the active condition. Hubs play an important role in the propagation and
generation of resume signaling. The following description is an outline of a general global resume sequence. A
complete description of the resume sequence, the special cases caused by selective suspend, and the role of the
hub are given in Section 11.9.
The host may signal resume (TDRSMDN) at any time. It must send the resume signaling for at least 20 ms and
then end the resume signaling in one of two ways, depending on the speed at which its port was operating when
it was suspended. If the port was in low-/full-speed when suspended, the resume signaling must be ended with a
standard, low-speed EOP (two low-speed bit times of SE0 followed by a J). If the port was operating in highspeed when it was suspended, the resume signaling must be ended with a transition to the high-speed idle state.
The 20 ms of resume signaling ensures that all devices in the network that are enabled to see the resume are
awakened. The connectivity established by the resume signaling is torn down by the End of Resume, which
prepares the hubs for normal operation. After resuming the bus, the host must begin sending bus traffic (at least
the SOF token) within 3 ms of the start of the idle state to keep the system from going back into the Suspend
state.
A device with remote wakeup capability may not generate resume signaling unless the bus has been
continuously in the Idle state for 5 ms (TWTRSM). This allows the hubs to get into their Suspend state and
prepare for propagating resume signaling. The remote wakeup device must hold the resume signaling for at
least 1 ms but for no more than 15 ms (TDRSMUP). At the end of this period, the device stops driving the bus
(puts its drivers into the high-impedance state and does not drive the bus to the J state).
If the hub upstream of a remote wakeup device is suspended, it will propagate the resume signaling to its
upstream facing port and to all of its enabled downstream facing ports, including the port that originally signaled
the resume. When a hub is propagating resume signaling from a downstream device, it may transition from the
idle state to K with a risetime faster than is normally allowed. The hub must begin this rebroadcast (TURSM) of
the resume signaling within 1 ms of receiving the original resume. The resume signal will propagate in this
manner upstream until it reaches the host or a non-suspended hub (refer to Section 11.9), which will reflect the
resume downstream and take control of resume timing. This hub is termed the controlling hub. Intermediate
hubs (hubs between the resume initiator and the controlling hub) drive resume (TDRSMUP) on their upstream
facing port for at least 1 ms during which time they also continue to drive resume on enabled downstream facing
ports. An intermediate hub will stop driving resume on the upstream facing port and reverse the direction of
connectivity from upstream to downstream within 15 ms after first asserting resume on its upstream facing port.
When all intermediate hubs have reversed connectivity, resume is being driven from the controlling hub through
all intermediate hubs and to all enabled ports. The controlling hub must rebroadcast the resume signaling within
1 ms (TURSM) and ensures that resume is signaled for at least 20 ms (TDRSMDN). The hub may then begin
normal operation by terminating the resume process as described above.
The USB System Software must provide a 10 ms resume recovery time (TRSMRCY) during which it will not
attempt to access any device connected to the affected (just-activated) bus segment.
Port connects and disconnects can also cause a hub to send a resume signal and awaken the system. These
events will cause a hub to send a resume signal only if the hub has been enabled as a remote-wakeup source.
Refer to Section 11.4.4 for more details.
Refer to Section 7.2.3 for a description of power control during suspend and resume.
If the hub port and device were operating in high-speed prior to suspend, they are required to "remember" that
they were previously operating in high-speed, and they must transition back to high-speed operation, without
arbitration, within two low-speed bit times of the K to SE0 transition. The inactivity timers must be started two
low-speed bit times after the K to SE0 transition. Note that the transition from SE0 to J which would normally
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occur at the end of full-speed resume signaling is omitted if the link was operating in high-speed at the time
when it was suspended.
It is required that the host begin sending SOF’s in time to prevent the high-speed tree from suspending.
7.1.8 Data Encoding/Decoding
The USB employs NRZI data encoding when transmitting packets. In NRZI encoding, a “1” is represented by
no change in level and a “0” is represented by a change in level. Figure 7-31 shows a data stream and the NRZI
equivalent. The high level represents the J state on the data lines in this and subsequent figures showing NRZI
encoding. A string of zeros causes the NRZI data to toggle each bit time. A string of ones causes long periods
with no transitions in the data.
'DWD
15=,
,GOH
J
K
,GOH
Figure 7-31. NRZI Data Encoding
7.1.9 Bit Stuffing
In order to ensure adequate signal transitions, bit stuffing is employed by the transmitting device when sending a
packet on USB (see Figure 7-32 and Figure 7-34). A zero is inserted after every six consecutive ones in the data
stream before the data is NRZI encoded, to force a transition in the NRZI data stream. This gives the receiver
logic a data transition at least once every seven bit times to guarantee the data and clock lock. Bit stuffing is
enabled beginning with the Sync Pattern. The data “one” that ends the Sync Pattern is counted as the first one in
a sequence. Bit stuffing by the transmitter is always enforced, except during high-speed EOP. If required by the
bit stuffing rules, a zero bit will be inserted even if it is the last bit before the end-of-packet (EOP) signal.
The receiver must decode the NRZI data, recognize the stuffed bits, and discard them.
7.1.9.1 Full-/low-speed
Full-/low-speed signaling uses bit stuffing throughout the packet without exception. If the receiver sees seven
consecutive ones anywhere in the packet, then a bit stuffing error has occurred and the packet should be ignored.
The time interval just before an EOP is a special case. The last data bit before the EOP can become stretched by
hub switching skews. This is known as dribble and can lead to the case illustrated in Figure 7-33, which shows
where dribble introduces a sixth bit that does not require a bit stuff. Therefore, the receiver must accept a packet
for which there are up to six full bit times at the port with no transitions prior to the EOP.
Data Encoding Sequence:
Raw Data
Sync Pattern
Packet Data
Stuffed Bit
Bit Stuffed Data
Sync Pattern
Packet Data
Six Ones
NRZI
Idle
Encoded Data
Sync Pattern
Packet Data
Figure 7-32. Bit Stuffing
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Universal Serial Bus Specification Revision 2.0
Transmitted
Data
0
0
1
1
1
1
1
EOP
EOP
J
CRC
Data from
Transmitter
Acceptable
Extra Bit,
No Error
EOP
CRC
Data at
Receiver
Extra
bit
EOP
Figure 7-33. Illustration of Extra Bit Preceding EOP (Full-/low-speed)
Power Up
No Packet
Transmission
Idle
Begin Packet Transmission
Reset Bit
Counter to 0
Get Next
Bit
=0
Bit
Value?
=1
Increment
the Counter
No
Counter
= 6?
Yes
Insert a
Zero Bit
Reset Bit
Counter to 0
No
Is Packet
Transfer
Done?
Yes
Figure 7-34. Flow Diagram for Bit Stuffing
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7.1.9.2 High-Speed
High-speed signaling uses bit stuffing throughout the packet, with the exception of the intentional bit stuff errors
used in the high-speed EOP as described in Section 7.1.13.2.
7.1.10 Sync Pattern
The SYNC pattern used for low-/full-speed transmission is required to be 3 KJ pairs followed by 2 K’s for a total
of eight symbols. Figure 7-35 shows the NRZI bit pattern, which is prefixed to each low-/full-speed packet.
6<1&3$77(51
15=,'DWD
(QFRGLQJ
,GOH
3,'
3,'
Figure 7-35. Sync Pattern (Low-/full-speed)
The SYNC pattern used for high-speed transmission is required to be 15 KJ pairs followed by 2 K’s, for a total
of 32 symbols. Hubs are allowed to drop up to 4 bits from the start of the SYNC pattern when repeating
packets. Hubs must not corrupt any repeated bits of the SYNC field, however. Thus, after being repeated by
5 hubs, a packet’s SYNC field may be as short as 12 bits.
7.1.11 Data Signaling Rate
The high-speed data rate (THSDRAT) is nominally 480.00 Mb/s, with a required bit rate accuracy of ±500 ppm.
For hosts, hubs, and high-speed capable functions, the required data-rate accuracy when transmitting at any
speed is ±0.05% (500 ppm). The full-speed rate for such hubs and functions is TFDRATHS. The low-speed rate for
such hubs is TLDRATHS (a low-speed function must not support high-speed).
The full-speed data rate is nominally 12.000 Mb/s. For full-speed only functions, the required data-rate when
transmitting (TFDRATE) is 12.000 Mb/s ±0.25% (2,500 ppm).
The low-speed data rate is nominally 1.50 Mb/s. For low-speed functions, the required data-rate when
transmitting (TLDRATE) is 1.50 Mb/s ±1.5% (15,000 ppm). This allows the use of resonators in low cost, lowspeed devices.
Hosts and hubs must be able to receive data from any compliant low-speed, full-speed, or high-speed source.
High-speed capable functions must be able to receive data from any compliant full-speed or high-speed source.
Full-speed only functions must be able to receive data from any compliant full-speed source. Low-speed only
functions must be able to receive data from any compliant low-speed source.
The above accuracy numbers include contributions from all sources:
•
Initial frequency accuracy
•
Crystal capacitive loading
•
Supply voltage on the oscillator
•
Temperature
•
Aging
7.1.12 Frame Interval
The USB defines a frame interval (TFRAME) to be 1.000 ms ±500 ns long. The USB defines a microframe
interval (THSFRAM) to be 125.0 µs ±62.5 ns long. The (micro)frame interval is measured from any point in an
SOF token in one (micro)frame to the same point in the SOF token of the next (micro)frame.
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Since the Host Controller and hubs must meet clock accuracy specification of ±0.05%, they will automatically
meet the frame interval requirements without the need for adjustment.
The frame interval repeatability, TRFI (difference in frame interval between two successive frames), must be less
than 0.5 full-speed bit times. The microframe interval repeatability, THSRFI (difference in the microframe
interval between two successive microframes, measured at the host), must be less than 4 high-speed bit times.
Each hub may introduce at most 4 additional high-speed bits of microframe jitter.
Hubs and certain full-/high-speed functions need to track the (micro)frame interval. They also are required to
have sufficient frame timing adjustment to compensate for their own frequency inaccuracy.
7.1.13 Data Source Signaling
This section covers the timing characteristics of data produced and sent from a port (the data source).
Section 7.1.14 covers the timing characteristics of data that is transmitted through the Hub Repeater section of a
hub. In this section, TPERIOD is defined as the actual period of the data rate that can have a range as defined in
Section 7.1.11.
7.1.13.1 Data Source Jitter
This section describes the maximum allowable data source jitter for low-speed, full-speed, and high-speed
signaling.
7.1.13.1.1 Low-/full-speed Data Source Jitter
The source of data can have some variation (jitter) in the timing of edges of the data transmitted. The time
between any set of data transitions is (N * TPERIOD) ± jitter time, where ‘N’ is the number of bits between the
transitions. The data jitter is measured with the same load used for maximum rise and fall times and is measured
at the crossover points of the data lines, as shown in Figure 7-36.
Crossover
Points
Differential
Data Lines
Jitter
Consecutive
Transitions
Integer multiples of TPERIOD
Paired
Transitions
Figure 7-36. Data Jitter Taxonomy
•
For full-speed transmissions, the jitter time for any consecutive differential data transitions must be within
±2.0 ns and within ±1.0 ns for any set of paired (JK-to-next JK transition or KJ-to-next KJ transition)
differential data transitions.
•
For low-speed transmissions, the jitter time for any consecutive differential data transitions must be within
±25 ns and within ±10 ns for any set of paired differential data transitions.
These jitter numbers include timing variations due to differential buffer delay and rise and fall time mismatches,
internal clock source jitter, and noise and other random effects.
7.1.13.1.2 High-speed Data Source Jitter
High-speed data within a single packet must be transmitted with no more jitter than is allowed by the eye
patterns defined in Section 7.1.2 when measured over a sliding window of 480 high-speed bit times.
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7.1.13.2 EOP Width
This section describes low-speed, full-speed, and high-speed EOP width.
7.1.13.2.1 Low-/full-speed EOP
The width of the SE0 in the EOP is approximately 2 * TPERIOD. The SE0 width is measured with the same load
used for maximum rise and fall times and is measured at the same level as the differential signal crossover
points of the data lines (see Figure 7-37).
TPERIOD
Differential
Data Lines
Data
Crossover
Level
SE0 for
EOP
Width
Figure 7-37. SE0 for EOP Width Timing
•
For full-speed transmissions, the SE0 for EOP width from the transmitter must be between 160 ns and
175 ns.
•
For low-speed transmissions, the transmitter’s SE0 for EOP width must be between 1.25 µs and 1.50 µs.
These ranges include timing variations due to differential buffer delay and rise and fall time mismatches and to
noise and other random effects.
A receiver must accept any valid EOP. Receiver design should note that the single-ended input threshold
voltage can be different from the differential crossover voltage and the SE0 transitions will in general be
asynchronous to the clock encoded in the NRZI stream.
•
A full-speed EOP may have the SE0 interval reduced to as little as 82 ns (TFEOPR) and a low-speed SE0
interval may be as short as 670 ns (TLEOPR).
A hub may tear down connectivity if it sees an SE0 of at least TFST or TLST followed by a transition to the J state.
A hub must tear down connectivity on any valid EOP.
7.1.13.2.2 High-speed EOP
In high-speed signaling, a bit stuff error is intentionally generated to indicate EOP. A receiver is required to
interpret any bit stuff error as an EOP.
For high-speed packets other than SOF's, the transmitted EOP delimiter is required to be an NRZ byte of
01111111 without bit stuffing. For example, if the last symbol prior to the EOP field is a J, this would lead to an
EOP of KKKKKKKK.
For high-speed SOF's, the transmitted EOP delimiter is required to be 5 NRZ bytes without bit stuffing,
consisting of 01111111 11111111 11111111 11111111 11111111. Thus if the last bit prior to the EOP field is a
J, this would lead to 40 K's on the wire, at the end of which the lines must return to the high-speed Idle state.
This extra EOP length is of no significance to a receiver; it is used for disconnect detection as discussed in
Section 7.1.7.3.
A hub may add at most 4 random bits to the end of the EOP field when repeating a packet. Thus after
5 repeaters, a packet can have up to 20 random bits following the EOP field. A hub, however, must not corrupt
any of the 8 (or 40 in the case of a SOF) required bits of the EOP field.
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7.1.14 Hub Signaling Timings
This section describes low-speed, full-speed, and high-speed hub signaling timings.
7.1.14.1 Low-/full-speed Hub Signaling Timings
The propagation of a full-speed, differential data signal through a hub is shown in Figure 7-38. The downstream
signaling is measured without a cable connected to the port and with the load used for measuring rise and fall
times. The total delay through the upstream cable and hub electronics must be a maximum of 70 ns (THDD1). If
the hub has a detachable USB cable, then the delay (THDD2) through hub electronics and the associated
transmission line must be a maximum of 44 ns to allow for a maximum cable delay of 26 ns (TFSCBL). The
delay through this hub is measured in both the upstream and downstream directions, as shown in Figure 7-38B,
from data line crossover at the input port to data line crossover at the output port.
Upstream End
of Cable
Data Line
Crossover
Point
Downstream
Port
50% Point of
Initial Swing
VSS
50% Point of
Initial Swing
Hub Delay
Downstream
70ns (max)
Downstream
Port
Hub Delay
Upstream
70ns (max)
Upstream End
of Cable
Data Line
Crossover
Point
VSS
A. Downstream Hub Delay
upstream end of cable
B. Upstream Hub Delay
upstream port
Host or
Hub
downstream port
Hub
plug
receptacle
Function
downstream signaling
upstream signaling
C. Measurement Points
Figure 7-38. Hub Propagation Delay of Full-speed Differential Signals
Low-speed propagation delay for differential signals is measured in the same fashion as for full-speed signaling.
The maximum low-speed hub delay is 300 ns (TLHDD). This allows for the slower low-speed buffer propagation
delay and rise and fall times. It also provides time for the hub to re-clock the low-speed data in the upstream
direction.
When the hub acts as a repeater, it must reproduce the received, full-speed signal accurately on its outputs. This
means that for differential signals, the propagation delays of a J-to-K state transition must match closely to the
delays of a K-to-J state transition. For full-speed propagation, the maximum difference allowed between these
two delays (THDJ1) (see Figure 7-38 and Figure 7-52) for a hub plus cable is ±3.0 ns. Similarly, the difference
in delay between any two J-to-K or K-to-J transitions through a hub (THDJ2) must be less than ±1.0 ns. For lowspeed propagation in the downstream direction, the corresponding allowable jitter (TLDHJ1) is ±45 ns and
(TLDHJ2) ±15 ns, respectively. For low-speed propagation in the upstream direction, the allowable jitter is
±45 ns in both cases (TLUHJ1 and TLUHJ2).
An exception to this case is the skew that can be introduced in the Idle-to-K state transition at SOP (TFSOP and
TLSOP) (refer to Section 7.1.7.4). In this case, the delay to the opposite port includes the time to enable the
output buffer. However, the delays should be closely matched to the normal hub delay and the maximum
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additional delay difference over a normal J-to-K transition is ±5.0 ns. This limits the maximum distortion of the
first bit in the packet.
Note: Because of this distortion of the SOP transition relative to the next K-to-J state transition, the first SYNC
field bit should not be used to synchronize the receiver to the data stream.
The EOP must be propagated through a hub in the same way as the differential signaling. The propagation delay
for sensing an SE0 must be no less than the greater of the J-to-K or K-to-J differential data delay (to avoid
truncating the last data bit in a packet), but not more than 15 ns greater than the larger of these differential delays
at full-speed and 200 ns at low-speed (to prevent creating a bit stuff error at the end of the packet). EOP delays
are shown in Figure 7-53.
Because the sense levels for the SE0 state are not at the midpoint of the signal swing, the width of SE0 state will
be changed as it passes through each hub. A hub may not change the width of the SE0 state in a full-speed EOP
by more than ±15 ns (TFHESK), as measured by the difference of the leading edge and trailing edge delays of the
SE0 state (see Figure 7-53). An SE0 from a low-speed device has long rise and fall times and is subject to
greater skew, but these conditions exist only on the cable from the low-speed device to the port to which it is
connected. Thereafter, the signaling uses full-speed buffers and their faster rise and fall times. The SE0 from
the low-speed device cannot be changed by more than ±300 ns (TLHESK) as it passes through the hub to which
the device is connected. This time allows for some signal conditioning in the low-speed transceiver to reduce its
sensitivity to noise.
7.1.14.2 High-speed Hub Signaling Timings
When a hub acts as a repeater for high-speed data, the delay of the hub (THSHDD) must not exceed 36 high-speed
bit times plus 4 ns (the trace delays allowed for the hub circuit board). This delay is measured from the last bit
of the SYNC field at the input connector to the last bit of the SYNC field at the output connector.
A high-speed hub repeater must digitally resynchronize the buffered data, so there is no allowance for
cumulative jitter (within a single packet) as a high-speed packet passes through multiple repeater stages. Within
a single packet, the jitter must not exceed the eye pattern templates defined in Section 7.1.2 over a sliding
window of 480 high-speed bit times.
Due to the data synchronization process, the propagation delay of a hub repeater is allowed to vary at most
5 high-speed bit times (THSHDV). The delay including this allowed variation must not exceed 36 high-speed bit
times plus 4 ns. (This allows for some uncertainty as to when an incoming packet arrives at the hub with respect
to the phase of the synchronization clock.)
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7.1.15 Receiver Data Jitter
This section describes low-speed, full-speed, and high-speed receiver data jitter.
7.1.15.1 Low-/full-speed Receiver Data Jitter
The data receivers for all types of devices must be able to properly decode the differential data in the presence of
jitter. The more of the bit cell that any data edge can occupy and still be decoded, the more reliable the data
transfer will be. Data receivers are required to decode differential data transitions that occur in a window plus
and minus a nominal quarter bit cell from the nominal (centered) data edge position. (A simple 4X oversampling state machine DPLL can be built that satisfies these requirements.) This requirement is derived in
Table 7-4 and Table 7-5. The tables assume a worst-case topology of five hubs between the host and device and
the worst-case number of seven bits between transitions. The derived numbers are rounded up for ease of
specification.
Jitter will be caused by the delay mismatches discussed above and by mismatches in the source and destination
data rates (frequencies). The receive data jitter budgets for full- and low-speed are given in Table 7-4 and
Table 7-5. These tables give the value and totals for each source of jitter for both consecutive (next) and paired
transitions. Note that the jitter component related to the source or destination frequency tolerance has been
allocated to the appropriate device (i.e., the source jitter includes bit shifts due to source frequency inaccuracy
over the worst-case data transition interval). The output driver jitter can be traded off against the device clock
accuracy in a particular implementation as long as the jitter specification is met.
The low-speed jitter budget table has an additional line in it because the jitter introduced by the hub to which the
low-speed device is attached is different from all the other devices in the data path. The remaining devices
operate with full-speed signaling conventions (though at low-speed data rate).
Table 7-4. Full-speed Jitter Budget
Jitter Source
Full-speed
Next Transition
Each (ns)
Total (ns)
Each (ns) Total (ns)
Source Driver Jitter
2.0
2.0
1.0
1.0
Source Frequency Tolerance (worst-case)
0.21/bit
1.5
0.21/bit
3.0
1.0
5.0
Source Jitter Total
Hub Jitter
3.5
3.0
Jitter Specification
Destination Frequency Tolerance
Receiver Jitter Budget
164
Paired Transition
15.0
4.0
18.5
0.21/bit
1.5
20.0
9.0
0.21/bit
3.0
12.0
Universal Serial Bus Specification Revision 2.0
Table 7-5. Low-speed Jitter Budget
Jitter Source
Low-speed Upstream
Next Transition
Paired Transition
Each (ns) Total (ns) Each (ns) Total (ns)
Function Driver Jitter
25.0
25.0
10.0
Function Frequency Tolerance (worst-case)
10.0/bit
70.0
10.0/bit
Source (Function) Jitter Total
95.0
10.0
140.0
150.0
Hub with Low-speed Device Jitter
45.0
45.0
45.0
45.0
Remaining (full-speed) Hubs’ Jitter
3.0
12.0
1.0
4.0
Jitter Specification
Host Frequency Tolerance
152.0
1.7/bit
Host Receiver Jitter Budget
199.0
12.0
1.7/bit
164.0
24.0
223.0
Low-speed Downstream
Next Transition
Paired Transition
Each (ns) Total (ns) Each (ns) Total (ns)
Host Driver Jitter
2.0
2.0
1.0
Host Frequency Tolerance (worst-case)
1.7/bit
12.0
1.7/bit
Source (Host) Jitter Total
14.0
1.0
24.0
25.0
Hub with Low-speed Device Jitter
45.0
45.0
15.0
15.0
Remaining (full-speed) Hubs’ Jitter
3.0
12.0
1.0
4.0
Jitter Spec
Function Frequency Tolerance
Function Receiver Jitter Budget
71.0
10.0/bit
44.0
70.0
10.0/bit
141.0
140.0
184.0
Note: This table describes the host transmitting at low-speed data rate using full-speed signaling to
a low-speed device through the maximum number of hubs. When the host is directly connected to
the low-speed device, it uses low-speed data rate and low-speed signaling, and the host has to meet
the source jitter listed in the “Jitter Specification” row.
7.1.15.2 High-speed Receiver Data Jitter
A high-speed capable receiver must reliably recover high-speed data when the waveforms at its inputs conform
to the receiver sensitivity eye pattern templates. The templates, which are called out in Section 7.1.2.2, specify
the horizontal and vertical eye pattern opening over a 480 bit time sliding window over the duration of a packet.
Thus, for example, a high-speed receiver within a function must reliably recover data with a peak to peak jitter
of 30%, measured at its B receptacle (as described by Template 4).
Such conformance is tested using Test Mode Test_Packet, as defined in Section 7.1.20.
-12
It is a recommended design guideline that a receiver’s BER should be <= 10 when the receiver sensitivity
requirement is met.
7.1.16 Cable Delay
The maximum total one-way signal propagation delay allowed is 30 ns. The allocation for cable delay is 26 ns.
A maximum delay of 3 ns is allowed from a Host or Hub Controller downstream facing transceiver to its
exterior downstream facing connector, while a maximum delay of 1 ns is allowed from the upstream facing
connector to the upstream facing transceiver of any device. For a standard USB detachable cable, the cable
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delay is measured from the Series A connector pins to the Series B connector pins and is no more than 26 ns.
For other cables, the delay is measured from the series A connector to the point where the cable is connected to
the device. The cable delay must also be less than 5.2 ns per meter.
The maximum one-way data delay on a full-speed cable is measured as shown in Figure 7-39.
One-way cable delay for low-speed cables must be less than 18 ns. It is measured as shown in Figure 7-40.
Traces on Board
Host/Hub
Hub/Device
Downstream
Port
Upstream
Port
3ns (max)
A-Connector
B-Connector
Total One-Way
Propagation Delay
30ns (max)
1ns (max)
Driver End
of Cable
50% Point of Initial Swing
VSS
Receiver
End of
Cable
One Way Cable
Delay 26ns
(max)
Data Line
Crossover
Point at input of
B-connector
VSS
Figure 7-39. Full-speed Cable Delay
Traces on Board
Host/Hub
Downstream
Port
Low-speed
Device
A-Connector + cable
18nS (max)
One-way Propagation Delay
Figure 7-40. Low-speed Cable Delay
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7.1.17 Cable Attenuation
USB cables must not exceed the loss figures shown in Table 7-6. Between the frequencies called out in the
table, the cable loss should be no more than is shown in the accompanying graph.
Table 7-6. Maximum Allowable Cable Loss
Frequency (MHz)
Attenuation (maximum) dB/cable
0.064
0.08
0.256
0.11
0.512
0.13
0.772
0.15
1.000
0.20
4.000
0.39
8.000
0.57
12.000
0.67
24.000
0.95
48.000
1.35
96.000
1.9
200.00
3.2
400.00
5.8
Maximum Allowable Attenuation (dB)
Maximum Allowable Cable Loss
0
-1
-2
-3
-4
-5
-6
Frequency (log scale) from 10KHz to 1GHz, 1 decade per division
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7.1.18 Bus Turn-around Time and Inter-packet Delay
This section describes low-speed, full-speed, and high-speed bus turn-around time and inter-packet delay.
7.1.18.1 Low-/Full-Speed Bus Turn-around Time and Inter-packet Delay
Inter-packet delays are measured from the SE0-to-J transition at the end of the EOP to the J-to-K transition that
starts the next packet.
A device must provide at least two bit times of inter-packet delay. The delay is measured at the responding
device with a bit time defined in terms of the response. This provides adequate time for the device sending the
EOP to drive J for one bit time and then turn off its output buffers.
The host must provide at least two bit times of J after the SE0 of an EOP and the start of a new packet (TIPD). If
a function is expected to provide a response to a host transmission, the maximum inter-packet delay for a
function or hub with a detachable (TRSPIPD1) cable is 6.5 bit times measured at the Series B receptacle. If the
device has a captive cable, the inter-packet delay (TRSPIPD2) must be less than 7.5 bit times as measured at the
Series A plug. These timings apply to both full-speed and low-speed devices and the bit times are referenced to
the data rate of the packet.
The maximum inter-packet delay for a host response is 7.5 bit times measured at the host’s port pins. There is
no maximum inter-packet delay between packets in unrelated transactions.
7.1.18.2 High-Speed Bus Turn-around Time and Inter-packet Delay
High-speed inter-packet delays are measured from time when the line returns to a high-speed Idle State at the
end of one packet to when the line leaves the high-speed Idle State at the start of the next packet.
When transmitting after receiving a packet, hosts and devices must provide an inter-packet delay of at least 8 bit
times (THSIPDOD) measured at their A or B connectors (receptacles or plugs).
Additionally, if a host is transmitting two packets in a row, the inter-packet delay must be a minimum of 88 bit
times (THSIPDSD), measured at the host’s A receptacle. This will guarantee an inter-packet delay of at least 32 bit
times at all devices (when receiving back to back packets). The maximum inter-packet delay provided by a host
is 192 bit times within a transaction (THSRSPIPD1) measured at the A receptacle. When a host responds to a
packet from a device, it will provide an inter-packet delay of at most 192 bit times measured at the A receptacle.
There is no maximum inter-packet delay between packets in unrelated transactions.
When a device with a detachable cable responds to a packet from a host, it will provide an inter-packet delay of
at most 192 bit times measured at the B receptacle. If the device has a captive cable, it will provide an interpacket delay of at most 192 bit times plus 52 ns (2 times the max cable length) measured at the cable's A plug
(THSRSPIPD2).
7.1.19 Maximum End-to-end Signal Delay
This section describes low-speed, full-speed, and high-speed end-to-end delay.
7.1.19.1 Low-/full-speed End-to-end Signal Delay
A device expecting a response to a transmission will invalidate the transaction if it does not see the start-ofpacket (SOP) transition within the timeout period after the end of the transmission (after the SE0-to-J state
transition in the EOP). This can occur between an IN token and the following data packet or between a data
packet and the handshake packet (refer to Chapter 8). The device expecting the response will not time out
before 16 bit times but will timeout before 18 bit times (measured at the data pins of the device from the SE0-toJ transition at the end of the EOP). The host will wait at least 18 bit times for a response to start before it will
start a new transaction.
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Figure 7-41 depicts the configuration of six signal hops (cables) that results in allowable worst-case signal delay.
The maximum propagation delay from the upstream end of a hub’s cable to any downstream facing connector on
that hub is 70 ns.
Host
Controller
Hub 1
Hub 2
Hub 3
Hub 4
Cable Delay + Hub Delay ≤ 70ns (each)
Hub 5
Function
Propagation Delay ≤ 30ns
Figure 7-41. Worst-case End-to-end Signal Delay Model for Low-/full-speed
7.1.19.2 High-Speed End-to-end Delay
A high-speed host or device expecting a response to a transmission must not timeout the transaction if the interpacket delay is less than 736 bit times, and it must timeout the transaction if no signaling is seen within 816 bit
times.
These timeout limits allow a response to be seen even for the worst-case round trip signal delay. In high-speed
mode, the worst-case round trip signal delay model is the sum of the following components:
12 max length cable delays (6 cables)
= 312 ns
10 max delay hubs (5 hubs)
= 40 ns + 360 bit times
1 max device response time
=
192 bit times
_________________________________________________________
Worst-case round trip delay
= 352 ns +552 bit times = 721 bit times
7.1.20 Test Mode Support
To facilitate compliance testing, host controllers, hubs, and high-speed capable functions must support the
following test modes:
•
Test mode Test_SE0_NAK: Upon command, a port’s transceiver must enter the high-speed receive mode
and remain in that mode until the exit action is taken. This enables the testing of output impedance, low
level output voltage, and loading characteristics. In addition, while in this mode, upstream facing ports (and
only upstream facing ports) must respond to any IN token packet with a NAK handshake (only if the packet
CRC is determined to be correct) within the normal allowed device response time. This enables testing of
the device squelch level circuitry and, additionally, provides a general purpose stimulus/response test for
basic functional testing.
•
Test mode Test_J: Upon command, a port’s transceiver must enter the high-speed J state and remain in that
state until the exit action is taken. This enables the testing of the high output drive level on the D+ line.
•
Test mode Test_K: Upon command, a port’s transceiver must enter the high-speed K state and remain in
that state until the exit action is taken. This enables the testing of the high output drive level on the D- line.
•
Test mode Test_Packet: Upon command, a port must repetitively transmit the following test packet until
the exit action is taken. This enables the testing of rise and fall times, eye patterns, jitter, and any other
dynamic waveform specifications.
The test packet is made up by concatenating the following strings. (Note: For J/K NRZI data, and for NRZ
data, the bit on the left is the first one transmitted. “S” indicates that a bit stuff occurs, which inserts an
“extra” NRZI data bit. “* N” is used to indicate N occurrences of a string of bits or symbols.)
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NRZI Symbols
(Fields)
NRZ Bit Strings
Number of NRZ Bits
{KJ * 15}, KK
{00000000 * 3}, 00000001
32
11000011
8
JKJKJKJK * 9
00000000 * 9
72
JJKKJJKK * 8
01010101 * 8
64
JJJJKKKK * 8
01110111 * 8
64
JJJJJJJKKKKKKK * 8
0, {111111S *15}, 111111
97
JJJJJJJK * 8
S, 111111S, {0111111S * 7}
55
{JKKKKKKK * 10}, JK
00111111, {S0111111 * 9}, S0
72
JJJKKKJJKKKKJKKK
0110110101110011
16
01111111
8
(SYNC)
KKJKJKKK
(DATA0 PID)
(CRC16)
JJJJJJJJ
(EOP)
A port in Test_Packet mode must send this packet repetitively. The inter-packet timing must be no less than
the minimum allowable inter-packet gap as defined in Section 7.1.18 and no greater than 125 µs.
•
Test mode Test_Force_Enable: Upon command, downstream facing hub ports (and only downstream
facing hub ports) must be enabled in high-speed mode, even if there is no device attached. Packets arriving
at the hub’s upstream facing port must be repeated on the port which is in this test mode. This enables
testing of the hub’s disconnect detection; the disconnect detect bit can be polled while varying the loading
on the port, allowing the disconnect detection threshold voltage to be measured.
Test Mode Entry and Exit
Test mode of a port is entered by using a device specific standard request (for an upstream facing port) or a port
specific hub class request (for a downstream facing port). The device standard request
SetFeature(TEST_MODE) is defined in Section 9.4.9. The hub class request SetPortFeature(PORT_TEST) is
defined in Section 11.24.2.13. All high-speed capable devices/hubs must support these requests. These requests
are not supported for non-high-speed devices.
The transition to test mode must be complete no later than 3 ms after the completion of the status stage of the
request.
For an upstream facing port, the exit action is to power cycle the device. For a downstream facing port, the exit
action is to reset the hub, as defined in Section 11.24.2.13.
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7.2 Power Distribution
This section describes the USB power distribution specification.
7.2.1 Classes of Devices
The power source and sink requirements of different device classes can be simplified with the introduction of the
concept of a unit load. A unit load is defined to be 100 mA. The number of unit loads a device can draw is an
absolute maximum, not an average over time. A device may be either low-power at one unit load or highpower, consuming up to five unit loads. All devices default to low-power. The transition to high-power is under
software control. It is the responsibility of software to ensure adequate power is available before allowing
devices to consume high-power.
The USB supports a range of power sourcing and power consuming agents; these include the following:
•
Root port hubs: Are directly attached to the USB Host Controller. Hub power is derived from the same
source as the Host Controller. Systems that obtain operating power externally, either AC or DC, must
supply at least five unit loads to each port. Such ports are called high-power ports. Battery-powered
systems may supply either one or five unit loads. Ports that can supply only one unit load are termed lowpower ports.
•
Bus-powered hubs: Draw all of their power for any internal functions and downstream facing ports from
VBUS on the hub’s upstream facing port. Bus-powered hubs may only draw up to one unit load upon
power-up and five unit loads after configuration. The configuration power is split between allocations to the
hub, any non-removable functions and the external ports. External ports in a bus-powered hub can supply
only one unit load per port regardless of the current draw on the other ports of that hub. The hub must be
able to supply this port current when the hub is in the Active or Suspend state.
•
Self-powered hubs: Power for the internal functions and downstream facing ports does not come from
VBUS. However, the USB interface of the hub may draw up to one unit load from VBUS on its upstream
facing port to allow the interface to function when the remainder of the hub is powered down. Hubs that
obtain operating power externally (from the USB) must supply five unit loads to each port. Batterypowered hubs may supply either one or five unit loads per port.
•
Low-power bus-powered functions: All power to these devices comes from VBUS. They may draw no
more than one unit load at any time.
•
High-power bus-powered functions: All power to these devices comes from VBUS. They must draw no
more than one unit load upon power-up and may draw up to five unit loads after being configured.
•
Self-powered functions: May draw up to one unit load from VBUS to allow the USB interface to function
when the remainder of the function is powered down. All other power comes from an external (to the USB)
source.
No device shall supply (source) current on VBUS at its upstream facing port at any time. From VBUS on its
upstream facing port, a device may only draw (sink) current. They may not provide power to the pull-up resistor
on D+/D- unless VBUS is present (see Section 7.1.5). When VBUS is removed, the device must remove power
from the D+/D- pull-up resistor within 10 seconds. On power-up, a device needs to ensure that its upstream
facing port is not driving the bus, so that the device is able to receive the reset signaling. Devices must also
ensure that the maximum operating current drawn by a device is one unit load, until configured. Any device that
draws power from the bus must be able to detect lack of activity on the bus, enter the Suspend state, and reduce
its current consumption from VBUS (refer to Section 7.2.3 and Section 9.2.5.1).
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7.2.1.1 Bus-powered Hubs
Bus-powered hub power requirements can be met with a power control circuit such as the one shown in
Figure 7-42. Bus-powered hubs often contain at least one non-removable function. Power is always available to
the hub’s controller, which permits host access to power management and other configuration registers during
the enumeration process. A non-removable function(s) may require that its power be switched, so that upon
power-up, the entire device (hub and non-removable functions) draws no more than one unit load. Power
switching on any non-removable function may be implemented either by removing its power or by shutting off
the clock. Switching on the non-removable function is not required if the aggregate power drawn by it and the
Hub Controller is less than one unit load. However, as long as the hub port associated with the function is in the
Power-off state, the function must be logically reset and the device must appear to be not connected. The total
current drawn by a bus-powered device is the sum of the current to the Hub Controller, any non-removable
function(s), and the downstream facing ports.
Figure 7-42 shows the partitioning of power based upon the maximum current draw (from upstream) of five unit
loads: one unit load for the Hub Controller and the non-removable function and one unit load for each of the
external downstream facing ports. If more than four external ports are required, then the hub will need to be
self-powered. If the non-removable function(s) and Hub Controller draw more than one unit load, then the
number of external ports must be appropriately reduced. Power control to a bus-powered hub may require a
regulator. If present, the regulator is always enabled to supply the Hub Controller. The regulator can also power
the non-removable functions(s). Inrush current limiting must also be incorporated into the regulator subsystem.
Upstream
Data Port
Downstream
Data Ports
Hub Controller
Iportctrl
On/Off
pstream V BUS
Regulator
5 unit loads
Non-removable
Function
1 unit load - Iportctrl
On/Off
Switch
1 unit load/port
Downstream VBUS
Figure 7-42. Compound Bus-powered Hub
Power to external downstream facing ports of a bus-powered hub must be switched. The Hub Controller
supplies a software controlled on/off signal from the host, which is in the “off” state when the device is powered
up or after reset signaling. When switched to the “on” state, the switch implements a soft turn-on function that
prevents excessive transient current from being drawn from upstream. The voltage drop across the upstream
cable, connectors, and switch in a bus-powered hub must not exceed 350 mV at maximum rated current.
7.2.1.2 Self-powered Hubs
Self-powered hubs have a local power supply that furnishes power to any non-removable functions and to all
downstream facing ports, as shown in Figure 7-43. Power for the Hub Controller, however, may be supplied
from the upstream VBUS (a “hybrid” powered hub) or the local power supply. The advantage of supplying the
Hub Controller from the upstream supply is that communication from the host is possible even if the device’s
power supply remains off. This makes it possible to differentiate between a disconnected and an unpowered
device. If the hub draws power for its upstream facing port from VBUS, it may not draw more than one unit
load.
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Upstream VBUS
1 unit load (max)
Regulator
Hub Controller
.
.
.
Downstream
Data Ports
Upstream
Data Port
Local Power
Supply
Regulator
Non-removable
Function
On/Off
.
.
Current Limit
5 unit loads/port
Current Limit
Downstream VBUS
.
.
Figure 7-43. Compound Self-powered Hub
The number of ports that can be supported is limited only by the address capability of the hub and the local
supply.
Self-powered hubs may experience loss of power. This may be the result of disconnecting the power cord or
exhausting the battery. Under these conditions, the hub may force a re-enumeration of itself as a bus-powered
hub. This requires the hub to implement port power switching on all external ports. When power is lost, the hub
must ensure that upstream current does not exceed low-power. All the rules of a bus-powered hub then apply.
7.2.1.2.1 Over-current Protection
The host and all self-powered hubs must implement over-current protection for safety reasons, and the hub must
have a way to detect the over-current condition and report it to the USB software. Should the aggregate current
drawn by a gang of downstream facing ports exceed a preset value, the over-current protection circuit removes
or reduces power from all affected downstream facing ports. The over-current condition is reported through the
hub to Host Controller, as described in Section 11.12.5. The preset value cannot exceed 5.0 A and must be
sufficiently above the maximum allowable port current such that transient currents (e.g., during power up or
dynamic attach or reconfiguration) do not trip the over-current protector. If an over-current condition occurs on
any port, subsequent operation of the USB is not guaranteed, and once the condition is removed, it may be
necessary to reinitialize the bus as would be done upon power-up. The over-current limiting mechanism must be
resettable without user mechanical intervention. Polymeric PTCs and solid-state switches are examples of
methods, which can be used for over-current limiting.
7.2.1.3 Low-power Bus-powered Functions
A low-power function is one that draws up to one unit load from the USB cable when operational. Figure 7-44
shows a typical bus-powered, low-power function, such as a mouse. Low-power regulation can be integrated
into the function silicon. Low-power functions must be capable of operating with input VBUS voltages as low as
4.40 V, measured at the plug end of the cable.
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Upstream
Data Port
Function
Upstream V BUS
Regulator
1 unit load (max)
Figure 7-44. Low-power Bus-powered Function
7.2.1.4 High-power Bus-powered Functions
A function is defined as being high-power if, when fully powered, it draws over one but no more than five unit
loads from the USB cable. A high-power function requires staged switching of power. It must first come up in
a reduced power state of less than one unit load. At bus enumeration time, its total power requirements are
obtained and compared against the available power budget. If sufficient power exists, the remainder of the
function may be powered on. A typical high-power function is shown in Figure 7-45. The function’s electronics
have been partitioned into two sections. The function controller contains the minimum amount of circuitry
necessary to permit enumeration and power budgeting. The remainder of the function resides in the function
block. High-power functions must be capable of operating in their low-power (one unit load) mode with an
input voltage as low as 4.40 V, so that it may be detected and enumerated even when plugged into a buspowered hub. They must also be capable of operating at full power (up to five unit loads) with a VBUS voltage
of 4.75 V, measured at the upstream plug end of the cable.
Upstream
Data Port
Function Controller
Function
On/Off
1 unit load
(max)
Upstream VBUS
Regulator
5 unit loads (max)
Figure 7-45. High-power Bus-powered Function
7.2.1.5 Self-powered Functions
Figure 7-46 shows a typical self-powered function. The function controller is powered either from the upstream
bus via a low-power regulator or from the local power supply. The advantage of the former scheme is that it
permits detection and enumeration of a self-powered function whose local power supply is turned off. The
maximum upstream power that the function controller can draw is one unit load, and the regulator block must
implement inrush current limiting. The amount of power that the function block may draw is limited only by the
local power supply. Because the local power supply is not required to power any downstream bus ports, it does
not need to implement current limiting, soft start, or power switching.
174
Universal Serial Bus Specification Revision 2.0
Upstream
Data Port
Function Controller
Upstream VBUS
Function
Regulator
1 unit load (max)
Local Power
Supply
Regulator
Figure 7-46. Self-powered Function
7.2.2 Voltage Drop Budget
The voltage drop budget is determined from the following:
•
The voltage supplied by high-powered hub ports is 4.75 V to 5.25 V.
•
The voltage supplied by low-powered hub ports is 4.4 V to 5.25 V.
•
Bus-powered hubs can have a maximum drop of 350 mV from their cable plug (where they attach to a
source of power) to their output port connectors (where they supply power).
•
The maximum voltage drop (for detachable cables) between the A-series plug and B-series plug on VBUS is
125 mV (VBUSD).
•
The maximum voltage drop for all cables between upstream and downstream on GND is 125 mV (VGNDD).
•
All hubs and functions must be able to provide configuration information with as little as 4.40 V at the
connector end of their upstream cables. Only low-power functions need to be operational with this
minimum voltage.
•
Functions drawing more than one unit load must operate with a 4.75 V minimum input voltage at the
connector end of their upstream cables.
Figure 7-47 shows the minimum allowable voltages in a worst-case topology consisting of a bus-powered hub
driving a bus-powered function.
Bus-powered
Hub
Host or
Powered Hub
4.750V
4.735V
4.640V
4.625V
*4.400V
Low-power
Function
4.397V
4.378V
4.500V
0.000V
0.015V
0.110V
0.125V
Referenced
to Source
4.375V
4.350V
0.000V
0.003V
0.022V
0.025V
Referenced
to Hub
*Under transient conditions, supply at hub can drop from 4.400V to 4.070V
Figure 7-47. Worst-case Voltage Drop Topology (Steady State)
175
Universal Serial Bus Specification Revision 2.0
7.2.3 Power Control During Suspend/Resume
Suspend current is a function of unit load allocation. All USB devices initially default to low-power. Lowpower devices or high-power devices operating at low-power are limited to 500 µA of suspend current. If the
device is configured for high-power and enabled as a remote wakeup source, it may draw up to 2.5 mA during
suspend. When computing suspend current, the current from VBUS through the bus pull-up and pull-down
resistors must be included. Configured bus-powered hubs may also consume a maximum of 2.5 mA, with
500 µA allocated to each available external port and the remainder available to the hub and its internal functions.
If a hub is not configured, it is operating as a low-power device and must limit its suspend current to 500 µA.
While in the Suspend state, a device may briefly draw more than the average current. The amplitude of the
current spike cannot exceed the device power allocation 100 mA (or 500 mA). A maximum of 1.0 second is
allowed for an averaging interval. The average current cannot exceed the average suspend current limit (ICCSH
or ICCSL, see Table 7-7) during any 1.0-second interval (TSUSAVG1). The profile of the current spike is
restricted so the transient response of the power supply (which may be an efficient, low-capacity, trickle power
supply) is not overwhelmed. The rising edge of the current spike must be no more than 100 mA/µs.
Downstream facing ports must be able to absorb the 500 mA peak current spike and meet the voltage droop
requirements defined for inrush current during dynamic attach (see Section 7.2.4.1). Figure 7-48 illustrates a
typical example profile for an averaging interval. If the supply to the pull-up resistor on D+/D- is derived from
VBUS, then the suspend current will never go to zero because the pull-up and pull-down resistors will always
draw power.
ICONFIGURED(max)
Edge rate must
not exceed
100mA/µs
Current Spike
ICCS(H|L)
I
0 mA
time
Averaging Interval
Figure 7-48. Typical Suspend Current Averaging Profile
Devices are responsible for handling the bus voltage reduction due to the inductive and resistive effects of the
cable. When a hub is in the Suspend state, it must still be able to provide the maximum current per port (one
unit load of current per port for bus-powered hubs and five unit loads per port for self-powered hubs). This is
necessary to support remote wakeup-capable devices that will power-up while the remainder of the system is
still suspended. Such devices, when enabled to do remote wakeup, must drive resume signaling upstream within
10 ms of starting to draw the higher, non-suspend current. Devices not capable of remote wakeup must draw the
higher current only when not suspended.
When devices wakeup, either by themselves (remote wakeup) or by seeing resume signaling, they must limit the
inrush current on VBUS. The target maximum droop in the hub VBUS is 330 mV. The device must have
sufficient on-board bypass capacitance or a controlled power-on sequence such that the current drawn from the
hub does not exceed the maximum current capability of the port at any time while the device is waking up.
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Universal Serial Bus Specification Revision 2.0
7.2.4 Dynamic Attach and Detach
The act of plugging or unplugging a hub or function must not affect the functionality of another device on other
segments of the network. Unplugging a function will stop the transaction between that function and the host.
However, the hub to which this function was attached will recover from this condition and will alert the host that
the port has been disconnected.
7.2.4.1 Inrush Current Limiting
When a function or hub is plugged into the network, it has a certain amount of on-board capacitance between
VBUS and ground. In addition, the regulator on the device may supply current to its output bypass capacitance
and to the function as soon as power is applied. Consequently, if no measures are taken to prevent it, there could
be a surge of current into the device which might pull the VBUS on the hub below its minimum operating level.
Inrush currents can also occur when a high-power function is switched into its high-power mode. This problem
must be solved by limiting the inrush current and by providing sufficient capacitance in each hub to prevent the
power supplied to the other ports from going out of tolerance. An additional motivation for limiting inrush
current is to minimize contact arcing, thereby prolonging connector contact life.
The maximum droop in the hub VBUS is 330 mV, or about 10% of the nominal signal swing from the function.
In order to meet this requirement, the following conditions must be met:
•
The maximum load (CRPB) that can be placed at the downstream end of a cable is 10 µF in parallel with
44 Ω. The 10 µF capacitance represents any bypass capacitor directly connected across the VBUS lines in
the function plus any capacitive effects visible through the regulator in the device. The 44 Ω resistance
represents one unit load of current drawn by the device during connect.
•
If more bypass capacitance is required in the device, then the device must incorporate some form of VBUS
surge current limiting, such that it matches the characteristics of the above load.
•
The hub downstream facing port VBUS power lines must be bypassed (CHPB) with no less than 120 µF of
low-ESR capacitance per hub. Standard bypass methods should be used to minimize inductance and
resistance between the bypass capacitors and the connectors to reduce droop. The bypass capacitors
themselves should have a low dissipation factor to allow decoupling at higher frequencies.
The upstream facing port of a hub is also required to meet the above requirements. Furthermore, a bus-powered
hub must provide additional surge limiting in the form of a soft-start circuit when it enables power to its
downstream facing ports.
A high-power bus-powered device that is switching from a lower power configuration to a higher power
configuration must not cause droop > 330 mV on the VBUS at its upstream hub. The device can meet this by
ensuring that changes in the capacitive load it presents do not exceed 10 µF.
Signal pins are protected from excessive currents during dynamic attach by being recessed in the connector such
that the power pins make contact first. This guarantees that the power rails to the downstream device are
referenced before the signal pins make contact. In addition, the signal lines are in a high-impedance state during
connect, so that no current flows for standard signal levels.
7.2.4.2 Dynamic Detach
When a device is detached from the network with power flowing in the cable, the inductance of the cable will
cause a large flyback voltage to occur on the open end of the device cable. This flyback voltage is not
destructive. Proper bypass measures on the hub ports will suppress any coupled noise. The frequency range of
this noise is inversely dependent on the length of the cable, to a maximum of 60 MHz for a one-meter cable.
This will require some low capacitance, very low inductance bypass capacitors on each hub port connector. The
flyback voltage and the noise it creates is also moderated by the bypass capacitance on the device end of the
cable. Also, there must be some minimum capacitance on the device end of the cable to ensure that the
177
Universal Serial Bus Specification Revision 2.0
inductive flyback on the open end of the cable does not cause the voltage on the device end to reverse polarity.
A minimum of 1.0 µF is recommended for bypass across VBUS.
7.3 Physical Layer
The physical layer specifications are described in the following subsections.
7.3.1 Regulatory Requirements
All USB devices should be designed to meet the applicable regulatory requirements.
7.3.2 Bus Timing/Electrical Characteristics
Table 7-7. DC Electrical Characteristics
Parameter
Symbol
Conditions
Min.
Max.
Units
Supply Voltage:
High-power Port
VBUS
Note 2, Section 7.2.1
4.75
5.25
V
Low-power Port
VBUS
Note 2, Section 7.2.1
4.40
5.25
V
High-power Hub Port (out)
ICCPRT
Section 7.2.1
500
mA
Low-power Hub Port (out)
ICCUPT
Section 7.2.1
100
mA
High-power Function (in)
ICCHPF
Section 7.2.1
500
mA
Low-power Function (in)
ICCLPF
Section 7.2.1
100
mA
Unconfigured Function/Hub (in)
ICCINIT
Section 7.2.1.4
100
mA
Suspended High-power Device
ICCSH
Section 7.2.3; Note 15
2.5
mA
Suspended Low-power Device
ICCSL
Section 7.2.3
500
µA
High (driven)
VIH
Note 4, Section 7.1.4
2.0
High (floating)
VIHZ
Note 4, Section 7.1.4
2.7
Low
VIL
Note 4, Section 7.1.4
Differential Input Sensitivity
VDI
|(D+)-(D-)|;
Figure 7-19; Note 4
0.2
Differential Common Mode
Range
VCM
Includes VDI range;
Figure 7-19; Note 4
0.8
2.5
V
VHSSQ
Section 7.1.7.2
(specification refers to
differential signal
amplitude)
100
150
mV
Supply Current:
Input Levels for Low-/full-speed:
V
3.6
V
0.8
V
V
Input Levels for High-speed:
High-speed squelch detection
threshold (differential signal
amplitude)
178
Universal Serial Bus Specification Revision 2.0
Table 7-7. DC Electrical Characteristics (Continued)
Parameter
High speed disconnect
detection threshold (differential
signal amplitude)
Symbol
VHSDSC
High-speed differential input
signaling levels
High-speed data signaling
common mode voltage range
(guideline for receiver)
Conditions
Section 7.1.7.2
(specification refers to
differential signal
amplitude)
Min.
Max.
Units
525
625
mV
Section 7.1.7.2
Specified by eye pattern
templates
VHSCM
Section 7.1.4.2
-50
500
mV
Low
VOL
Note 4, 5, Section 7.1.1
0.0
0.3
V
High (Driven)
VOH
Note 4, 6, Section 7.1.1
2.8
3.6
V
SE1
VOSE1
Section 7.1.1
0.8
Output Signal Crossover
Voltage
VCRS
Measured as in
Figure 7-8; Note 10
1.3
2.0
High-speed idle level
VHSOI
Section 7.1.7.2
-10.0
10.0
mV
High-speed data signaling high
VHSOH
Section 7.1.7.2
360
440
mV
High-speed data signaling low
VHSOL
Section 7.1.7.2
-10.0
10.0
mV
Chirp J level (differential
voltage)
VCHIRPJ
Section 7.1.7.2
700
1100
mV
Chirp K level (differential
voltage)
VCHIRPK
Section 7.1.7.2
-900
-500
mV
Downstream Facing Port
Bypass Capacitance (per hub)
CHPB
VBUS to GND,
Section 7.2.4.1
Upstream Facing Port Bypass
Capacitance
CRPB
VBUS to GND; Note 9,
Section 7.2.4.1
Output Levels for Low-/full-speed:
V
V
Output Levels for High-speed:
Decoupling Capacitance:
µF
120
1.0
10.0
µF
Input Capacitance for Low-/full-speed:
Downstream Facing Port
CIND
Note 2; Section 7.1.6.1
150
pF
Upstream Facing Port (w/o
cable)
CINUB
Note 3; Section 7.1.6.1
100
pF
Transceiver edge rate control
capacitance
CEDGE
Section 7.1.6.1
75
pF
179
Universal Serial Bus Specification Revision 2.0
Table 7-7. DC Electrical Characteristics (Continued)
Parameter
Symbol
Conditions
Min.
Max.
Units
Input Impedance for High-speed:
TDR spec for high-speed
termination
Section 7.1.6.2
Terminations:
Bus Pull-up Resistor on
Upstream Facing Port
RPU
1.5 kΩ ±5%
Section 7.1.5
1.425
1.575
kΩ
Bus Pull-down Resistor on
Downstream Facing Port
RPD
15 kΩ ±5%
Section 7.1.5
14.25
15.75
kΩ
Input impedance exclusive of
pullup/pulldown (for low-/fullspeed)
ZINP
Section 7.1.6
300
Termination voltage for
upstream facing port pullup
(RPU)
VTERM
Section 7.1.5
3.0
VHSTERM
Section 7.1.6.2
kΩ
3.6
V
Terminations in High-speed:
Termination voltage in highspeed
-10
10
mV
Table 7-8. High-speed Source Electrical Characteristics
Parameter
Symbol
Conditions
Min.
Max.
Units
Driver Characteristics:
Rise Time (10% - 90%)
THSR
Section 7.1.2
500
ps
Fall Time (10% - 90%)
THSF
Section 7.1.2
500
ps
Driver waveform requirements
Specified by eye pattern
templates in Section 7.1.2
ZHSDRV
Section 7.1.1.1
40.5
49.5
Ω
High-speed Data Rate
THSDRAT
Section 7.1.11
479.760
480.240
Mb/s
Microframe Interval
THSFRAM
Section 7.1.12
124.9375
125.0625
µs
Consecutive Microframe
Interval Difference
THSRFI
Section 7.1.12
Driver Output Resistance
(which also serves as highspeed termination)
Clock Timings:
4 highspeed bit
times
High-speed Data Timings:
Data source jitter
Receiver jitter tolerance
180
Source and receiver jitter specified by the eye pattern
templates in Section 7.1.2.2
Universal Serial Bus Specification Revision 2.0
Table 7-9. Full-speed Source Electrical Characteristics
Parameter
Symbol
Conditions
Min.
Max.
Units
Driver Characteristics:
Rise Time
TFR
Figure 7-8; Figure 7-9
4
20
ns
Fall Time
TFF
Figure 7-8; Figure 7-9
4
20
ns
Differential Rise and Fall Time
Matching
TFRFM
(TFR/TFF) Note 10,
Section 7.1.2
90
Driver Output Resistance for
driver which is not high-speed
capable
ZDRV
Section 7.1.1.1
28
44
111.11
%
Ω
Clock Timings:
Full-speed Data Rate for hubs
and devices which are highspeed capable
TFDRATHS
Average bit rate,
Section 7.1.11
11.9940
12.0060
Mb/s
Full-speed Data Rate for
devices which are not highspeed capable
TFDRATE
Average bit rate,
Section 7.1.11
11.9700
12.0300
Mb/s
Frame Interval
TFRAME
Section 7.1.12
0.9995
1.0005
ms
Consecutive Frame Interval
Jitter
TRFI
No clock adjustment
Section 7.1.12
42
ns
Full-speed Data Timings:
Source Jitter Total (including
frequency tolerance):
To Next Transition
For Paired Transitions
Source Jitter for Differential
Transition to SE0 Transition
TDJ1
TDJ2
TFDEOP
Receiver Jitter:
To Next Transition
For Paired Transitions
Note 7, 8, 12, 10;
Measured as in
Figure 7-49;
Note 8; Figure 7-50;
Note 11
-3.5
-4
3.5
4
ns
ns
-2
5
ns
-18.5
-9
18.5
9
ns
ns
Note 8; Figure 7-51
TJR1
TJR2
Source SE0 interval of EOP
TFEOPT
Figure 7-50
Receiver SE0 interval of EOP
TFEOPR
Note 13; Section 7.1.13.2;
Figure 7-50
Width of SE0 interval during
differential transition
TFST
Section 7.1.4
160
175
82
ns
ns
14
ns
181
Universal Serial Bus Specification Revision 2.0
Table 7-10. Low-speed Source Electrical Characteristics
Parameter
Symbol
Conditions
Min.
Max.
Units
Driver Characteristics:
Transition Time:
TLR
TLF
Measured as in Figure 7-8
75
75
300
300
ns
ns
Rise and Fall Time Matching
TLRFM
(TLR/TLF) Note 10
80
125
%
Upstream Facing Port
(w/cable, low-speed only)
CLINUA
Note 1; Section 7.1.6
200
450
pF
Rise Time
Fall Time
Clock Timings:
Low-speed Data Rate for hubs
which are high-speed capable
TLDRATHS
Section 7.1.11
1.49925
1.50075
Mb/s
Low-speed Data Rate for
devices which are not highspeed capable
TLDRATE
Section 7.1.11
1.4775
1.5225
Mb/s
Low-speed Data Timings:
Upstream facing port source
Jitter Total (including frequency
tolerance):
To Next Transition
For Paired Transitions
Upstream facing port source
Jitter for Differential Transition
to SE0 Transition
Note 7, 8; Figure 7-49
TUDJ1
TUDJ2
TLDEOP
Upstream facing port differential
Receiver Jitter:
To Next Transition
For Paired Transitions
Downstream facing port source
Jitter Total (including frequency
tolerance):
Note 8; Figure 7-50;
Note 11
-95
-150
95
150
ns
ns
-40
100
ns
-75
-45
75
45
ns
ns
-25
-14
25
14
ns
ns
Note 8; Figure 7-51
TDJR1
TDJR2
Note 7, 8; Figure 7-49
TDDJ1
TDDJ2
To Next Transition
For Paired Transitions
Downstream facing port source
Jitter for Differential Transition
to SE0 Transition
Note 8; Figure 7-50;
Note 11
Downstream facing port
Differential Receiver Jitter:
Note 8; Figure 7-50
To Next Transition
For Paired Transitions
182
TUJR1
TUJR2
ns
-152
-200
152
200
ns
ns
Universal Serial Bus Specification Revision 2.0
Table 7-10. Low-speed Source Electrical Characteristics (Continued)
Parameter
Symbol
Conditions
Source SE0 interval of EOP
TLEOPT
Figure 7-50
Receiver SE0 interval of EOP
TLEOPR
Note 13; Section 7.1.13.2;
Figure 7-50
Width of SE0 interval during
differential transition
TLST
Section 7.1.4
Min.
Max.
Units
1.25
1.50
µs
670
ns
210
ns
Table 7-11. Hub/Repeater Electrical Characteristics
Parameter
Symbol
Conditions
Min.
Max.
Units
Full-speed Hub Characteristics (as measured at connectors):
Driver Characteristics:
(Refer to Table 7-9)
Upstream facing port and
downstream facing ports
configured as full-speed
Hub Differential Data Delay:
Note 7, 8
(with cable)
(without cable)
THDD1
THDD2
Hub Differential Driver Jitter:
(including cable)
To Next Transition
For Paired Transitions
Figure 7-52A
Figure 7-52B
70
44
ns
ns
-3
-1
3
1
ns
ns
Note 7, 8; Figure 7-52,
Section 7.1.14
THDJ1
THDJ2
Data Bit Width Distortion after SOP
TFSOP
Note 8; Figure 7-52
-5
5
ns
Hub EOP Delay Relative to THDD
TFEOPD
Note 8; Figure 7-53
0
15
ns
Hub EOP Output Width Skew
TFHESK
Note 8; Figure 7-53
-15
15
ns
300
ns
Low-speed Hub Characteristics (as measured at connectors):
Driver Characteristics:
(Refer to Table 7-10)
Hub Differential Data Delay
Downstream facing ports
configured as low-speed
TLHDD
Hub Differential Driver Jitter
(including cable):
Note 7, 8; Figure 7-52
Note 7, 8; Figure 7-52
Downstream facing port :
To Next Transition
For Paired Transitions
TLDHJ1
TLDHJ2
-45
-15
45
15
ns
ns
TLUHJ1
TLUHJ2
-45
-45
45
45
ns
ns
Upstream facing port:
To Next Transition
For Paired Transitions
Data Bit Width Distortion after SOP
TLSOP
Note 8; Figure 7-52
-60
60
ns
Hub EOP Delay Relative to THDD
TLEOPD
Note 8; Figure 7-53
0
200
ns
Hub EOP Output Width Skew
TLHESK
Note 8; Figure 7-53
-300
+300
ns
183
Universal Serial Bus Specification Revision 2.0
Table 7-11. Hub/Repeater Electrical Characteristics (Continued)
Parameter
Symbol
Conditions
Min.
Max.
High-speed Hub Characteristics (as measured at connectors):
Driver Characteristics:
(Refer to Table 7-8)
Hub Data Delay (without cable):
Upstream facing port and
downstream facing ports
configured as high-speed
THSHDD
Hub Data Jitter:
Hub Delay Variation Range:
184
Section 7.1.14.2
36 highspeed bit
times +
4 ns
Specified by eye patterns
in Section 7.1.2.2
THSHDV
Section 7.1.14.2
5 highspeed bit
times
Units
Universal Serial Bus Specification Revision 2.0
Table 7-12. Cable Characteristics (Note 14)
Parameter
Symbol
Conditions
Min
Max
Units
VBUS Voltage drop for
detachable cables
VBUSD
Section 7.2.2
125
mV
GND Voltage drop (for all
cables)
VGNDD
Section 7.2.2
125
mV
Differential Cable Impedance
(full-/high-speed)
ZO
(90 Ω ±15%);
76.5
103.5
Ω
Common mode cable
impedance (full-/high-speed)
ZCM
(30 Ω ±30%);
21.0
39.0
Ω
26
18
ns
ns
100
ps
2
pF
Cable Delay (one way)
Full-/high-speed
Low-speed
Section 7.1.16
TFSCBL
TLSCBL
Cable Skew
TSKEW
Section 7.1.3
Unmated Contact Capacitance
CUC
Section 6.7
Cable loss
Specified by table and
graph in Section 7.1.17
Note 1:
Measured at A plug.
Note 2:
Measured at A receptacle.
Note 3:
Measured at B receptacle.
Note 4:
Measured at A or B connector.
Note 5:
Measured with RL of 1.425 kΩ to 3.6 V.
Note 6:
Measured with RL of 14.25 kΩ to GND.
Note 7:
Timing difference between the differential data signals.
Note 8:
Measured at crossover point of differential data signals.
Note 9:
The maximum load specification is the maximum effective capacitive load allowed that meets the target
VBUS drop of 330 mV.
Note 10: Excluding the first transition from the Idle state.
Note 11: The two transitions should be a (nominal) bit time apart.
Note 12: For both transitions of differential signaling.
Note 13: Must accept as valid EOP.
Note 14: Single-ended capacitance of D+ or D- is the capacitance of D+/D- to all other conductors and, if present,
shield in the cable. That is, to measure the single-ended capacitance of D+, short D-, VBUS, GND, and
the shield line together and measure the capacitance of D+ to the other conductors.
Note 15: For high power devices (non-hubs) when enabled for remote wakeup.
185
Universal Serial Bus Specification Revision 2.0
Table 7-13. Hub Event Timings
Event Description
Symbol
Time to detect a downstream
facing port connect event
Awake Hub
Suspended Hub
TDCNN
Time to detect a disconnect event
at a hub’s downstream facing port
TDDIS
Duration of driving resume to a
downstream port; only from a
controlling hub
Conditions
Min
Unit
Section 11.5 and
Section 7.1.7.3
2.5
2.5
TDRSMDN
Max
Section 7.1.7.3
Nominal; Section
7.1.7.7 and
Section 11.5
2
2000
12000
2.5
20
µs
µs
µs
ms
Time from detecting downstream
resume to rebroadcast
TURSM
Section 7.1.7.7
1.0
Duration of driving reset to a
downstream facing port
TDRST
Only for a
SetPortFeature
(PORT_RESET)
request;
Section 7.1.7.5 and
Section 11.5
10
Overall duration of driving reset to
downstream facing port, root hub
TDRSTR
Only for root hubs;
Section 7.1.7.5
50
Maximum interval between reset
segments used to create TDRSTR
TRHRSI
Only for root hubs;
each reset pulse must
be of length TDRST;
Section 7.1.7.5
Time to detect a long K from
upstream
TURLK
Section 11.6
Time to detect a long SE0 from
upstream
TURLSE0
Section 11.6
Duration of repeating SE0
upstream (for low-/full-speed
repeater)
TURPSE0
Section 11.6
23
FS bit
times
Duration of sending SE0 upstream
after EOF1 (for low-/full-speed
repeater)
TUDEOP
Optional
Section 11.6
2
FS bit
times
20
ms
ms
ms
3
ms
2.5
100
µs
2.5
10000
µs
Inter-packet Delay (for highspeed) for packets traveling in
same direction
THSIPDSD
Section 7.1.18.2
88
bit
times
Inter-packet Delay (for highspeed) for packets traveling in
opposite direction
THSIPDOD
Section 7.1.18.2
8
bit
times
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Universal Serial Bus Specification Revision 2.0
Table 7-13. Hub Event Timings (Continued)
Event Description
Symbol
Inter-packet delay for device/root
hub response w/detachable cable
for high-speed
THSRSPIPD1
Conditions
Min
Section 7.1.18.2
Max
192
Unit
bit
times
Reset Handshake Protocol:
Time for which a Chirp J or Chirp
K must be continuously detected
(filtered) by hub or device during
Reset handshake
TFILT
Section 7.1.7.5
Time after end of device Chirp K
by which hub must start driving
first Chirp K in the hub’s chirp
sequence
TWTDCH
Section 7.1.7.5
Time for which each individual
Chirp J or Chirp K in the chirp
sequence is driven downstream
by hub during reset
TDCHBIT
Section 7.1.7.5
Time before end of reset by which
a hub must end its downstream
chirp sequence
TDCHSE0
Section 7.1.7.5
µs
2.5
100
µs
40
60
µs
100
500
µs
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Universal Serial Bus Specification Revision 2.0
Table 7-14. Device Event Timings
Parameter
Symbol
Time from internal power good to
device pulling D+/D- beyond VIHZ
(min) (signaling attach)
TSIGATT
Debounce interval provided by
USB system software after attach
TATTDB
Conditions
Min
Max
Units
100
ms
100
ms
10
ms
1
s
Figure 7-29
Figure 7-29
Maximum time a device can draw
power >suspend power when bus
is continuously in idle state
T2SUSP
Section 7.1.7.6
Maximum duration of suspend
averaging interval
TSUSAVGI
Section 7.2.3
Period of idle bus before device
can initiate resume
TWTRSM
Device must be
remote-wakeup
enabled
Section 7.1.7.5
5
Duration of driving resume
upstream
TDRSMUP
Section 7.1.7.7
1
Resume Recovery Time
TRSMRCY
Provided by USB
System Software;
Section 7.1.7.7
Time to detect a reset from
upstream for non high-speed
capable devices
TDETRST
Section 7.1.7.5
Reset Recovery Time
TRSTRCY
Section 7.1.7.5
Inter-packet Delay (for low-/fullspeed)
TIPD
Section 7.1.18
Inter-packet delay for device
response w/detachable cable for
low-/full-speed
TRSPIPD1
Section 7.1.18
6.5
bit
times
Inter-packet delay for device
response w/captive cable for low/full-speed
TRSPIPD2
Section 7.1.18
7.5
bit
times
188
ms
15
10
2.5
ms
ms
10000
10
2
µs
ms
bit
times
Universal Serial Bus Specification Revision 2.0
Table 7-14. Device Event Timings (Continued)
Parameter
SetAddress() Completion Time
Symbol
Conditions
Min
Max
Units
TDSETADDR
Section 9.2.6.3
50
ms
TDRQCMPLTND
Section 9.2.6.4
50
ms
Time to deliver first and
subsequent (except last) data for
standard request
TDRETDATA1
Section 9.2.6.4
500
ms
Time to deliver last data for
standard request
TDRETDATAN
Section 9.2.6.4
50
ms
Time to complete standard
request with no data
Inter-packet delay for device
response w/captive cable (highspeed)
THSRSPIPD2
Section 7.1.18.2
192 bit times
+ 52 ns
SetAddress() Completion Time
TDSETADDR
Section 9.2.6.3
50
ms
Time to complete standard
request with no data
TDRQCMPLTND
Section 9.2.6.4
50
ms
Time for which a suspended highspeed capable device must see a
continuous SE0 before beginning
the high-speed detection
handshake
TFILTSE0
Section 7.1.7.5
2.5
Time a high-speed capable device
operating in non-suspended fullspeed must wait after start of SE0
before beginning the high-speed
detection handshake
TWTRSTFS
Section 7.1.7.5
2.5
Time a high-speed capable device
operating in high-speed must wait
after start of SE0 before reverting
to full-speed
TWTREV
Section 7.1.7.5
3.0
Time a device must wait after
reverting to full-speed before
sampling the bus state for SE0
and beginning the high-speed
detection handshake
TWTRSTHS
Section 7.1.7.5
Reset Handshake Protocol:
100
µs
3000
3.125
875
µs
ms
µs
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Universal Serial Bus Specification Revision 2.0
Table 7-14. Device Event Timings (Continued)
Parameter
Symbol
Conditions
Minimum duration of a Chirp K
from a high-speed capable device
within the reset protocol
TUCH
Section 7.1.7.5
Time after start of SE0 by which a
high-speed capable device is
required to have completed its
Chirp K within the reset protocol
TUCHEND
Section 7.1.7.5
Time between detection of
downstream chirp and entering
high-speed state
TWTHS
Section 7.1.7.5
Time after end of upstream chirp
at which device reverts to fullspeed default state if no
downstream chirp is detected
TWTFS
Section 7.1.7.5
190
Min
Max
1.0
ms
7.0
500
1.0
Units
2.5
ms
µs
ms
Universal Serial Bus Specification Revision 2.0
7.3.3 Timing Waveforms
TPERIOD
Differential
Data Lines
Crossover
Points
Consecutive
Transitions
N * TPERIOD + TxDJ1
Paired
Transitions
N * TPERIOD + TxDJ2
Figure 7-49. Differential Data Jitter for Low-/full-speed
TPERIOD
Differential
Data Lines
Crossover Point
Extended
Crossover
Point
Diff. Data-toSE0 Skew
N * TPERIOD + TxDEOP
Source EOP Width:
TFEOPT
TLEOPT
Receiver EOP Width: TFEOPR,
TLEOPR
Figure 7-50. Differential-to-EOP Transition Skew and EOP Width for Low-/full-speed
TPERIOD
Differential
Data Lines
TxJR
TxJR1
TxJR2
Consecutive
Transitions
N * TPERIOD + TxJR1
Paired
Transitions
N * TPERIOD + TxJR2
Figure 7-51. Receiver Jitter Tolerance for Low-/full-speed
TPERIOD is the data rate of the receiver that can have the range as defined in Section 7.1.11.
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Universal Serial Bus Specification Revision 2.0
Upstream
End of
Cable
Crossover
Point
Upstream
Port of hub
50% Point of
Initial Swing
VSS
VSS
Downstream
Port of hub
Hub Delay
Downstream
THDD1
Hub Delay
Downstream
THDD2
Downstream
Port of hub
50% Point of
Initial Swing
VSS
VSS
A. Downstream Hub Delay with Cable
Downstream
Port of hub
B. Downstream Hub Delay without Cable
Crossover
Point
VSS
Upstream
Port or End
of Cable
Hub Delay
Upstream
THDD1
THDD2
Crossover
Point
VSS
C. Upstream Hub Delay with or without Cable
Hub Differential Jitter:
T HDJ1 = T HDDx(J) - T HDDx(K) or T HDDx(K) - T HDDx(J) Consecutive Transitions
T HDJ2 = T HDDx(J) - T HDDx(J) or T HDDx(K) - T HDDx(K) Paired Transitions
Bit after SOP Width Distortion (same as data jitter for SOP and next J transition):
T FSOP = T HDDx(next J) - T HDDx(SOP)
Low-speed timings are determined in the same way for:
T LHDD, T LDHJ1, T LDJH2, T LUHJ1, TLUJH2, and T LSOP
Figure 7-52. Hub Differential Delay, Differential Jitter, and SOP Distortion for Low-/full-speed
Measurement locations referenced in Figure 7-52 and Figure 7-53 are specified in Figure 7-38.
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Universal Serial Bus Specification Revision 2.0
Upstream
End of
Cable
VSS
50% Point of
Initial Swing
Crossover
Point
Extended
Upstream
Port of hub
TEOP-
VSS
TEOP+
Downstream
Port of hub
TEOP-
Downstream
Port of hub
TEOP
VSS
VSS
A. Downstream EOP Delay with Cable
B. Downstream EOP Delay without Cable
Crossover
Point
Extended
Downstream
Port
VSS
TEOPUpstream
Port or
End of Cable
TEOP+
Crossover
Point
Extended
VSS
C. Upstream EOP Delay with or Without Cable
EOP Delay:
TFEOPD = TEOPy - THDDx
(TEOPy means that this equation applies to TEOP- and TEOP+)
EOP Skew:
TFHESK = TEOP+ - TEOPLow-speed timings are determined in the same way for:
TLEOPD and TLHESK
Figure 7-53. Hub EOP Delay and EOP Skew for Low-/full-speed
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Universal Serial Bus Specification Revision 2.0
Chapter 8
Protocol Layer
This chapter presents a bottom-up view of the USB protocol, starting with field and packet definitions. This
is followed by a description of packet transaction formats for different transaction types. Link layer flow
control and transaction level fault recovery are then covered. The chapter finishes with a discussion of retry
synchronization, babble, loss of bus activity recovery, and high-speed PING protocol.
8.1 Byte/Bit Ordering
Bits are sent out onto the bus least-significant bit (LSb) first, followed by the next LSb, through to the mostsignificant bit (MSb) last. In the following diagrams, packets are displayed such that both individual bits
and fields are represented (in a left to right reading order) as they would move across the bus.
Multiple byte fields in standard descriptors, requests, and responses are interpreted as and moved over the
bus in little-endian order, i.e., LSB to MSB.
8.2 SYNC Field
All packets begin with a synchronization (SYNC) field, which is a coded sequence that generates a
maximum edge transition density. It is used by the input circuitry to align incoming data with the local
clock. A SYNC from an initial transmitter is defined to be eight bits in length for full/low-speed and 32 bits
for high-speed. Received SYNC fields may be shorter as described in Chapter 7. SYNC serves only as a
synchronization mechanism and is not shown in the following packet diagrams (refer to Section 7.1.10).
The last two bits in the SYNC field are a marker that is used to identify the end of the SYNC field and, by
inference, the start of the PID.
8.3 Packet Field Formats
Field formats for the token, data, and handshake packets are described in the following section. Packet bit
definitions are displayed in unencoded data format. The effects of NRZI coding and bit stuffing have been
removed for the sake of clarity. All packets have distinct Start- and End-of-Packet delimiters. The Start-ofPacket (SOP) delimiter is part of the SYNC field, and the End-of-Packet (EOP) delimiter is described in
Chapter 7.
8.3.1 Packet Identifier Field
A packet identifier (PID) immediately follows the SYNC field of every USB packet. A PID consists of a
four-bit packet type field followed by a four-bit check field as shown in Figure 8-1. The PID indicates the
type of packet and, by inference, the format of the packet and the type of error detection applied to the
packet. The four-bit check field of the PID ensures reliable decoding of the PID so that the remainder of the
packet is interpreted correctly. The PID check field is generated by performing a one’s complement of the
packet type field. A PID error exists if the four PID check bits are not complements of their respective
packet identifier bits.
(LSb)
PID
(MSb)
0
PID
1
PID
2
PID
3
PID
0
PID
1
PID
2
PID
3
Figure 8-1. PID Format
The host and all functions must perform a complete decoding of all received PID fields. Any PID received
with a failed check field or which decodes to a non-defined value is assumed to be corrupted and it, as well
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Universal Serial Bus Specification Revision 2.0
as the remainder of the packet, is ignored by the packet receiver. If a function receives an otherwise valid
PID for a transaction type or direction that it does not support, the function must not respond. For example,
an IN-only endpoint must ignore an OUT token. PID types, codings, and descriptions are listed in
Table 8-1.
Table 8-1. PID Types
PID Type
PID Name
Token
OUT
0001B
Address + endpoint number in host-to-function
transaction
IN
1001B
Address + endpoint number in function-to-host
transaction
SOF
0101B
Start-of-Frame marker and frame number
SETUP
1101B
Address + endpoint number in host-to-function
transaction for SETUP to a control pipe
DATA0
0011B
Data packet PID even
DATA1
1011B
Data packet PID odd
DATA2
0111B
Data packet PID high-speed, high bandwidth isochronous
transaction in a microframe (see Section 5.9.2 for more
information)
MDATA
1111B
Data packet PID high-speed for split and high bandwidth
isochronous transactions (see Sections 5.9.2, 11.20, and
11.21 for more information)
ACK
0010B
Receiver accepts error-free data packet
NAK
1010B
Receiving device cannot accept data or transmitting
device cannot send data
STALL
1110B
Endpoint is halted or a control pipe request is not
supported
NYET
0110B
No response yet from receiver (see Sections 8.5.1 and
11.17-11.21)
PRE
1100B
(Token) Host-issued preamble. Enables downstream bus
traffic to low-speed devices.
ERR
1100B
(Handshake) Split Transaction Error Handshake (reuses
PRE value)
SPLIT
1000B
(Token) High-speed Split Transaction Token (see
Section 8.4.2)
PING
0100B
Reserved
0000B
(Token) High-speed flow control probe for a bulk/control
endpoint (see Section 8.5.1)
Data
Handshake
Special
PID<3:0>*
Description
Reserved PID
*Note: PID bits are shown in MSb order. When sent on the USB, the rightmost bit (bit 0) will be sent first.
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Universal Serial Bus Specification Revision 2.0
PIDs are divided into four coding groups: token, data, handshake, and special, with the first two transmitted
PID bits (PID<0:1>) indicating which group. This accounts for the distribution of PID codes.
8.3.2 Address Fields
Function endpoints are addressed using two fields: the function address field and the endpoint field. A
function needs to fully decode both address and endpoint fields. Address or endpoint aliasing is not
permitted, and a mismatch on either field must cause the token to be ignored. Accesses to non-initialized
endpoints will also cause the token to be ignored.
8.3.2.1 Address Field
The function address (ADDR) field specifies the function, via its address, that is either the source or
destination of a data packet, depending on the value of the token PID. As shown in Figure 8-2, a total of
128 addresses are specified as ADDR<6:0>. The ADDR field is specified for IN, SETUP, and OUT tokens
and the PING and SPLIT special token. By definition, each ADDR value defines a single function. Upon
reset and power-up, a function’s address defaults to a value of zero and must be programmed by the host
during the enumeration process. Function address zero is reserved as the default address and may not be
assigned to any other use.
(MSb)
(LSb)
Addr
0
Addr
1
Addr
2
Addr
3
Addr
4
Addr
5
Addr
6
Figure 8-2. ADDR Field
8.3.2.2 Endpoint Field
An additional four-bit endpoint (ENDP) field, shown in Figure 8-3, permits more flexible addressing of
functions in which more than one endpoint is required. Except for endpoint address zero, endpoint numbers
are function-specific. The endpoint field is defined for IN, SETUP, and OUT tokens and the PING special
token. All functions must support a control pipe at endpoint number zero (the Default Control Pipe). Lowspeed devices support a maximum of three pipes per function: a control pipe at endpoint number zero plus
two additional pipes (either two control pipes, a control pipe and a interrupt endpoint, or two interrupt
endpoints). Full-speed and high-speed functions may support up to a maximum of 16 IN and OUT
endpoints.
(MSb)
(LSb)
Endp
0
Endp
1
Endp
2
Endp
3
Figure 8-3. Endpoint Field
8.3.3 Frame Number Field
The frame number field is an 11-bit field that is incremented by the host on a per-frame basis. The frame
number field rolls over upon reaching its maximum value of 7FFH and is sent only in SOF tokens at the
start of each (micro)frame.
8.3.4 Data Field
The data field may range from zero to 1,024 bytes and must be an integral number of bytes. Figure 8-4
shows the format for multiple bytes. Data bits within each byte are shifted out LSb first.
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Universal Serial Bus Specification Revision 2.0
(MSb)
(LSb)
D7
D0
D1
D2
D3
D4
D5
Byte N
Byte N-1
D6
(MSb)
(LSb)
D7
D0
Byte N+1
Figure 8-4. Data Field Format
Data packet size varies with the transfer type, as described in Chapter 5.
8.3.5 Cyclic Redundancy Checks
Cyclic redundancy checks (CRCs) are used to protect all non-PID fields in token and data packets. In this
context, these fields are considered to be protected fields. The PID is not included in the CRC check of a
packet containing a CRC. All CRCs are generated over their respective fields in the transmitter before bit
stuffing is performed. Similarly, CRCs are decoded in the receiver after stuffed bits have been removed.
Token and data packet CRCs provide 100% coverage for all single- and double-bit errors. A failed CRC is
considered to indicate that one or more of the protected fields is corrupted and causes the receiver to ignore
those fields and, in most cases, the entire packet.
For CRC generation and checking, the shift registers in the generator and checker are seeded with an allones pattern. For each data bit sent or received, the high order bit of the current remainder is XORed with
the data bit and then the remainder is shifted left one bit and the low-order bit set to zero. If the result of
that XOR is one, then the remainder is XORed with the generator polynomial.
When the last bit of the checked field is sent, the CRC in the generator is inverted and sent to the checker
MSb first. When the last bit of the CRC is received by the checker and no errors have occurred, the
remainder will be equal to the polynomial residual.
A CRC error exists if the computed checksum remainder at the end of a packet reception does not match the
residual.
Bit stuffing requirements must be met for the CRC, and this includes the need to insert a zero at the end of a
CRC if the preceding six bits were all ones.
8.3.5.1 Token CRCs
A five-bit CRC field is provided for tokens and covers the ADDR and ENDP fields of IN, SETUP, and
OUT tokens or the time stamp field of an SOF token. The PING and SPLIT special tokens also include a
five-bit CRC field. The generator polynomial is:
5
2
G(X) = X + X + 1
The binary bit pattern that represents this polynomial is 00101B. If all token bits are received without error,
the five-bit residual at the receiver will be 01100B.
8.3.5.2 Data CRCs
The data CRC is a 16-bit polynomial applied over the data field of a data packet. The generating
polynomial is:
16
15
2
G(X) = X + X + X + 1
The binary bit pattern that represents this polynomial is 1000000000000101B. If all data and CRC bits are
received without error, the 16-bit residual will be 1000000000001101B.
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Universal Serial Bus Specification Revision 2.0
8.4 Packet Formats
This section shows packet formats for token, data, and handshake packets. Fields within a packet are
displayed in these figures in the order in which bits are shifted out onto the bus.
8.4.1 Token Packets
Figure 8-5 shows the field formats for a token packet. A token consists of a PID, specifying either IN,
OUT, or SETUP packet type and ADDR and ENDP fields. The PING special token packet also has the
same fields as a token packet. For OUT and SETUP transactions, the address and endpoint fields uniquely
identify the endpoint that will receive the subsequent Data packet. For IN transactions, these fields uniquely
identify which endpoint should transmit a Data packet. For PING transactions, these fields uniquely
identify which endpoint will respond with a handshake packet. Only the host can issue token packets. An
IN PID defines a Data transaction from a function to the host. OUT and SETUP PIDs define Data
transactions from the host to a function. A PING PID defines a handshake transaction from the function to
the host.
(lsb)
(msb)
Field
PID
ADDR
ENDP
CRC5
Bits
8
7
4
5
Figure 8-5. Token Format
Token packets have a five-bit CRC that covers the address and endpoint fields as shown above. The CRC
does not cover the PID, which has its own check field. Token and SOF packets are delimited by an EOP
after three bytes of packet field data. If a packet decodes as an otherwise valid token or SOF but does not
terminate with an EOP after three bytes, it must be considered invalid and ignored by the receiver.
8.4.2 Split Transaction Special Token Packets
USB defines a special token for split transactions: SPLIT. This is a 4 byte token packet compared to other
normal 3 byte token packets. The split transaction token packet provides additional transaction types with
additional transaction specific information. The split transaction token is used to support split transactions
between the host controller communicating with a hub operating at high speed with full-/low-speed devices
to some of its downstream facing ports. There are two split transactions defined that use the SPLIT special
token: a start-split transaction (SSPLIT) and a complete-split transaction (CSPLIT). A field in the SPLIT
special token, described in the following sections, indicates the specific split transaction.
8.4.2.1 Split Transactions
A high-speed split transaction is used only between the host controller and a hub when the hub has full/low-speed devices attached to it. This high-speed split transaction is used to initiate a full-/low-speed
transaction via the hub and some full-/low-speed device endpoint. The high-speed split transaction also
allows the completion status of the full-/low-speed transaction to be retrieved from the hub. This approach
allows the host controller to start a full-/low-speed transaction via a high-speed transaction and then
continue with other high-speed transactions without having to wait for the full-/low-speed transaction to
proceed/complete at the slower speed. See Chapter 11 for more details about the state machines and
transaction definitions of split transactions.
A high-speed split transaction has two parts: a start-split and a complete-split. Split transactions are only
defined to be used between the host controller and a hub. No other high-speed or full-/low-speed devices
ever use split transactions.
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Figure 8-6 shows the packets composing a generic start-split transaction. There are two packets in the token
phase: the SPLIT special token and a full-/low-speed token. Depending on the direction of data transfer and
whether a handshake is defined for the transaction type, the token phase is optionally followed by a data
packet and a handshake packet. Start split transactions can consist of 2, 3, or 4 packets as determined by the
specific transfer type and data direction.
SSPLIT
Token
FS/LS Token
DATAx
Handshake
Token Phase
Figure 8-6. Packets in a Start-split Transaction
Figure 8-7 shows the packets composing a generic complete-split transaction. There are two packets in the
token phase: the SPLIT special token and a full-/low-speed token. A data or handshake packet follows the
token phase packets in the complete-split depending on the data transfer direction and specific transaction
type. Complete split transactions can consist of 2 or 3 packets as determined by the specific transfer type
and data direction.
DATAx
CSPLIT
Token
FS/LS Token
or
Handshake
Token Phase
Figure 8-7. Packets in a Complete-split Transaction
The results of a split transaction are returned by a complete-split transaction. Figure 8-8 shows this
conceptual “conversion” for an example interrupt IN transfer type. The host issues a start-split (indicated
with 1) to the hub and then can proceed with other high-speed transactions. The start-split causes the hub to
issue a full-/low-speed IN token sometime later (indicated by 2). The device responds to the IN token (in
this example) with a data packet and the hub responds with a handshake to the device. Finally, the host
sometime later issues a complete-split (indicated by 3) to retrieve the data provided by the device. Note that
in the example, the hub provided the full-/low-speed handshake (ACK in this example) to the device
endpoint before the complete-split, and the complete-split did not provide a high-speed handshake to the
hub.
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Universal Serial Bus Specification Revision 2.0
1
Start
Split
2
SSPLIT
Full/Low-Speed
IN Token
Host
Hub
CSPLIT
3
Complete
Split
IN Token
Data0
Device
ACK
IN Token
Data0
High-Speed
Bus
Full-/Low-Speed
Bus
Figure 8-8. Relationship of Interrupt IN Transaction to High-speed Split Transaction
A normal full-/low-speed OUT transaction is similarly conceptually “converted” into start-split and
complete-split transactions. Figure 8-9 shows this “conversion” for an example interrupt OUT transfer
type. The host issues a start-split transaction consisting of a SSPLIT special token, an OUT token, and
a DATA packet. The hub sometime later issues the OUT token and DATA packet on the full-/lowspeed bus. The device responds with a handshake. Sometime later, the host issues the complete-split
transaction and the hub responds with the results (either full-/low-speed data or handshake) provided by
the device.
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Universal Serial Bus Specification Revision 2.0
1
Start
Split
SSPLIT
2
OUT Token
Full/Low-speed
Data0
OUT Token
Host
Hub
CSPLIT
3
Device
Data0
ACK
OUT Token
Complete
Split
ACK
High-Speed
Bus
Full-/Low-Speed
Bus
Figure 8-9. Relationship of Interrupt OUT Transaction to High-speed Split OUT Transaction
The next two sections describe the fields composing the detailed start- and complete-split token packets.
Figure 8-10 and Figure 8-12 show the fields in the split-transaction token packet. The SPLIT special token
follows the general token format and starts with a PID field (after a SYNC) and ends with a CRC5 field
(and EOP). Start-split and complete-split token packets are both 4 bytes long. SPLIT transactions must
only originate from the host. The start-split token is defined in Section 8.4.2.2 and the complete-split token
is defined in Section 8.4.2.3.
8.4.2.2 Start-Split Transaction Token
(lsb)
Field
Bits
SPLIT Hub SC Port S E ET
PID Addr
8
7
1 7 1 1 2
(msb)
CRC5
5
Figure 8-10. Start-split (SSPLIT) Token
The Hub addr field contains the USB device address of the hub supporting the specified full-/low-speed
device for this full-/low-speed transaction. This field has the same definition as the ADDR field definition
in Section 8.3.2.1.
A SPLIT special token packet with the SC (Start/Complete) field set to zero indicates that this is a start-split
transaction (SSPLIT).
The Port field contains the port number of the target hub for which this full-/low-speed transaction is
destined. As shown in Figure 8-11, a total of 128 ports are specified as PORT<6:0>. The host must
correctly set the port field for single and multiple TT hub implementations. A single TT hub
implementation may ignore the port field.
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(LSb)
Port
(MSb)
0
Port
1
Port
2
Port
3
Port
4
Port
5
Port
6
Figure 8-11. Port Field
The S (Speed) field specifies the speed for this interrupt or control transaction as follows:
•
0 – Full speed
•
1 – Low speed
For bulk IN/OUT and isochronous IN start-splits, the S field must be set to zero. For bulk/control IN/OUT,
interrupt IN/OUT, and isochronous IN start-splits, the E field must be set to zero.
1
For full-speed isochronous OUT start-splits, the S (Start) and E (End) fields specify how the high-speed
data payload corresponds to data for a full-speed data packet as shown in Table 8-2.
Table 8-2. Isochronous OUT Payload Continuation Encoding
S
E
High-speed to Full-speed Data Relation
0
0
High-speed data is the middle of the fullspeed data payload
0
1
High-speed data is the end of the full-speed
data payload
1
0
High-speed data is the beginning of the fullspeed data payload
1
1
High-speed data is all of the full-speed data
payload.
Isochronous OUT start-split transactions use these encodings to allow the hub to detect various error cases
due to lack of receiving start-split transactions for an endpoint with a data payload that requires multiple
start-splits. For example, a large full-speed data payload may require three start-split transactions: a startsplit/beginning, a start-split/middle and a start-split/end. If any of these transactions is not received by the
hub, it will either ignore the full-speed transaction (if the start-split/beginning is not received), or it will
force an error for the corresponding full-speed transaction (if one of the other two transactions are not
received). Other error conditions can be detected by not receiving a start-split during a microframe.
The ET (Endpoint Type) field specifies the endpoint type of the full-/low-speed transaction as shown in
Table 8-3.
1
The S bit can be reused for these encodings since isochronous transactions must not be low speed.
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Table 8-3. Endpoint Type Values in Split Special Token
ET value
(msb:lsb)
Endpoint
Type
00
Control
01
Isochronous
10
Bulk
11
Interrupt
This field tells the hub which split transaction state machine to use for this full-/low-speed transaction.
The full-/low-speed device address and endpoint number information is contained in the normal token
packet that follows the SPLIT special token packet.
8.4.2.3 Complete-Split Transaction Token
(lsb)
(msb)
Field SPLIT Hub
PID Addr
Bits
8
7
SC Port S U ET
1
7
1 1 2
CRC5
5
Figure 8-12. Complete-split (CSPLIT) Transaction Token
A SPLIT special token packet with the SC field set to one indicates that this is a complete-split transaction
(CSPLIT).
The U bit is reserved/unused and must be reset to zero(0B).
The other fields of the complete-split token packet have the same definitions as for the start-split token
packet.
8.4.3 Start-of-Frame Packets
Start-of-Frame (SOF) packets are issued by the host at a nominal rate of once every 1.00 ms ±0.0005 ms for
a full-speed bus and 125 µs ±0.0625 µs for a high-speed bus. SOF packets consist of a PID indicating
packet type followed by an 11-bit frame number field as illustrated in Figure 8-13.
(lsb)
(msb)
Field
PID
FrameNumber
CRC5
Bits
8
11
5
Figure 8-13. SOF Packet
The SOF token comprises the token-only transaction that distributes an SOF marker and accompanying
frame number at precisely timed intervals corresponding to the start of each frame. All high-speed and fullspeed functions, including hubs, receive the SOF packet. The SOF token does not cause any receiving
function to generate a return packet; therefore, SOF delivery to any given function cannot be guaranteed.
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The SOF packet delivers two pieces of timing information. A function is informed that an SOF has
occurred when it detects the SOF PID. Frame timing sensitive functions, that do not need to keep track of
frame number (e.g., a full-speed operating hub), need only decode the SOF PID; they can ignore the frame
number and its CRC. If a function needs to track frame number, it must comprehend both the PID and the
time stamp. Full-speed devices that have no particular need for bus timing information may ignore the SOF
packet.
8.4.3.1 USB Frames and Microframes
USB defines a full-speed 1 ms frame time indicated by a Start Of Frame (SOF) packet each and every 1ms
period with defined jitter tolerances. USB also defines a high-speed microframe with a 125 µs frame time
with related jitter tolerances (See Chapter 7). SOF packets are generated (by the host controller or hub
transaction translator) every 1ms for full-speed links. SOF packets are also generated after the next seven
125 µs periods for high-speed links.
Figure 8-14 shows the relationship between microframes and frames.
Full / Low-Speed Frame Size (1 ms)
1 ms
1 ms
Full-Speed USB Frame Ticks
Full-Speed Isochronous Data Payload
High-Speed Micro-Frames (125 us)
USB 2.0 Micro-Frame Ticks
(1/8th Full-Speed Frame)
High-Speed Isochronous Data Payload
Figure 8-14. Relationship between Frames and Microframes
High-speed devices see an SOF packet with the same frame number eight times (every 125 µs) during each
1 ms period. If desired, a high-speed device can locally determine a particular microframe “number” by
detecting the SOF that had a different frame number than the previous SOF and treating that as the zeroth
microframe. The next seven SOFs with the same frame number can be treated as microframes 1 through 7.
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8.4.4 Data Packets
A data packet consists of a PID, a data field containing zero or more bytes of data, and a CRC as shown in
Figure 8-15. There are four types of data packets, identified by differing PIDs: DATA0, DATA1, DATA2
and MDATA. Two data packet PIDs (DATA0 and DATA1) are defined to support data toggle
synchronization (refer to Section 8.6). All four data PIDs are used in data PID sequencing for high
bandwidth high-speed isochronous endpoints (refer to Section 5.9). Three data PIDs (MDATA, DATA0,
DATA1) are used in split transactions (refer to Sections 11.17-11.21).
(lsb)
(msb)
Field
PID
DATA
CRC16
Bits
8
0-8192
16
Figure 8-15. Data Packet Format
Data must always be sent in integral numbers of bytes. The data CRC is computed over only the data field
in the packet and does not include the PID, which has its own check field.
The maximum data payload size allowed for low-speed devices is 8 bytes. The maximum data payload size
for full-speed devices is 1023. The maximum data payload size for high-speed devices is 1024 bytes.
8.4.5 Handshake Packets
Handshake packets, as shown in Figure 8-16, consist of only a PID. Handshake packets are used to report
the status of a data transaction and can return values indicating successful reception of data, command
acceptance or rejection, flow control, and halt conditions. Only transaction types that support flow control
can return handshakes. Handshakes are always returned in the handshake phase of a transaction and may be
returned, instead of data, in the data phase. Handshake packets are delimited by an EOP after one byte of
packet field. If a packet decodes as an otherwise valid handshake but does not terminate with an EOP after
one byte, it must be considered invalid and ignored by the receiver.
(lsb) (msb)
Field
PID
Bits
8
Figure 8-16. Handshake Packet
There are four types of handshake packets and one special handshake packet:
206
•
ACK indicates that the data packet was received without bit stuff or CRC errors over the data field and
that the data PID was received correctly. ACK may be issued either when sequence bits match and the
receiver can accept data or when sequence bits mismatch and the sender and receiver must
resynchronize to each other (refer to Section 8.6 for details). An ACK handshake is applicable only in
transactions in which data has been transmitted and where a handshake is expected. ACK can be
returned by the host for IN transactions and by a function for OUT, SETUP, or PING transactions.
•
NAK indicates that a function was unable to accept data from the host (OUT) or that a function has no
data to transmit to the host (IN). NAK can only be returned by functions in the data phase of IN
transactions or the handshake phase of OUT or PING transactions. The host can never issue NAK.
Universal Serial Bus Specification Revision 2.0
NAK is used for flow control purposes to indicate that a function is temporarily unable to transmit or
receive data, but will eventually be able to do so without need of host intervention.
•
STALL is returned by a function in response to an IN token or after the data phase of an OUT or in
response to a PING transaction (see Figure 8-30 and Figure 8-38). STALL indicates that a function is
unable to transmit or receive data, or that a control pipe request is not supported. The state of a
function after returning a STALL (for any endpoint except the default endpoint) is undefined. The host
is not permitted to return a STALL under any condition.
The STALL handshake is used by a device in one of two distinct occasions. The first case, known as
“functional stall,” is when the Halt feature associated with the endpoint is set. (The Halt feature is
specified in Chapter 9 of this document.) A special case of the functional stall is the “commanded
stall.” Commanded stall occurs when the host explicitly sets the endpoint’s Halt feature, as detailed in
Chapter 9. Once a function’s endpoint is halted, the function must continue returning STALL until the
condition causing the halt has been cleared through host intervention.
The second case, known as “protocol stall,” is detailed in Section 8.5.3. Protocol stall is unique to
control pipes. Protocol stall differs from functional stall in meaning and duration. A protocol STALL
is returned during the Data or Status stage of a control transfer, and the STALL condition terminates at
the beginning of the next control transfer (Setup). The remainder of this section refers to the general
case of a functional stall.
•
NYET is a high-speed only handshake that is returned in two circumstances. It is returned by a highspeed endpoint as part of the PING protocol described later in this chapter. NYET may also be
returned by a hub in response to a split-transaction when the full-/low-speed transaction has not yet
been completed or the hub is otherwise not able to handle the split-transaction. See Chapter 11 for
more details.
•
ERR is a high-speed only handshake that is returned to allow a high-speed hub to report an error on a
full-/low-speed bus. It is only returned by a high-speed hub as part of the split transaction protocol.
See Chapter 11 for more details.
8.4.6 Handshake Responses
Transmitting and receiving functions must return handshakes based upon an order of precedence detailed in
Table 8-4 through Table 8-6. Not all handshakes are allowed, depending on the transaction type and
whether the handshake is being issued by a function or the host. Note that if an error occurs during the
transmission of the token to the function, the function will not respond with any packets until the next token
is received and successfully decoded.
8.4.6.1 Function Response to IN Transactions
Table 8-4 shows the possible responses a function may make in response to an IN token. If the function is
unable to send data, due to a halt or a flow control condition, it issues a STALL or NAK handshake,
respectively. If the function is able to issue data, it does so. If the received token is corrupted, the function
returns no response.
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Table 8-4. Function Responses to IN Transactions
Token Received
Corrupted
Function Tx
Endpoint Halt
Feature
Function Can
Transmit Data
Action Taken
Yes
Don’t care
Don’t care
Return no response
No
Set
Don’t care
Issue STALL handshake
No
Not set
No
Issue NAK handshake
No
Not set
Yes
Issue data packet
8.4.6.2 Host Response to IN Transactions
Table 8-5 shows the host response to an IN transaction. The host is able to return only one type of
handshake: ACK. If the host receives a corrupted data packet, it discards the data and issues no response.
If the host cannot accept data from a function, (due to problems such as internal buffer overrun) this
condition is considered to be an error and the host returns no response. If the host is able to accept data and
the data packet is received error-free, the host accepts the data and issues an ACK handshake.
Table 8-5. Host Responses to IN Transactions
Data Packet
Corrupted
Host Can
Accept Data
Handshake Returned by Host
Yes
N/A
Discard data, return no response
No
No
Discard data, return no response
No
Yes
Accept data, issue ACK
8.4.6.3 Function Response to an OUT Transaction
Handshake responses for an OUT transaction are shown in Table 8-6. Assuming successful token decode, a
function, upon receiving a data packet, may return any one of the three handshake types. If the data packet
was corrupted, the function returns no handshake. If the data packet was received error-free and the
function’s receiving endpoint is halted, the function returns STALL. If the transaction is maintaining
sequence bit synchronization and a mismatch is detected (refer to Section 8.6 for details), then the function
returns ACK and discards the data. If the function can accept the data and has received the data error-free,
it returns ACK. If the function cannot accept the data packet due to flow control reasons, it returns NAK.
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Table 8-6. Function Responses to OUT Transactions in Order of Precedence
Data Packet
Corrupted
Receiver
Halt
Feature
Sequence Bits
Match
Function Can
Accept Data
Handshake Returned
by Function
Yes
N/A
N/A
N/A
None
No
Set
N/A
N/A
STALL
No
Not set
No
N/A
ACK
No
Not set
Yes
Yes
ACK
No
Not set
Yes
No
NAK
8.4.6.4 Function Response to a SETUP Transaction
SETUP defines a special type of host-to-function data transaction that permits the host to initialize an
endpoint’s synchronization bits to those of the host. Upon receiving a SETUP token, a function must accept
the data. A function may not respond to a SETUP token with either STALL or NAK, and the receiving
function must accept the data packet that follows the SETUP token. If a non-control endpoint receives a
SETUP token, it must ignore the transaction and return no response.
8.5 Transaction Packet Sequences
The packets that comprise a transaction varies depending on the endpoint type. There are four endpoint
types: bulk, control, interrupt, and isochronous.
A host controller and device each require different state machines to correctly sequence each type of
transaction. Figures in the following sections show state machines that define the correct sequencing of
packets within a transaction of each type. The diagrams should not be taken as a required implementation,
but to specify the required behavior.
Figure 8-17 shows the legend for the state machine diagrams. A circle with a three-line border indicates a
reference to another (hierarchical) state machine. A circle with a two-line border indicates an initial state.
A circle with a single-line border represents a simple state.
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State
Hierarchy
- Contains other state machines
Initial
State
- Initial state of a state machine
State
- State in a state machine
- Entry and exit of state machine
&
Condition
Actions
- Joint used to connect transitions
- Transition: taken when condition
is true and performs actions
Figure 8-17. Legend for State Machines
The “tab” shapes with arrows are the entry or exit (respectively in the legend) to/from the state machine.
The entry/exit relates to another state in a state machine at a higher level in the state machine hierarchy.
A diamond (joint) is used to join several transitions to a common point. A joint allows a single input
transition with multiple output transitions or multiple input transitions and a single output transition. All
conditions on the transitions of a path involving a joint must be true for the path to be taken. A path is
simply a sequence of transitions involving one or more joints.
A transition is labeled with a block with a line in the middle separating the (upper) condition and the (lower)
actions. The condition is required to be true to take the transition. The syntax for actions and conditions is
VHDL. The actions are performed if the transition is taken. A circle includes a name in bold and
optionally one or more actions that are performed upon entry to the state.
The host controller and device state machines are in a context as shown in Figure 8-18. The host controller
determines the next transaction to run for an endpoint and issues a command (HC_cmd) to the host
controller state machines. This causes the host controller state machines to issue one or more packets to
move over the downstream bus (HSD1).
The device receives these packets from the bus (HSD2), reacts to the received packet, and interacts with its
function(s) via the state of the corresponding endpoint (in the EP_array). Then the device may respond with
a packet on the upstream bus (HSU1). The host controller state machines can receive a packet from the bus
(HSU2) and provide a result of the transaction back to the host controller (HC_resp). The details of what
packets are sent on the bus is determined by the transfer type for the endpoint and what bus activity the state
machines observe.
The state machines are presented in a hierarchical form. Figure 8-19 shows the top level state machines for
the host controller. The non-split transactions are presented in the remainder of this chapter. The split
transaction state machines (HC_Do_start and HC_Do_complete) are described and shown in Chapter 11.
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Transaction
commands
HC_cmd
Transaction
Results
Host
Controller
HC_resp
Host state machines
HSD1
HSU2
Downstream
Bus
Upstream
Bus
HSD2
HSU1
Device state machines
Ep array
Device
Functions
Figure 8-18. State Machine Context Overview
HC_Process_command
HC_Do_start
HC_Do_complete
HC_Do_nonsplit
Figure 8-19. Host Controller Top Level Transaction State Machine Hierarchy Overview
The host controller state machines are located in the host controller. The host controller causes packets to
be issued downstream (labeled as HSD1) and it receives upstream packets (labeled as HSU2).
The device state machines are located in the device. The device causes packets to be issued upstream
(labeled as HSU1) and it receives downstream packets (labeled as HSD2).
The host controller has commands that tell it what transaction to issue next for an endpoint. The host
controller tracks transactions for several endpoints. The host controller state machines sequence to
determine what the host controller needs to do next for the current endpoint. The device has a state for each
of its endpoints. The device state machines sequence to determine what reaction the device has to a
transaction.
The appendix includes some declarations that were used in constructing the state machines and may be
useful in understanding additional details of the state machines. There are several pseudo-code procedures
and functions for conditions and actions. Simple descriptions of them are also included in the appendix.
Figure 8-20 shows an overview of the overall state machine hierarchy for the host controller for the nonsplit transaction types. Figure 8-21 shows the hierarchy of the device state machines. The state machines
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common to endpoint types are presented first. The lowest level endpoint type specific state machines are
presented in each following endpoint type section.
HC_Do_nonsplit
HC_HS_BCO
HC_Do_BCINTO
HC_Do_BCINTI
HC_Do_IsochO
HC_Do_IsochI
Figure 8-20. Host Controller Non-split Transaction State Machine Hierarchy Overview
Device_Process_trans
Dev_do_OUT
Dev_Do_IsochO
Dev_Do_BCINTO
Dev_HS_BCO
Dev_do_IN
Dev_Do_IsochI
Dev_Do_BCINTI
Dev_HS_ping
Figure 8-21. Device Transaction State Machine Hierarchy Overview
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Global Actions
Concurrent Statements
Architecture Declarations
Signals Status
SIGNAL SCOPE
hsu1
OUT
device INT
token
INT
Package List
ieee
std_logic_1164
ieee
numeric_std
usb2statemachines behav_package
Packet_ready(HSD2)
No_packet
Wait_for_packet(
HSD2, none);
State Register Statements
DEFAULT
(BULK,NAK,0,0,ok,in_dir,TRUE,ALLDATA,FALSE,FA
’0’
Process Declarations
’0’
Device_process_Trans
Save(HSD2, token);
Figure 8-22. Device Top Level State Machine
token.PID /= tokenOUT and
token.PID /= tokenIN and
token.PID /= tokenSETUP and
token.PID /= ping and
(token.PID = ping and
not device.HS)
Device_do_OUT
token.PID = tokenOUT or
token.PID = tokenSETUP
token.PID = tokenIN
Device_do_IN
device.HS and
token.PID = ping
Dev_HS_ping
Device_process_trans
Figure 8-23. Device_process_Trans State Machine
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token.PID = tokenSETUP and
device.ep(token.endpt).ep_type /= control
(token.PID = tokenSETUP and
device.ep(token.endpt).ep_type = control) or
token.PID = tokenOUT
Dev_Do_IsochO
device.ep(token.endpt).ep_type = isochronous
&
(not device.HS and
(device.ep(token.endpt).ep_type = bulk or
device.ep(token.endpt).ep_type = control)) or
device.ep(token.endpt).ep_type = interrupt
Dev_Do_BCINTO
device.HS and
(device.ep(token.endpt).ep_type = bulk or
device.ep(token.endpt).ep_type = control)
Dev_HS_BCO
Device_Do_OUT
Figure 8-24. Dev_do_OUT State Machine
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Dev_Do_IsochI
device.ep(token.endpt).ep_type = isochronous
device.ep(token.endpt).ep_type = bulk or
device.ep(token.endpt).ep_type = control or
device.ep(token.endpt).ep_type = interrupt
Dev_Do_BCINTI
Device_Do_IN
Figure 8-25. Dev_do_IN State Machine
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HC_Do_IsochI
HC_cmd.ep_type = isochronous
&
HC_cmd.ep_type = bulk or
HC_cmd.ep_type = control or
HC_cmd.ep_type = interrupt
HC_Do_BCINTI
HC_cmd.direction = in_dir
HC_cmd.direction = out_dir
HC_Do_IsochO
HC_cmd.ep_type = isochronous
&
(not HC_cmd.HS and
(HC_cmd.ep_type = bulk or
HC_cmd.ep_type = control)) or
HC_cmd.ep_type = interrupt
HC_Do_BCINTO
HC_cmd.HS and
(HC_cmd.ep_type = bulk or
HC_cmd.ep_type = control)
HC_HS_BCO
HC_Do_nonsplit
Figure 8-26. HC_Do_nonsplit State Machine
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8.5.1 NAK Limiting via Ping Flow Control
Full-/low-speed devices can have bulk/control endpoints that take time to process their data and, therefore,
respond to OUT transactions with a NAK handshake. This handshake response indicates that the endpoint
did not accept the data because it did not have space for the data. The host controller is expected to retry the
transaction at some future time when the endpoint has space available. Unfortunately, by the time the
endpoint NAKs, most of the full-/low-speed bus time for the transaction had been used. This means that the
full-/low-speed bus has poor utilization when there is a high frequency of NAK’d OUT transactions.
High-speed devices must support an improved NAK mechanism for Bulk OUT and Control endpoints and
transactions. Control endpoints must support this protocol for an OUT transaction in the data and status
stages. The control Setup stage must not support the PING protocol.
This mechanism allows the device to tell the host controller whether it has sufficient endpoint space for the
next OUT transaction. If the device endpoint does not have space, the host controller can choose to delay a
transaction attempt for this endpoint and instead try some other transaction. This can lead to improved bus
utilization. The mechanism avoids using bus time to send data until the host controller knows that the
endpoint has space for the data.
The host controller queries the high-speed device endpoint with a PING special token. The PING special
token packet is a normal token packet as shown in Figure 8-5. The endpoint either responds to the PING
with a NAK or an ACK handshake.
A NAK handshake indicates that the endpoint does not have space for a wMaxPacketSize data payload. The
host controller will retry the PING at some future time to query the endpoint again. A device can respond to
a PING with a NAK for long periods of time. A NAK response is not a reason for the host controller to
retire a transfer request. If a device responds with a NAK in a (micro)frame, the host controller may choose
to issue the next transaction in the next bInterval specified for the endpoint. However, the device must be
prepared to receive PINGs as sequential transactions, e.g., one immediately after the other.
An ACK handshake indicates the endpoint has space for a wMaxPacketSize data payload. The host
controller must generate an OUT transaction with a DATA phase as the next transaction to the endpoint.
The host controller may generate other transactions to other devices or endpoints before the OUT/DATA
transaction for this endpoint.
If the endpoint responds to the OUT/DATA transaction with an ACK handshake, this means the endpoint
accepted the data successfully and has room for another wMaxPacketSize data payload. The host controller
continues with OUT/DATA transactions (which are not required to be the next transactions on the bus) as
long as it has transactions to generate.
If the endpoint instead responds to the OUT/DATA transaction with a NYET handshake, this means that the
endpoint accepted the data but does not have room for another wMaxPacketSize data payload. The host
controller must return to using a PING token until the endpoint indicates it has space.
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HSD2.x or
not device.ep(token.endpt).space_avail
(not HSD2.x) and
HSD2.CRC16 = ok and
device.ep(token.endpt).space_avail
&
Dev_accept_data;
HSD2.x /=
device.ep(token.endpt).toggle and
HSD2.CRC16 = ok
token.PID = tokenSETUP and
HSD2.PID = datax
HSD2.x = device.ep(token.endpt).toggle and
HSD2.CRC16 = ok and
device.ep(token.endpt).space_avail
&
Dopkt
Dev_accept_data;
Issue_packet(HSU1, ACK);
token.PID = tokenOUT and
HSD2.PID = datax
HSD2.x = device.ep(token.endpt).toggle and
HSD2.CRC16 = ok and
not device.ep(token.endpt).space_avail
Dchkpkt2
Issue_packet(HSU1, NAK);
Packet_ready(HSD2)
Dev_wait_Odata
Wait_for_packet(
HSD2, ITG);
device.ep(token.endpt).ep_trouble
Issue_packet(HSU1, STALL);
(HSD2.PID = datax and
HSD2.CRC16 = bad) or
HSD2.PID /= datax or
HSD2.timeout
Dev_Do_BCINTO
Figure 8-27. Host High-speed Bulk OUT/Control Ping State Machine
8.5.1.1 NAK Responses to OUT/DATA During PING Protocol
The endpoint may also respond to the OUT/DATA transaction with a NAK handshake. This means that the
endpoint did not accept the data and does not have space for a wMaxPacketSize data payload at this time.
The host controller must return to using a PING token until the endpoint indicates it has space.
A NAK response is expected to be an unusual occurrence. A high-speed bulk/control endpoint must specify
its maximum NAK rate in its endpoint descriptor. The endpoint is allowed to NAK at most one time each
bInterval period. A NAK suggests that the endpoint responded to a previous OUT or PING with an
inappropriate handshake, or that the endpoint transitioned into a state where it (temporarily) could not
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accept data. An endpoint can use a bInterval of zero to indicate that it never NAKs. An endpoint must
always be able to accept a PING from the host, even if it never NAKs.
If a timeout occurs after the data phase, the host must return to using a PING token. Note that a transition
back to the PING state does not affect the data toggle state of the transaction data phase.
Figure 8-27 shows the host controller state machine for the interactions and transitions between PING and
OUT/DATA tokens and the allowed ACK, NAK, and NYET handshakes for the PING mechanism.
Figure 8-29 shows the device endpoint state machine for PING based on the buffer space the endpoint has
available.
not device.ep(token.endpt).space_avail
&
Issue_packet(HSU1, NAK);
device.ep(token.endpt).space_avail
Issue_packet(HSU1, ACK);
device.ep(token.endpt).ep_trouble
Issue_packet(HSU1, STALL);
Not allowed for control
setup transaction
Dev_HS_ping
Figure 8-28. Dev_HS_ping State Machine
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HSD2.x = device.ep(token.endpt).toggle and
HSD2.CRC16 = ok and
not device.ep(token.endpt).space_avail
Issue_packet(HSU1, NAK);
HSD2.x /= device.ep(token.endpt).toggle and
HSD2.CRC16 = ok
&
HSD2.x = device.ep(token.endpt).toggle and
HSD2.CRC16 = ok and
device.ep(token.endpt).space_avail
&
Dev_accept_data;
device.ep(token.endpt).space_avail
Issue_packet(HSU1, ACK);
Dspace
not device.ep(token.endpt).space_avail
Issue_packet(HSU1, NYET);
HSD2.PID = datax
Dchkpkt
device.ep(token.endpt).ep_trouble
Packet_ready(HSD2)
Dev_wait_Odata1
Wait_for_packet(
HSD2, ITG);
Issue_packet(HSU1, STALL);
(HSD2.PID = datax and
HSD2.CRC16 = bad) or
HSD2.PID /= datax or
HSD2.timeout
Dev_HS_BCO
Figure 8-29. Device High-speed Bulk OUT /Control State Machine
Full-/low-speed devices/endpoints must not support the PING protocol. Host controllers must not support
the PING protocol for full-/low-speed devices.
Note: The PING protocol is also not included as part of the split-transaction protocol definition. Some
split-transactions have equivalent flow control without using PING. Other split-transactions will not benefit
from PING as defined. In any case, split-transactions that can return a NAK handshake have small data
payloads which should have minor high-speed bus impact. Hubs must support PING on their control
endpoint, but PING is not defined for the split-transactions that are used to communicate with full-/lowspeed devices supported by a hub.
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8.5.2 Bulk Transactions
Bulk transaction types are characterized by the ability to guarantee error-free delivery of data between the
host and a function by means of error detection and retry. Bulk transactions use a three-phase transaction
consisting of token, data, and handshake packets as shown in Figure 8-30. Under certain flow control and
halt conditions, the data phase may be replaced with a handshake resulting in a two-phase transaction in
which no data is transmitted. The PING and NYET packets must only be used with devices operating at
high-speed.
Idle
High-speed OUT only
oken
IN
PING
OUT
Error
DATA0/
DATA1
ata
NAK
DATA0/
DATA1
STALL
Error
Error
ACK
NAK
STALL
Idle
Idle
High-speed only
andshake
ACK
Data
Error
NYET
ACK
NAK
STALL
Data
Error
Idle
Host
Function
Figure 8-30. Bulk Transaction Format
When the host is ready to receive bulk data, it issues an IN token. The function endpoint responds by
returning either a data packet or, should it be unable to return data, a NAK or STALL handshake. NAK
indicates that the function is temporarily unable to return data, while STALL indicates that the endpoint is
permanently halted and requires USB System Software intervention. If the host receives a valid data
packet, it responds with an ACK handshake. If the host detects an error while receiving data, it returns no
handshake packet to the function.
When the host is ready to transmit bulk data, it first issues an OUT token packet followed by a data packet
(or PING special token packet, see Section 8.5.1). If the data is received without error by the function, it
will return one of three (or four including NYET, for a device operating at high-speed) handshakes:
•
ACK indicates that the data packet was received without errors and informs the host that it may send
the next packet in the sequence.
•
NAK indicates that the data was received without error but that the host should resend the data because
the function was in a temporary condition preventing it from accepting the data (e.g., buffer full).
•
If the endpoint was halted, STALL is returned to indicate that the host should not retry the transmission
because there is an error condition on the function.
If the data packet was received with a CRC or bit stuff error, no handshake is returned.
Figure 8-31 and Figure 8-32 show the host and device state machines respectively for bulk, control, and
interrupt OUT full/low-speed transactions. Figure 8-27, Figure 8-28, and Figure 8-29 show the state
machines for high-speed transactions. Figure 8-33 and Figure 8-34 show the host and device state machines
respectively for bulk, control, and interrupt IN transactions.
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Universal Serial Bus Specification Revision 2.0
(HSU2.PID /= STALL and
HSU2.PID /= NAK and
HSU2.PID /= ACK) or
HSU2.timeout
Wait_resp
Wait_for_packet(
HSU2, ITG);
BCI_error
IncError;
Packet_ready(HSU2)
ErrorCount < 3
&
Issue_packet(HSD1, datax);
RespondHC(Do_same_cmd);
ErrorCount >= 3
RespondHC(Do_halt);
Do_data
not HC_cmd.setup
Issue_packet(
HSD1, tokenOUT);
HSU2.PID = STALL
RespondHC(Do_halt);
HC_cmd.setup
Issue_packet(HSD1, tokensetup);
HSU2.PID = NAK
RespondHC(Do_same_cmd);
HSU2.PID = ACK
Do_token
RespondHC(Do_next_cmd);
Not allowed for control
setup transaction
HC_Do_BCINTO
Figure 8-31. Bulk/Control/Interrupt OUT Transaction Host State Machine
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Universal Serial Bus Specification Revision 2.0
HSD2.x or
not device.ep(token.endpt).space_avail
(not HSD2.x) and
HSD2.CRC16 = ok and
device.ep(token.endpt).space_avail
&
Dev_accept_data;
HSD2.x /=
device.ep(token.endpt).toggle and
HSD2.CRC16 = ok
token.PID = tokenSETUP and
HSD2.PID = datax
HSD2.x = device.ep(token.endpt).toggle and
HSD2.CRC16 = ok and
device.ep(token.endpt).space_avail
&
Dopkt
Dev_accept_data;
Issue_packet(HSU1, ACK);
token.PID = tokenOUT and
HSD2.PID = datax
HSD2.x = device.ep(token.endpt).toggle and
HSD2.CRC16 = ok and
not device.ep(token.endpt).space_avail
Dchkpkt2
Issue_packet(HSU1, NAK);
Packet_ready(HSD2)
Dev_wait_Odata
Wait_for_packet(
HSD2, ITG);
device.ep(token.endpt).ep_trouble
Issue_packet(HSU1, STALL);
(HSD2.PID = datax and
HSD2.CRC16 = bad) or
HSD2.PID /= datax or
HSD2.timeout
Dev_Do_BCINTO
Figure 8-32. Bulk/Control/Interrupt OUT Transaction Device State Machine
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Universal Serial Bus Specification Revision 2.0
(HSU2.PID /= NAK and
HSU2.PID /= STALL and
HSU2.PID /= datax) or
(HSU2.PID = datax and
HSU2.CRC16 = bad) or
HSU2.timeout
&
Packet_ready(HSU2)
BCII_error
IncError;
ErrorCount < 3
RespondHC(Do_same_cmd);
Wait_data
Wait_for_packet(
HSU2, ITG);
HSU2.PID = STALL
RespondHC(Do_halt);
ErrorCount >= 3
RespondHC(Do_halt);
Issue_packet(HSD1, tokenIN);
HSU2.PID = NAK
RespondHC(Do_same_cmd);
HSU2.PID = datax and
HSU2.CRC16 = ok and
HSU2.x /= HC_cmd.toggle
HSU2.PID = datax and
HSU2.CRC16 = ok and
HSU2.x = HC_cmd.toggle
Issue_packet(HSD1, ACK);
RespondHC(Do_same_cmd);
HC_Accept_data;
Donext
Issue_packet(HSD1, ACK);
RespondHC(Do_next_cmd);
HC_Do_BCINTI
Figure 8-33. Bulk/Control/Interrupt IN Transaction Host State Machine
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Universal Serial Bus Specification Revision 2.0
device.ep(token.endpt).ep_trouble
Issue_packet(HSU1, STALL);
device.ep(token.endpt).data_avail
Issue_packet(HSU1, datax);
not device.ep(token.endpt).data_avail
Issue_packet(HSU1, NAK);
Dev_resp
Wait_for_packet(
HSD2, ITG);
HSD2.PID = ACK
RespondDev(Do_next_data);
Packet_ready(HSD2)
&
HSD2.PID /= ACK or
HSD2.timeout
Dev_Do_BCINTI
Figure 8-34. Bulk/Control/Interrupt IN Transaction Device State Machine
Figure 8-35 shows the sequence bit and data PID usage for bulk reads and writes. Data packet
synchronization is achieved via use of the data sequence toggle bits and the DATA0/DATA1 PIDs. A bulk
endpoint’s toggle sequence is initialized to DATA0 when the endpoint experiences any configuration event
(configuration events are explained in Sections 9.1.1.5 and 9.4.5). Data toggle on an endpoint is NOT
initialized as the direct result of a short packet transfer or the retirement of an IRP.
Bulk
Write
OUT (0)
DATA0
Bulk
Read
IN (0)
DATA0
OUT (1)
...
DATA1
IN (1)
OUT (0/1)
DATA0/1
...
DATA1
IN (0/1)
DATA0/1
Figure 8-35. Bulk Reads and Writes
The host always initializes the first transaction of a bus transfer to the DATA0 PID with a configuration
event. The second transaction uses a DATA1 PID, and successive data transfers alternate for the remainder
of the bulk transfer. The data packet transmitter toggles upon receipt of ACK, and the receiver toggles upon
receipt and acceptance of a valid data packet (refer to Section 8.6).
8.5.3 Control Transfers
Control transfers minimally have two transaction stages: Setup and Status. A control transfer may
optionally contain a Data stage between the Setup and Status stages. During the Setup stage, a SETUP
transaction is used to transmit information to the control endpoint of a function. SETUP transactions are
similar in format to an OUT but use a SETUP rather than an OUT PID. Figure 8-36 shows the SETUP
transaction format. A SETUP always uses a DATA0 PID for the data field of the SETUP transaction. The
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Universal Serial Bus Specification Revision 2.0
function receiving a SETUP must accept the SETUP data and respond with ACK; if the data is corrupted,
discard the data and return no handshake.
Idle
Token
SETUP
Data
DATA0
Handshake
Error
ACK
Idle
Host
Function
Figure 8-36. Control SETUP Transaction
The Data stage, if present, of a control transfer consists of one or more IN or OUT transactions and follows
the same protocol rules as bulk transfers. All the transactions in the Data stage must be in the same
direction (i.e., all INs or all OUTs). The amount of data to be sent during the data stage and its direction are
specified during the Setup stage. If the amount of data exceeds the prenegotiated data packet size, the data
is sent in multiple transactions (INs or OUTs) that carry the maximum packet size. Any remaining data is
sent as a residual in the last transaction.
The Status stage of a control transfer is the last transaction in the sequence. The status stage transactions
follow the same protocol sequence as bulk transactions. Status stage for devices operating at high-speed
also includes the PING protocol. A Status stage is delineated by a change in direction of data flow from the
previous stage and always uses a DATA1 PID. If, for example, the Data stage consists of OUTs, the status
is a single IN transaction. If the control sequence has no Data stage, then it consists of a Setup stage
followed by a Status stage consisting of an IN transaction.
Figure 8-37 shows the transaction order, the data sequence bit value, and the data PID types for control read
and write sequences. The sequence bits are displayed in parentheses.
Setup
Stage
Control
Write
SETUP (0)
DATA0
Control
Read
No-data
Control
Data
Stage
OUT (1)
DATA1
SETUP (0)
IN (1)
DATA0
DATA1
Setup
Stage
Status
Stage
SETUP (0)
IN (1)
DATA0
DATA1
OUT (0)
Status
Stage
...
DATA0
IN (0)
DATA0
OUT (0/1)
DATA0/1
...
DATA1
IN (0/1)
OUT (1)
DATA0/1
DATA1
Figure 8-37. Control Read and Write Sequences
226
IN (1)
Universal Serial Bus Specification Revision 2.0
When a STALL handshake is sent by a control endpoint in either the Data or Status stages of a control
transfer, a STALL handshake must be returned on all succeeding accesses to that endpoint until a SETUP
PID is received. The endpoint is not required to return a STALL handshake after it receives a subsequent
SETUP PID. For the default endpoint, if an ACK handshake is returned for the SETUP transaction, the host
expects that the endpoint has automatically recovered from the condition that caused the STALL and the
endpoint must operate normally.
8.5.3.1 Reporting Status Results
The Status stage reports to the host the outcome of the previous Setup and Data stages of the transfer. Three
possible results may be returned:
•
The command sequence completed successfully.
•
The command sequence failed to complete.
•
The function is still busy completing the command.
Status reporting is always in the function-to-host direction. Table 8-7 summarizes the type of responses
required for each. Control write transfers return status information in the data phase of the Status stage
transaction. Control read transfers return status information in the handshake phase of a Status stage
transaction, after the host has issued a zero-length data packet during the previous data phase.
Table 8-7. Status Stage Responses
Control Write Transfer
Control Read Transfer
(sent during data phase)
(sent during handshake phase)
Function completes
Zero-length data packet
ACK handshake
Function has an error
STALL handshake
STALL handshake
Function is busy
NAK handshake
NAK handshake
Status Response
For control reads, the host must send either an OUT token or PING special token (for a device operating at
high-speed) to the control pipe to initiate the Status stage. The host may only send a zero-length data packet
in this phase but the function may accept any length packet as a valid status inquiry. The pipe’s handshake
response to this data packet indicates the current status. NAK indicates that the function is still processing
the command and that the host should continue the Status stage. ACK indicates that the function has
completed the command and is ready to accept a new command. STALL indicates that the function has an
error that prevents it from completing the command.
For control writes, the host sends an IN token to the control pipe to initiate the Status stage. The function
responds with either a handshake or a zero-length data packet to indicate its current status. NAK indicates
that the function is still processing the command and that the host should continue the Status stage; return of
a zero-length packet indicates normal completion of the command; and STALL indicates that the function
cannot complete the command. The function expects the host to respond to the data packet in the Status
stage with ACK. If the function does not receive ACK, it remains in the Status stage of the command and
will continue to return the zero-length data packet for as long as the host continues to send IN tokens.
If during a Data stage a command pipe is sent more data or is requested to return more data than was
indicated in the Setup stage (see Section 8.5.3.2), it should return STALL. If a control pipe returns STALL
during the Data stage, there will be no Status stage for that control transfer.
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Universal Serial Bus Specification Revision 2.0
8.5.3.2 Variable-length Data Stage
A control pipe may have a variable-length data phase in which the host requests more data than is contained
in the specified data structure. When all of the data structure is returned to the host, the function should
indicate that the Data stage is ended by returning a packet that is shorter than the MaxPacketSize for the
pipe. If the data structure is an exact multiple of wMaxPacketSize for the pipe, the function will return a
zero-length packet to indicate the end of the Data stage.
8.5.3.3 Error Handling on the Last Data Transaction
If the ACK handshake on an IN transaction is corrupted, the function and the host will temporarily disagree
on whether the transaction was successful. If the transaction is followed by another IN, the toggle retry
mechanism will detect the mismatch and recover from the error. If the ACK was on the last IN of a Data
stage, the toggle retry mechanism cannot be used and an alternative scheme must be used.
The host that successfully received the data of the last IN will send ACK. Later, the host will issue an OUT
token to start the Status stage of the transfer. If the function did not receive the ACK that ended the Data
stage, the function will interpret the start of the Status stage as verification that the host successfully
received the data. Control writes do not have this ambiguity. If an ACK handshake on an OUT gets
corrupted, the host does not advance to the Status stage and retries the last data instead. A detailed analysis
of retry policy is presented in Section 8.6.4.
8.5.3.4 STALL Handshakes Returned by Control Pipes
Control pipes have the unique ability to return a STALL handshake due to function problems in control
transfers. If the device is unable to complete a command, it returns a STALL in the Data and/or Status
stages of the control transfer. Unlike the case of a functional stall, protocol stall does not indicate an error
with the device. The protocol STALL condition lasts until the receipt of the next SETUP transaction, and
the function will return STALL in response to any IN or OUT transaction on the pipe until the SETUP
transaction is received. In general, protocol stall indicates that the request or its parameters are not
understood by the device and thus provides a mechanism for extending USB requests.
A control pipe may also support functional stall as well, but this is not recommended. This is a
degenerative case, because a functional stall on a control pipe indicates that it has lost the ability to
communicate with the host. If the control pipe does support functional stall, then it must possess a Halt
feature, which can be set or cleared by the host. Chapter 9 details how to treat the special case of a Halt
feature on a control pipe. A well-designed device will associate all of its functions and Halt features with
non-control endpoints. The control pipes should be reserved for servicing USB requests.
8.5.4 Interrupt Transactions
Interrupt transactions may consist of IN or OUT transfers. Upon receipt of an IN token, a function may
return data, NAK, or STALL. If the endpoint has no new interrupt information to return (i.e., no interrupt is
pending), the function returns a NAK handshake during the data phase. If the Halt feature is set for the
interrupt endpoint, the function will return a STALL handshake. If an interrupt is pending, the function
returns the interrupt information as a data packet. The host, in response to receipt of the data packet, issues
either an ACK handshake if data was received error-free or returns no handshake if the data packet was
received corrupted. Figure 8-38 shows the interrupt transaction format.
Section 5.9.1 contains additional information about high-speed, high-bandwidth interrupt endpoints. Such
endpoints use multiple transactions in a microframe as defined in that section. Each transaction for a highbandwidth endpoint follows the transaction format shown in Figure 8-38.
228
Universal Serial Bus Specification Revision 2.0
Idle
Token
Data
IN
DATA0/
DATA1
OUT
NAK
DATA0/
DATA1
STALL
Error
Idle
Error
Handshake
ACK
Data
Error
NAK
ACK
STALL
Data
Error
Idle
Host
Function
Figure 8-38. Interrupt Transaction Format
When an endpoint is using the interrupt transfer mechanism for actual interrupt data, the data toggle
protocol must be followed. This allows the function to know that the data has been received by the host and
the event condition may be cleared. This “guaranteed” delivery of events allows the function to only send
the interrupt information until it has been received by the host rather than having to send the interrupt data
every time the function is polled and until the USB System Software clears the interrupt condition. When
used in the toggle mode, an interrupt endpoint is initialized to the DATA0 PID by any configuration event
on the endpoint and behaves the same as the bulk transactions shown in Figure 8-35.
8.5.5 Isochronous Transactions
Isochronous transactions have a token and data phase, but no handshake phase, as shown in Figure 8-39.
The host issues either an IN or an OUT token followed by the data phase in which the endpoint (for INs) or
the host (for OUTs) transmits data. Isochronous transactions do not support a handshake phase or retry
capability.
Idle
IN
OUT
DATAx
DATAx
Token
Data
Error
Idle
Host
Function
See Note Below
Figure 8-39. Isochronous Transaction Format
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Universal Serial Bus Specification Revision 2.0
Note: A full-speed device or Host Controller should be able to accept either DATA0 or DATA1 PIDs in
data packets. A full-speed device or Host Controller should only send DATA0 PIDs in data packets. A
high-speed Host Controller must be able to accept and send DATA0, DATA1, DATA2, or MDATA PIDs in
data packets. A high-speed device with at most 1 transaction per microframe must only send DATA0 PIDs
in data packets. A high-speed device with high-bandwith endpoints (e.g., one that has more than 1
transaction per microframe) must be able to accept and/or send DATA0, DATA1, DATA2, or MDATA
PIDs in data packets.
Full-speed isochronous transactions do not support toggle sequencing. High-speed isochronous transactions
with a single transaction per microframe do not support toggle sequencing. High bandwidth, high-speed
isochronous transactions support data PID sequencing (see Section 5.9.1 for more details).
Figure 8-40 and Figure 8-41 show the host and device state machines respectively for isochronous OUT
transactions. Figure 8-42 and Figure 8-43 show the host and device state machines respectively for
isochronous IN transactions.
Issue_packet(HSD1, tokenOUT);
H_IDodata
Issue_packet(HSD1, datax);
H_IDo_next
RespondHC(Do_next_cmd);
HC_Do_IsochO
Figure 8-40. Isochronous OUT Transaction Host State Machine
230
Universal Serial Bus Specification Revision 2.0
HSD2.PID /= datax or
(HSD2.PID = datax and
HSD2.CRC16 = bad) or
HSD2.timeout
&
Dev_Record_error;
Packet_ready(HSD2)
HSD2.PID = datax and
HSD2.CRC16 = ok
DDo_IOdata
Dev_Accept_data;
Dev_wait_data
Wait_for_packet(
HSD2, ITG);
RespondDev(Do_next_data);
Dev_Do_IsochO
Figure 8-41. Isochronous OUT Transaction Device State Machine
&
HSU2.PID = datax and
HSU2.CRC16 = ok
HC_Accept_data;
Packet_ready(HSU2)
HSU2.PID /= datax or
(HSU2.PID = datax and
HSU2.CRC16 = bad) or
HSU2.timeout
Wait_IsochI_resp
Wait_for_packet(
HSU2, ITG);
H_IIDo_next
Record_error;
&
Issue_packet(HSD1, tokenIN);
RespondHC(Do_next_cmd);
HC_Do_IsochI
Figure 8-42. Isochronous IN Transaction Host State Machine
231
Universal Serial Bus Specification Revision 2.0
Issue_packet(HSU1, datax); -- data0
D_Do_IInext
RespondDev(Do_next_data);
Dev_Do_IsochI
Figure 8-43. Isochronous IN Transaction Device State Machine
8.6 Data Toggle Synchronization and Retry
The USB provides a mechanism to guarantee data sequence synchronization between data transmitter and
receiver across multiple transactions. This mechanism provides a means of guaranteeing that the handshake
phase of a transaction was interpreted correctly by both the transmitter and receiver. Synchronization is
achieved via use of the DATA0 and DATA1 PIDs and separate data toggle sequence bits for the data
transmitter and receiver. Receiver sequence bits toggle only when the receiver is able to accept data and
receives an error-free data packet with the correct data PID. Transmitter sequence bits toggle only when the
data transmitter receives a valid ACK handshake. The data transmitter and receiver must have their
sequence bits synchronized at the start of a transaction. The synchronization mechanism used varies with
the transaction type. Data toggle synchronization is not supported for isochronous transfers.
The state machines contained in this chapter and in Chapter 11 describe data toggle synchronization in a
more compact form. Instead of explicitly identifying DATA0 and DATA1, it uses a value “DATAx” to
represent either/both DATA0/DATA1 PIDs. In some cases where the specific data PID is important,
another variable labeled “x” is used that has the value 0 for DATA0 and 1 for DATA1.
High-speed, high-bandwidth isochronous and interrupt endpoints support a similar but different data
synchronization technique called data PID sequencing. That technique is used instead of data toggle
synchronization. Section 5.9.1 defines data PID sequencing.
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Universal Serial Bus Specification Revision 2.0
8.6.1 Initialization via SETUP Token
Host
Device
SETUP
Rx
(X)
Tx
(X-1)
DATA0
Tx
(1)
Accept
data
Rx
(X->1)
ACK
Rx
(1)
Tx
(1)
Figure 8-44. SETUP Initialization
Control transfers use the SETUP token for initializing host and function sequence bits. Figure 8-44 shows
the host issuing a SETUP packet to a function followed by an OUT transaction. The numbers in the circles
represent the transmitter and receiver sequence bits. The function must accept the data and return ACK.
When the function accepts the transaction, it must set its sequence bit so that both the host’s and function’s
sequence bits are equal to one at the end of the SETUP transaction.
8.6.2 Successful Data Transactions
Figure 8-45 shows the case where two successful transactions have occurred. For the data transmitter, this
means that it toggles its sequence bit upon receipt of ACK. The receiver toggles its sequence bit only if it
receives a valid data packet and the packet’s data PID matches the current value of its sequence bit. The
transmitter only toggles its sequence bit after it receives an ACK to a data packet.
During each transaction, the receiver compares the transmitter sequence bit (encoded in the data packet PID
as either DATA0 or DATA1) with its receiver sequence bit. If data cannot be accepted, the receiver must
issue NAK and the sequence bits of both the transmitter and receiver remain unchanged. If data can be
accepted and the receiver’s sequence bit matches the PID sequence bit, then data is accepted and the
sequence bit is toggled. Two-phase transactions in which there is no data packet leave the transmitter and
receiver sequence bits unchanged.
DATA0
Tx
(0)
DATA1
Accept Rx
data
(0->1)
Tx
(1)
ACK
Rx
(1->0)
ACK
Rx
(1)
Tx
(0->1)
Accept
data
Transfer i
Rx
(0)
Tx
(1->0)
Transfer i + 1
Figure 8-45. Consecutive Transactions
8.6.3 Data Corrupted or Not Accepted
If data cannot be accepted or the received data packet is corrupted, the receiver will issue a NAK or STALL
handshake, or timeout, depending on the circumstances, and the receiver will not toggle its sequence bit.
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Universal Serial Bus Specification Revision 2.0
Figure 8-46 shows the case where a transaction is NAKed and then retried. Any non-ACK handshake or
timeout will generate similar retry behavior. The transmitter, having not received an ACK handshake, will
not toggle its sequence bit. As a result, a failed data packet transaction leaves the transmitter’s and
receiver’s sequence bits synchronized and untoggled. The transaction will then be retried and, if successful,
will cause both transmitter and receiver sequence bits to toggle.
DATA0
DATA0
Reject
data
Tx
(0)
Accept
data
Tx
(0)
Rx
(0->0)
ACK
NAK
Rx
(0)
Tx
(0->0)
Rx
(0->1)
Rx
(1)
Tx
(0->1)
Transfer i
Retry
Transfer i
Figure 8-46. NAKed Transaction with Retry
8.6.4 Corrupted ACK Handshake
The transmitter is the last and only agent to know for sure whether a transaction has been successful, due to
its receiving an ACK handshake. A lost or corrupted ACK handshake can lead to a temporary loss of
synchronization between transmitter and receiver as shown in Figure 8-47. Here the transmitter issues a
valid data packet, which is successfully acquired by the receiver; however, the ACK handshake is corrupted.
DATA0
Tx
(0)
Accept
data
DATA0
Rx
(0->1)
Tx
(0)
Failed ACK
Transfer i
Rx
(1)
Tx
(1)
ACK
Rx
(1)
Tx
(0->0)
Ignore
data
DATA1
ACK
Rx
(1)
Tx
(0->1)
Transfer i
(retried)
Rx
(1->0)
Tx
(1->0)
Rx
(0)
Transfer i + 1
Figure 8-47. Corrupted ACK Handshake with Retry
At the end of transaction i, there is a temporary loss of coherency between transmitter and receiver, as
evidenced by the mismatch between their respective sequence bits. The receiver has received good data, but
the transmitter does not know whether it has successfully sent data. On the next transaction, the transmitter
will resend the previous data using the previous DATA0 PID. The receiver’s sequence bit and the data PID
will not match, so the receiver knows that it has previously accepted this data. Consequently, it discards the
incoming data packet and does not toggle its sequence bit. The receiver then issues ACK, which causes the
transmitter to regard the retried transaction as successful. Receipt of ACK causes the transmitter to toggle
its sequence bit. At the beginning of transaction i+1, the sequence bits have toggled and are again
synchronized.
The data transmitter must guarantee that any retried data packet is identical (same length and content) as
that sent in the original transaction. If the data transmitter is unable, because of problems such as a buffer
underrun condition, to transmit the identical amount of data as was in the original data packet, it must abort
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the transaction by generating a bit stuffing violation for full-/low-speed. An error for high-speed must be
forced by taking the currently calculated CRC and complementing it before transmitting it. This causes a
detectable error at the receiver and guarantees that a partial packet will not be interpreted as a good packet.
The transmitter should not try to force an error at the receiver by sending a constant known bad CRC. A
combination of a bad packet with a “bad” CRC may be interpreted by the receiver as a good packet.
8.6.5 Low-speed Transactions
The USB supports signaling at three speeds: high-speed signaling at 480 Mb/s, full-speed signaling at
12.0 Mb/s, and low-speed signaling at 1.5 Mb/s. Hubs isolate high-speed signaling from full-/low-speed
signaling environments.
Within a full-/low-speed signaling environment, hubs disable downstream bus traffic to all ports to which
low-speed devices are attached during full-speed downstream signaling. This is required both for EMI
reasons and to prevent any possibility that a low-speed device might misinterpret downstream a full-speed
packet as being addressed to it.
Figure 8-48 shows an IN low-speed transaction in which the host (or TT) issues a token and handshake and
receives a data packet.
Hub disables lowspeed port outputs
Hub enables lowspeed port outputs
Preamble
sent at full-speed
SYNC
PID
Token sent at low-speed
Hub setup
SYNC
PID
ENDP
...
EOP
Data packet sent at low-speed
SYNC
PID
Preamble
sent at full-speed
SYNC
PID
DATA
CRC
EOP
Hub disables lowspeed port outputs
Hub enables lowspeed port outputs
Handshake sent at low-speed
Hub setup
SYNC
PID
EOP
Figure 8-48. Low-speed Transaction
All downstream packets transmitted to low-speed devices within a full-/low-speed signaling environment
require a preamble. Preambles are never used in a high-speed signaling environment. The preamble
consists of a SYNC followed by a PRE PID, both sent at full-speed. Hubs must comprehend the PRE PID;
all other USB devices may ignore it and treat it as undefined. At the end of the preamble PID, the host (or
TT) drives the bus to the Idle state for at least one full-speed bit time. This Idle period on the bus is termed
the hub setup interval and lasts for at least four full-speed bit times. During this hub setup interval, hubs
must drive their full-speed and low-speed ports to their respective Idle states. Hubs must be ready to repeat
low-speed signaling on low-speed ports before the end of the hub setup interval. Low-speed connectivity
rules are summarized below:
1.
Low-speed devices are identified during the connection process, and the hub ports to which they are
connected are identified as low-speed.
2.
All downstream low-speed packets must be prefaced with a preamble (sent at full-speed), which turns
on the output buffers on low-speed hub ports.
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3.
Low-speed hub port output buffers are turned off upon receipt of EOP and are not turned on again until
a preamble PID is detected.
4.
Upstream connectivity is not affected by whether a hub port is full- or low-speed.
Low-speed signaling begins with the host (or TT) issuing SYNC at low-speed, followed by the remainder of
the packet. The end of the packet is identified by an End-of-Packet (EOP), at which time all hubs tear down
connectivity and disable any ports to which low-speed devices are connected. Hubs do not switch ports for
upstream signaling; low-speed ports remain enabled in the upstream direction for both low-speed and fullspeed signaling.
Low-speed and full-speed transactions maintain a high degree of protocol commonality. However, lowspeed signaling does have certain limitations which include:
•
Data payload is limited to eight bytes, maximum.
•
Only interrupt and control types of transfers are supported.
•
The SOF packet is not received by low-speed devices.
8.7 Error Detection and Recovery
The USB permits reliable end-to-end communication in the presence of errors on the physical signaling
layer. This includes the ability to reliably detect the vast majority of possible errors and to recover from
errors on a transaction-type basis. Control transactions, for example, require a high degree of data
reliability; they support end-to-end data integrity using error detection and retry. Isochronous transactions,
by virtue of their bandwidth and latency requirements, do not permit retries and must tolerate a higher
incidence of uncorrected errors.
8.7.1 Packet Error Categories
The USB employs three error detection mechanisms: bit stuff violations, PID check bits, and CRCs. Bit
stuff violations are defined in Section 7.1.9. PID errors are defined in Section 8.3.1. CRC errors are
defined in Section 8.3.5.
With the exception of the SOF token, any packet that is received corrupted causes the receiver to ignore it
and discard any data or other field information that came with the packet. Table 8-8 lists error detection
mechanisms, the types of packets to which they apply, and the appropriate packet receiver response.
Table 8-8. Packet Error Types
Field
236
Error
Action
PID
PID Check, Bit Stuff
Ignore packet
Address
Bit Stuff, Address CRC
Ignore token
Frame Number
Bit Stuff, Frame Number CRC
Ignore Frame Number field
Data
Bit Stuff, Data CRC
Discard data
Universal Serial Bus Specification Revision 2.0
8.7.2 Bus Turn-around Timing
Neither the device nor the host will send an indication that a received packet had an error. This absence of
positive acknowledgement is considered to be the indication that there was an error. As a consequence of
this method of error reporting, the host and USB function need to keep track of how much time has elapsed
from when the transmitter completes sending a packet until it begins to receive a response packet. This time
is referred to as the bus turn-around time. Devices and hosts require turn-around timers to measure this
time.
For full-/low-speed transactions, the timer starts counting on the SE0-to-‘J’ transition of the EOP strobe and
stops counting when the Idle-to-‘K’ SOP transition is detected. For high-speed transactions, the timer starts
counting when the data lines return to the squelch level and stops counting when the data lines leave the
squelch level.
The device bus turn-around time is defined by the worst case round trip delay plus the maximum device
response delay (refer to Sections 7.1.18 and 7.1.19 for specific bus turn-around times). If a response is not
received within this worst case timeout, then the transmitter considers that the packet transmission has
failed.
Timeout is used and interpreted as a transaction error condition for many transfer types. If the host wishes
to indicate an error condition for a transaction via a timeout, it must wait the full bus turn-around time
before issuing the next token to ensure that all downstream devices have timed out.
As shown in Figure 8-49, the device uses its bus turn-around timer between token and data or data and
handshake phases. The host uses its timer between data and handshake or token and data phases.
If the host receives a corrupted data packet, it may require additional wait time before sending out the next
token. This additional wait interval guarantees that the host properly handles false EOPs.
OUT/SETUP
Data
device waits
host waits
Data
IN
host waits
Handshake
Handshake
device waits
Figure 8-49. Bus Turn-around Timer Usage
8.7.3 False EOPs
False EOPs must be handled in a manner which guarantees that the packet currently in progress completes
before the host or any other device attempts to transmit a new packet. If such an event were to occur, it
would constitute a bus collision and have the ability to corrupt up to two consecutive transactions.
Detection of false EOP relies upon the fact that a packet into which a false EOP has been inserted will
appear as a truncated packet with a CRC failure. (The last 16 bits of the data packet will have a very low
probability of appearing to be a correct CRC.)
The host and devices handle false EOP situations differently. When a device receives a corrupted data
packet, it issues no response and waits for the host to send the next token. This scheme guarantees that the
device will not attempt to return a handshake while the host may still be transmitting a data packet. If a
false EOP has occurred, the host data packet will eventually end, and the device will be able to detect the
next token. If a device issues a data packet that gets corrupted with a false EOP, the host will ignore the
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packet and not issue the handshake. The device, expecting to see a handshake from the host, will timeout
the transaction.
If the host receives a corrupted full-/low-speed data packet, it assumes that a false EOP may have occurred
and waits for 16 bit times to see if there is any subsequent upstream traffic. If no bus transitions are
detected within the 16 bit interval and the bus remains in the Idle state, the host may issue the next token.
Otherwise, the host waits for the device to finish sending the remainder of its full-/low-speed packet.
Waiting 16 bit times guarantees two conditions:
•
The first condition is to make sure that the device has finished sending its packet. This is guaranteed by
a timeout interval (with no bus transitions) greater than the worst case six-bit time bit stuff interval.
•
The second condition is that the transmitting device’s bus turn-around timer must be guaranteed to
expire.
Note that the timeout interval is transaction speed sensitive. For full-speed transactions, the host must wait
full-speed bit times; for low-speed transactions, it must wait low-speed bit times.
If the host receives a corrupted high-speed data packet, it ignores any data until the data lines return to the
squelch level before issuing the next token. For high-speed transactions, the host does not need to wait
additional time (beyond the normal inter-transaction gap time) after the data lines return to the squelch
level.
If the host receives a data packet with a valid CRC, it assumes that the packet is complete and requires no
additional delay (beyond normal inter-transaction gap time) in issuing the next token.
8.7.4 Babble and Loss of Activity Recovery
The USB must be able to detect and recover from conditions which leave it waiting indefinitely for a
full-/low-speed EOP or which leave the bus in something other than the Idle state at the end of a
(micro)frame.
•
Full-/low-speed loss of activity (LOA) is characterized by an SOP followed by lack of bus activity (bus
remains driven to a ‘J’ or ‘K’) and no EOP at the end of a frame.
•
Full-/low-speed babble is characterized by an SOP followed by the presence of bus activity past the end
of a frame.
•
High-speed babble/LOA is characterized by the data lines being at an unsquelched level at the end of a
microframe.
LOA and babble have the potential to either deadlock the bus or delay the beginning of the next
(micro)frame. Neither condition is acceptable, and both must be prevented from occurring. As the USB
component responsible for controlling connectivity, hubs are responsible for babble/LOA detection and
recovery. All USB devices that fail to complete their transmission at the end of a (micro)frame are
prevented from transmitting past a (micro)frame’s end by having the nearest hub disable the port to which
the offending device is attached. Details of the hub babble/LOA recovery mechanism appear in
Section 11.2.5.
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Chapter 9
USB Device Framework
A USB device may be divided into three layers:
•
The bottom layer is a bus interface that transmits and receives packets.
•
The middle layer handles routing data between the bus interface and various endpoints on the device.
An endpoint is the ultimate consumer or provider of data. It may be thought of as a source or sink for
data.
•
The top layer is the functionality provided by the serial bus device, for instance, a mouse or ISDN
interface.
This chapter describes the common attributes and operations of the middle layer of a USB device. These
attributes and operations are used by the function-specific portions of the device to communicate through
the bus interface and ultimately with the host.
9.1
USB Device States
A USB device has several possible states. Some of these states are visible to the USB and the host, while
others are internal to the USB device. This section describes those states.
9.1.1 Visible Device States
This section describes USB device states that are externally visible (see Figure 9-1). Table 9-1 summarizes
the visible device states.
Note: USB devices perform a reset operation in response to reset signaling on the upstream facing port.
When reset signaling has completed, the USB device is reset.
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Attached
Hub Reset
Hub
or
Deconfigured Configured
Powered
Power
Interruption
Bus
Inactive
Suspended
Bus Activity
Reset
Default
Reset
Bus
Inactive
Suspended
Bus Activity
Address
Assigned
Address
Bus
Inactive
Suspended
Bus Activity
Device
Device
Deconfigured Configured
Configured
Bus
Inactive
Bus Activity
Figure 9-1. Device State Diagram
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Table 9-1. Visible Device States
Attached
Powered Default
Address
Configured
Suspended
State
No
--
--
--
--
--
Device is not attached to
the USB. Other attributes
are not significant.
Yes
No
--
--
--
--
Device is attached to the
USB, but is not powered.
Other attributes are not
significant.
Yes
Yes
No
--
--
--
Device is attached to the
USB and powered, but
has not been reset.
Yes
Yes
Yes
No
--
--
Device is attached to the
USB and powered and
has been reset, but has
not been assigned a
unique address. Device
responds at the default
address.
Yes
Yes
Yes
Yes
No
--
Device is attached to the
USB, powered, has been
reset, and a unique
device address has been
assigned. Device is not
configured.
Yes
Yes
Yes
Yes
Yes
No
Device is attached to the
USB, powered, has been
reset, has a unique
address, is configured,
and is not suspended.
The host may now use
the function provided by
the device.
Yes
Yes
--
--
--
Yes
Device is, at minimum,
attached to the USB and
is powered and has not
seen bus activity for 3 ms.
It may also have a unique
address and be
configured for use.
However, because the
device is suspended, the
host may not use the
device’s function.
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9.1.1.1 Attached
A USB device may be attached or detached from the USB. The state of a USB device when it is detached
from the USB is not defined by this specification. This specification only addresses required operations and
attributes once the device is attached.
9.1.1.2 Powered
USB devices may obtain power from an external source and/or from the USB through the hub to which they
are attached. Externally powered USB devices are termed self-powered. Although self-powered devices
may already be powered before they are attached to the USB, they are not considered to be in the Powered
state until they are attached to the USB and VBUS is applied to the device.
A device may support both self-powered and bus-powered configurations. Some device configurations
support either power source. Other device configurations may be available only if the device is selfpowered. Devices report their power source capability through the configuration descriptor. The current
power source is reported as part of a device’s status. Devices may change their power source at any time,
e.g., from self- to bus-powered. If a configuration is capable of supporting both power modes, the power
maximum reported for that configuration is the maximum the device will draw from VBUS in either mode.
The device must observe this maximum, regardless of its mode. If a configuration supports only one power
mode and the power source of the device changes, the device will lose its current configuration and address
and return to the Powered state. If a device is self-powered and its current configuration requires more than
100 mA, then if the device switches to being bus-powered, it must return to the Address state. Self-powered
hubs that use VBUS to power the Hub Controller are allowed to remain in the Configured state if local
power is lost. Refer to Section 11.13 for details.
A hub port must be powered in order to detect port status changes, including attach and detach. Buspowered hubs do not provide any downstream power until they are configured, at which point they will
provide power as allowed by their configuration and power source. A USB device must be able to be
addressed within a specified time period from when power is initially applied (refer to Chapter 7). After an
attachment to a port has been detected, the host may enable the port, which will also reset the device
attached to the port.
9.1.1.3 Default
After the device has been powered, it must not respond to any bus transactions until it has received a reset
from the bus. After receiving a reset, the device is then addressable at the default address.
When the reset process is complete, the USB device is operating at the correct speed (i.e., low-/full-/highspeed). The speed selection for low- and full-speed is determined by the device termination resistors. A
device that is capable of high-speed operation determines whether it will operate at high-speed as a part of
the reset process (see Chapter 7 for more details).
A device capable of high-speed operation must reset successfully at full-speed when in an electrical
environment that is operating at full-speed. After the device is successfully reset, the device must also
respond successfully to device and configuration descriptor requests and return appropriate information.
The device may or may not be able to support its intended functionality when operating at full-speed.
9.1.1.4 Address
All USB devices use the default address when initially powered or after the device has been reset. Each
USB device is assigned a unique address by the host after attachment or after reset. A USB device
maintains its assigned address while suspended.
A USB device responds to requests on its default pipe whether the device is currently assigned a unique
address or is using the default address.
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9.1.1.5 Configured
Before a USB device’s function may be used, the device must be configured. From the device’s
perspective, configuration involves correctly processing a SetConfiguration() request with a non-zero
configuration value. Configuring a device or changing an alternate setting causes all of the status and
configuration values associated with endpoints in the affected interfaces to be set to their default values.
This includes setting the data toggle of any endpoint using data toggles to the value DATA0.
9.1.1.6 Suspended
In order to conserve power, USB devices automatically enter the Suspended state when the device has
observed no bus traffic for a specified period (refer to Chapter 7). When suspended, the USB device
maintains any internal status, including its address and configuration.
All devices must suspend if bus activity has not been observed for the length of time specified in
Chapter 7. Attached devices must be prepared to suspend at any time they are powered, whether they have
been assigned a non-default address or are configured. Bus activity may cease due to the host entering a
suspend mode of its own. In addition, a USB device shall also enter the Suspended state when the hub port
it is attached to is disabled. This is referred to as selective suspend.
A USB device exits suspend mode when there is bus activity. A USB device may also request the host to
exit suspend mode or selective suspend by using electrical signaling to indicate remote wakeup. The ability
of a device to signal remote wakeup is optional. If a USB device is capable of remote wakeup signaling, the
device must support the ability of the host to enable and disable this capability. When the device is reset,
remote wakeup signaling must be disabled.
9.1.2 Bus Enumeration
When a USB device is attached to or removed from the USB, the host uses a process known as bus
enumeration to identify and manage the device state changes necessary. When a USB device is attached to
a powered port, the following actions are taken:
1.
The hub to which the USB device is now attached informs the host of the event via a reply on its status
change pipe (refer to Section 11.12.3 for more information). At this point, the USB device is in the
Powered state and the port to which it is attached is disabled.
2.
The host determines the exact nature of the change by querying the hub.
3.
Now that the host knows the port to which the new device has been attached, the host then waits for at
least 100 ms to allow completion of an insertion process and for power at the device to become stable.
The host then issues a port enable and reset command to that port. Refer to Section 7.1.7.5 for
sequence of events and timings of connection through device reset.
4.
The hub performs the required reset processing for that port (see Section 11.5.1.5). When the reset
signal is released, the port has been enabled. The USB device is now in the Default state and can draw
no more than 100 mA from VBUS. All of its registers and state have been reset and it answers to the
default address.
5.
The host assigns a unique address to the USB device, moving the device to the Address state.
6.
Before the USB device receives a unique address, its Default Control Pipe is still accessible via the
default address. The host reads the device descriptor to determine what actual maximum data payload
size this USB device’s default pipe can use.
7.
The host reads the configuration information from the device by reading each configuration zero to
n-1, where n is the number of configurations. This process may take several milliseconds to complete.
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8.
Based on the configuration information and how the USB device will be used, the host assigns a
configuration value to the device. The device is now in the Configured state and all of the endpoints in
this configuration have taken on their described characteristics. The USB device may now draw the
amount of VBUS power described in its descriptor for the selected configuration. From the device’s
point of view, it is now ready for use.
When the USB device is removed, the hub again sends a notification to the host. Detaching a device
disables the port to which it had been attached. Upon receiving the detach notification, the host will update
its local topological information.
9.2
Generic USB Device Operations
All USB devices support a common set of operations. This section describes those operations.
9.2.1 Dynamic Attachment and Removal
USB devices may be attached and removed at any time. The hub that provides the attachment point or port
is responsible for reporting any change in the state of the port.
The host enables the hub port where the device is attached upon detection of an attachment, which also has
the effect of resetting the device. A reset USB device has the following characteristics:
•
Responds to the default USB address
•
Is not configured
•
Is not initially suspended
When a device is removed from a hub port, the hub disables the port where the device was attached and
notifies the host of the removal.
9.2.2 Address Assignment
When a USB device is attached, the host is responsible for assigning a unique address to the device. This is
done after the device has been reset by the host, and the hub port where the device is attached has been
enabled.
9.2.3 Configuration
A USB device must be configured before its function(s) may be used. The host is responsible for
configuring a USB device. The host typically requests configuration information from the USB device to
determine the device’s capabilities.
As part of the configuration process, the host sets the device configuration and, where necessary, selects the
appropriate alternate settings for the interfaces.
Within a single configuration, a device may support multiple interfaces. An interface is a related set of
endpoints that present a single feature or function of the device to the host. The protocol used to
communicate with this related set of endpoints and the purpose of each endpoint within the interface may be
specified as part of a device class or vendor-specific definition.
In addition, an interface within a configuration may have alternate settings that redefine the number or
characteristics of the associated endpoints. If this is the case, the device must support the GetInterface()
request to report the current alternate setting for the specified interface and SetInterface() request to select
the alternate setting for the specified interface.
Within each configuration, each interface descriptor contains fields that identify the interface number and
the alternate setting. Interfaces are numbered from zero to one less than the number of concurrent interfaces
supported by the configuration. Alternate settings range from zero to one less than the number of alternate
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settings for a specific interface. The default setting when a device is initially configured is alternate setting
zero.
In support of adaptive device drivers that are capable of managing a related group of USB devices, the
device and interface descriptors contain Class, SubClass, and Protocol fields. These fields are used to
identify the function(s) provided by a USB device and the protocols used to communicate with the
function(s) on the device. A class code is assigned to a group of related devices that has been characterized
as a part of a USB Class Specification. A class of devices may be further subdivided into subclasses, and,
within a class or subclass, a protocol code may define how the Host Software communicates with the
device.
Note: The assignment of class, subclass, and protocol codes must be coordinated but is beyond the scope of
this specification.
9.2.4 Data Transfer
Data may be transferred between a USB device endpoint and the host in one of four ways. Refer to
Chapter 5 for the definition of the four types of transfers. An endpoint number may be used for different
types of data transfers in different alternate settings. However, once an alternate setting is selected
(including the default setting of an interface), a USB device endpoint uses only one data transfer method
until a different alternate setting is selected.
9.2.5 Power Management
Power management on USB devices involves the issues described in the following sections.
9.2.5.1 Power Budgeting
USB bus power is a limited resource. During device enumeration, a host evaluates a device’s power
requirements. If the power requirements of a particular configuration exceed the power available to the
device, Host Software shall not select that configuration.
USB devices shall limit the power they consume from VBUS to one unit load or less until configured.
Suspended devices, whether configured or not, shall limit their bus power consumption as defined in
Chapter 7. Depending on the power capabilities of the port to which the device is attached, a USB device
may be able to draw up to five unit loads from VBUS after configuration.
9.2.5.2 Remote Wakeup
Remote wakeup allows a suspended USB device to signal a host that may also be suspended. This notifies
the host that it should resume from its suspended mode, if necessary, and service the external event that
triggered the suspended USB device to signal the host. A USB device reports its ability to support remote
wakeup in a configuration descriptor. If a device supports remote wakeup, it must also allow the capability
to be enabled and disabled using the standard USB requests.
Remote wakeup is accomplished using electrical signaling described in Section 7.1.7.7.
9.2.6 Request Processing
With the exception of SetAddress() requests (see Section 9.4.6), a device may begin processing of a request
as soon as the device returns the ACK following the Setup. The device is expected to “complete”
processing of the request before it allows the Status stage to complete successfully. Some requests initiate
operations that take many milliseconds to complete. For requests such as this, the device class is required to
define a method other than Status stage completion to indicate that the operation has completed. For
example, a reset on a hub port takes at least 10 ms to complete. The SetPortFeature(PORT_RESET) (see
Chapter 11) request “completes” when the reset on the port is initiated. Completion of the reset operation is
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signaled when the port’s status change is set to indicate that the port is now enabled. This technique
prevents the host from having to constantly poll for a completion when it is known that the request will take
a relatively long period of time.
9.2.6.1 Request Processing Timing
All devices are expected to handle requests in a timely manner. USB sets an upper limit of 5 seconds as the
upper limit for any command to be processed. This limit is not applicable in all instances. The limitations
are described in the following sections. It should be noted that the limitations given below are intended to
encompass a wide range of implementations. If all devices in a USB system used the maximum allotted
time for request processing, the user experience would suffer. For this reason, implementations should
strive to complete requests in times that are as short as possible.
9.2.6.2 Reset/Resume Recovery Time
After a port is reset or resumed, the USB System Software is expected to provide a “recovery” interval of
10 ms before the device attached to the port is expected to respond to data transfers. The device may ignore
any data transfers during the recovery interval.
After the end of the recovery interval (measured from the end of the reset or the end of the EOP at the end
of the resume signaling), the device must accept data transfers at any time.
9.2.6.3 Set Address Processing
After the reset/resume recovery interval, if a device receives a SetAddress() request, the device must be able
to complete processing of the request and be able to successfully complete the Status stage of the request
within 50 ms. In the case of the SetAddress() request, the Status stage successfully completes when the
device sends the zero-length Status packet or when the device sees the ACK in response to the Status stage
data packet.
After successful completion of the Status stage, the device is allowed a SetAddress() recovery interval of
2 ms. At the end of this interval, the device must be able to accept Setup packets addressed to the new
address. Also, at the end of the recovery interval, the device must not respond to tokens sent to the old
address (unless, of course, the old and new address is the same).
9.2.6.4 Standard Device Requests
For standard device requests that require no Data stage, a device must be able to complete the request and
be able to successfully complete the Status stage of the request within 50 ms of receipt of the request. This
limitation applies to requests to the device, interface, or endpoint.
For standard device requests that require data stage transfer to the host, the device must be able to return the
first data packet to the host within 500 ms of receipt of the request. For subsequent data packets, if any, the
device must be able to return them within 500 ms of successful completion of the transmission of the
previous packet. The device must then be able to successfully complete the status stage within 50 ms after
returning the last data packet.
For standard device requests that require a data stage transfer to the device, the 5-second limit applies. This
means that the device must be capable of accepting all data packets from the host and successfully
completing the Status stage if the host provides the data at the maximum rate at which the device can accept
it. Delays between packets introduced by the host add to the time allowed for the device to complete the
request.
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9.2.6.5 Class-specific Requests
Unless specifically exempted in the class document, all class-specific requests must meet the timing
limitations for standard device requests. If a class document provides an exemption, the exemption may
only be specified on a request-by-request basis.
A class document may require that a device respond more quickly than is specified in this section. Faster
response may be required for standard and class-specific requests.
9.2.6.6 Speed Dependent Descriptors
A device capable of operation at high-speed can operate in either full- or high-speed. The device always
knows its operational speed due to having to manage its transceivers correctly as part of reset processing
(See Chapter 7 for more details on reset). A device also operates at a single speed after completing the reset
sequence. In particular, there is no speed switch during normal operation. However, a high-speed capable
device may have configurations that are speed dependent. That is, it may have some configurations that are
only possible when operating at high-speed or some that are only possible when operating at full-speed.
High-speed capable devices must support reporting their speed dependent configurations.
A high-speed capable device responds with descriptor information that is valid for the current operating
speed. For example, when a device is asked for configuration descriptors, it only returns those for the
current operating speed (e.g., full speed). However, there must be a way to determine the capabilities for
both high- and full-speed operation.
Two descriptors allow a high-speed capable device to report configuration information about the other
operating speed. The two descriptors are: the (other_speed) device_qualifier descriptor and the
other_speed_configuration descriptor. These two descriptors are retrieved by the host by using the
GetDescriptor request with the corresponding descriptor type values.
Note: These descriptors are not retrieved unless the host explicitly issues the corresponding GetDescriptor
requests. If these two requests are not issued, the device would simply appear to be a single speed device.
Devices that are high-speed capable must set the version number in the bcdUSB field of their descriptors to
0200H. This indicates that such devices support the other_speed requests defined by USB 2.0. A device
with descriptor version numbers less than 0200H should cause a Request Error response (see next section) if
it receives these other_speed requests. A USB 1.x device (i.e., one with a device descriptor version less
than 0200H) should not be issued the other_speed requests.
9.2.7 Request Error
When a request is received by a device that is not defined for the device, is inappropriate for the current
setting of the device, or has values that are not compatible with the request, then a Request Error exists.
The device deals with the Request Error by returning a STALL PID in response to the next Data stage
transaction or in the Status stage of the message. It is preferred that the STALL PID be returned at the next
Data stage transaction, as this avoids unnecessary bus activity.
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9.3
USB Device Requests
All USB devices respond to requests from the host on the device’s Default Control Pipe. These requests are
made using control transfers. The request and the request’s parameters are sent to the device in the Setup
packet. The host is responsible for establishing the values passed in the fields listed in Table 9-2. Every
Setup packet has eight bytes.
Table 9-2. Format of Setup Data
Offset
Field
Size
0
bmRequestType
1
Value
Bitmap
Description
Characteristics of request:
D7:
Data transfer direction
0 = Host-to-device
1 = Device-to-host
D6...5:
Type
0 = Standard
1 = Class
2 = Vendor
3 = Reserved
D4...0:
Recipient
0 = Device
1 = Interface
2 = Endpoint
3 = Other
4...31 = Reserved
1
bRequest
1
Value
Specific request (refer to Table 9-3)
2
wValue
2
Value
Word-sized field that varies according to
request
4
wIndex
2
Index or
Offset
Word-sized field that varies according to
request; typically used to pass an index or
offset
6
wLength
2
Count
Number of bytes to transfer if there is a
Data stage
9.3.1 bmRequestType
This bitmapped field identifies the characteristics of the specific request. In particular, this field identifies
the direction of data transfer in the second phase of the control transfer. The state of the Direction bit is
ignored if the wLength field is zero, signifying there is no Data stage.
The USB Specification defines a series of standard requests that all devices must support. These are
enumerated in Table 9-3. In addition, a device class may define additional requests. A device vendor may
also define requests supported by the device.
Requests may be directed to the device, an interface on the device, or a specific endpoint on a device. This
field also specifies the intended recipient of the request. When an interface or endpoint is specified, the
wIndex field identifies the interface or endpoint.
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9.3.2 bRequest
This field specifies the particular request. The Type bits in the bmRequestType field modify the meaning of
this field. This specification defines values for the bRequest field only when the bits are reset to zero,
indicating a standard request (refer to Table 9-3).
9.3.3 wValue
The contents of this field vary according to the request. It is used to pass a parameter to the device, specific
to the request.
9.3.4 wIndex
The contents of this field vary according to the request. It is used to pass a parameter to the device, specific
to the request.
The wIndex field is often used in requests to specify an endpoint or an interface. Figure 9-2 shows the
format of wIndex when it is used to specify an endpoint.
D7
D6
Direction
D15
D5
D4
D3
Reserved (Reset to zero)
D14
D13
D2
D1
D0
Endpoint Number
D12
D11
D10
D9
D8
Reserved (Reset to zero)
Figure 9-2. wIndex Format when Specifying an Endpoint
The Direction bit is set to zero to indicate the OUT endpoint with the specified Endpoint Number and to one
to indicate the IN endpoint. In the case of a control pipe, the request should have the Direction bit set to
zero but the device may accept either value of the Direction bit.
Figure 9-3 shows the format of wIndex when it is used to specify an interface.
D7
D6
D5
D4
D3
D2
D1
D0
D10
D9
D8
Interface Number
D15
D14
D13
D12
D11
Reserved (Reset to zero)
Figure 9-3. wIndex Format when Specifying an Interface
9.3.5 wLength
This field specifies the length of the data transferred during the second phase of the control transfer. The
direction of data transfer (host-to-device or device-to-host) is indicated by the Direction bit of the
bmRequestType field. If this field is zero, there is no data transfer phase.
On an input request, a device must never return more data than is indicated by the wLength value; it may
return less. On an output request, wLength will always indicate the exact amount of data to be sent by the
host. Device behavior is undefined if the host should send more data than is specified in wLength.
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9.4
Standard Device Requests
This section describes the standard device requests defined for all USB devices. Table 9-3 outlines the
standard device requests, while Table 9-4 and Table 9-5 give the standard request codes and descriptor
types, respectively.
USB devices must respond to standard device requests, even if the device has not yet been assigned an
address or has not been configured.
Table 9-3. Standard Device Requests
bmRequestType
bRequest
wValue
wIndex
wLength
Data
00000000B
00000001B
00000010B
CLEAR_FEATURE
Feature
Selector
Zero
Interface
Endpoint
Zero
None
10000000B
GET_CONFIGURATION
Zero
Zero
One
Configuration
Value
10000000B
GET_DESCRIPTOR
Descriptor
Type and
Descriptor
Index
Zero or
Language
ID
Descriptor
Length
Descriptor
10000001B
GET_INTERFACE
Zero
Interface
One
Alternate
Interface
10000000B
10000001B
10000010B
GET_STATUS
Zero
Zero
Interface
Endpoint
Two
Device,
Interface, or
Endpoint
Status
00000000B
SET_ADDRESS
Device
Address
Zero
Zero
None
00000000B
SET_CONFIGURATION
Configuration
Value
Zero
Zero
None
00000000B
SET_DESCRIPTOR
Descriptor
Type and
Descriptor
Index
Zero or
Language
ID
Descriptor
Length
Descriptor
00000000B
00000001B
00000010B
SET_FEATURE
Feature
Selector
Zero
Interface
Endpoint
Zero
None
00000001B
SET_INTERFACE
Alternate
Setting
Interface
Zero
None
10000010B
SYNCH_FRAME
Zero
Endpoint
Two
Frame Number
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Table 9-4. Standard Request Codes
bRequest
Value
GET_STATUS
0
CLEAR_FEATURE
1
Reserved for future use
2
SET_FEATURE
3
Reserved for future use
4
SET_ADDRESS
5
GET_DESCRIPTOR
6
SET_DESCRIPTOR
7
GET_CONFIGURATION
8
SET_CONFIGURATION
9
GET_INTERFACE
10
SET_INTERFACE
11
SYNCH_FRAME
12
Table 9-5. Descriptor Types
Descriptor Types
Value
DEVICE
1
CONFIGURATION
2
STRING
3
INTERFACE
4
ENDPOINT
5
DEVICE_QUALIFIER
6
OTHER_SPEED_CONFIGURATION
7
1
INTERFACE_POWER
8
1
The INTERFACE_POWER descriptor is defined in the current revision of the USB Interface Power
Management Specification.
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Feature selectors are used when enabling or setting features, such as remote wakeup, specific to a device,
interface, or endpoint. The values for the feature selectors are given in Table 9-6.
Table 9-6. Standard Feature Selectors
Feature Selector
DEVICE_REMOTE_WAKEUP
ENDPOINT_HALT
TEST_MODE
Recipient
Value
Device
1
Endpoint
0
Device
2
If an unsupported or invalid request is made to a USB device, the device responds by returning STALL in
the Data or Status stage of the request. If the device detects the error in the Setup stage, it is preferred that
the device returns STALL at the earlier of the Data or Status stage. Receipt of an unsupported or invalid
request does NOT cause the optional Halt feature on the control pipe to be set. If for any reason, the device
becomes unable to communicate via its Default Control Pipe due to an error condition, the device must be
reset to clear the condition and restart the Default Control Pipe.
9.4.1 Clear Feature
This request is used to clear or disable a specific feature.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
00000000B
00000001B
00000010B
CLEAR_FEATURE
Feature
Selector
Zero
Interface
Endpoint
Zero
None
Feature selector values in wValue must be appropriate to the recipient. Only device feature selector values
may be used when the recipient is a device, only interface feature selector values may be used when the
recipient is an interface, and only endpoint feature selector values may be used when the recipient is an
endpoint.
Refer to Table 9-6 for a definition of which feature selector values are defined for which recipients.
A ClearFeature() request that references a feature that cannot be cleared, that does not exist, or that
references an interface or endpoint that does not exist, will cause the device to respond with a Request
Error.
If wLength is non-zero, then the device behavior is not specified.
Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
This request is valid when the device is in the Address state; references to interfaces
or to endpoints other than endpoint zero shall cause the device to respond with a
Request Error.
Configured state:
This request is valid when the device is in the Configured state.
Note: The Test_Mode feature cannot be cleared by the ClearFeature() request.
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9.4.2 Get Configuration
This request returns the current device configuration value.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
10000000B
GET_CONFIGURATION
Zero
Zero
One
Configuration
Value
If the returned value is zero, the device is not configured.
If wValue, wIndex, or wLength are not as specified above, then the device behavior is not specified.
Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
The value zero must be returned.
Configured state:
The non-zero bConfigurationValue of the current configuration must be returned.
9.4.3 Get Descriptor
This request returns the specified descriptor if the descriptor exists.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
10000000B
GET_DESCRIPTOR
Descriptor
Type and
Descriptor
Index
Zero or
Language ID
(refer to
Section 9.6.7)
Descriptor
Length
Descriptor
The wValue field specifies the descriptor type in the high byte (refer to Table 9-5) and the descriptor index
in the low byte. The descriptor index is used to select a specific descriptor (only for configuration and
string descriptors) when several descriptors of the same type are implemented in a device. For example, a
device can implement several configuration descriptors. For other standard descriptors that can be retrieved
via a GetDescriptor() request, a descriptor index of zero must be used. The range of values used for a
descriptor index is from 0 to one less than the number of descriptors of that type implemented by the device.
The wIndex field specifies the Language ID for string descriptors or is reset to zero for other descriptors.
The wLength field specifies the number of bytes to return. If the descriptor is longer than the wLength field,
only the initial bytes of the descriptor are returned. If the descriptor is shorter than the wLength field, the
device indicates the end of the control transfer by sending a short packet when further data is requested. A
short packet is defined as a packet shorter than the maximum payload size or a zero length data packet (refer
to Chapter 5).
The standard request to a device supports three types of descriptors: device (also device_qualifier),
configuration (also other_speed_configuration), and string. A high-speed capable device supports the
device_qualifier descriptor to return information about the device for the speed at which it is not operating
(including wMaxPacketSize for the default endpoint and the number of configurations for the other speed).
The other_speed_configuration returns information in the same structure as a configuration descriptor, but
for a configuration if the device were operating at the other speed. A request for a configuration descriptor
returns the configuration descriptor, all interface descriptors, and endpoint descriptors for all of the
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interfaces in a single request. The first interface descriptor follows the configuration descriptor. The
endpoint descriptors for the first interface follow the first interface descriptor. If there are additional
interfaces, their interface descriptor and endpoint descriptors follow the first interface’s endpoint
descriptors. Class-specific and/or vendor-specific descriptors follow the standard descriptors they extend or
modify.
All devices must provide a device descriptor and at least one configuration descriptor. If a device does not
support a requested descriptor, it responds with a Request Error.
Default state:
This is a valid request when the device is in the Default state.
Address state:
This is a valid request when the device is in the Address state.
Configured state:
This is a valid request when the device is in the Configured state.
9.4.4 Get Interface
This request returns the selected alternate setting for the specified interface.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
10000001B
GET_INTERFACE
Zero
Interface
One
Alternate
Setting
Some USB devices have configurations with interfaces that have mutually exclusive settings. This request
allows the host to determine the currently selected alternate setting.
If wValue or wLength are not as specified above, then the device behavior is not specified.
If the interface specified does not exist, then the device responds with a Request Error.
Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
A Request Error response is given by the device.
Configured state:
This is a valid request when the device is in the Configured state.
9.4.5 Get Status
This request returns status for the specified recipient.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
10000000B
10000001B
10000010B
GET_STATUS
Zero
Zero
Interface
Endpoint
Two
Device,
Interface, or
Endpoint
Status
The Recipient bits of the bmRequestType field specify the desired recipient. The data returned is the current
status of the specified recipient.
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If wValue or wLength are not as specified above, or if wIndex is non-zero for a device status request, then
the behavior of the device is not specified.
If an interface or an endpoint is specified that does not exist, then the device responds with a Request Error.
Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
If an interface or an endpoint other than endpoint zero is specified, then the device
responds with a Request Error.
Configured state:
If an interface or endpoint that does not exist is specified, then the device responds
with a Request Error.
A GetStatus() request to a device returns the information shown in Figure 9-4.
D7
D6
D5
D4
D3
D2
Reserved (Reset to zero)
D15
D14
D13
D12
D11
D10
D1
D0
Remote
Wakeup
Self
Powered
D9
D8
Reserved (Reset to zero)
Figure 9-4. Information Returned by a GetStatus() Request to a Device
The Self Powered field indicates whether the device is currently self-powered. If D0 is reset to zero, the
device is bus-powered. If D0 is set to one, the device is self-powered. The Self Powered field may not be
changed by the SetFeature() or ClearFeature() requests.
The Remote Wakeup field indicates whether the device is currently enabled to request remote wakeup. The
default mode for devices that support remote wakeup is disabled. If D1 is reset to zero, the ability of the
device to signal remote wakeup is disabled. If D1 is set to one, the ability of the device to signal remote
wakeup is enabled. The Remote Wakeup field can be modified by the SetFeature() and ClearFeature()
requests using the DEVICE_REMOTE_WAKEUP feature selector. This field is reset to zero when the
device is reset.
A GetStatus() request to an interface returns the information shown in Figure 9-5.
D7
D6
D5
D4
D3
D2
D1
D0
D10
D9
D8
Reserved (Reset to zero)
D15
D14
D13
D12
D11
Reserved (Reset to zero)
Figure 9-5. Information Returned by a GetStatus() Request to an Interface
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A GetStatus() request to an endpoint returns the information shown in Figure 9-6.
D7
D6
D5
D4
D3
D2
D1
Reserved (Reset to zero)
D15
D14
D13
D12
D0
Halt
D11
D10
D9
D8
Reserved (Reset to zero)
Figure 9-6. Information Returned by a GetStatus() Request to an Endpoint
The Halt feature is required to be implemented for all interrupt and bulk endpoint types. If the endpoint is
currently halted, then the Halt feature is set to one. Otherwise, the Halt feature is reset to zero. The Halt
feature may optionally be set with the SetFeature(ENDPOINT_HALT) request. When set by the
SetFeature() request, the endpoint exhibits the same stall behavior as if the field had been set by a hardware
condition. If the condition causing a halt has been removed, clearing the Halt feature via a
ClearFeature(ENDPOINT_HALT) request results in the endpoint no longer returning a STALL. For
endpoints using data toggle, regardless of whether an endpoint has the Halt feature set, a
ClearFeature(ENDPOINT_HALT) request always results in the data toggle being reinitialized to DATA0.
The Halt feature is reset to zero after either a SetConfiguration() or SetInterface() request even if the
requested configuration or interface is the same as the current configuration or interface.
It is neither required nor recommended that the Halt feature be implemented for the Default Control Pipe.
However, devices may set the Halt feature of the Default Control Pipe in order to reflect a functional error
condition. If the feature is set to one, the device will return STALL in the Data and Status stages of each
standard request to the pipe except GetStatus(), SetFeature(), and ClearFeature() requests. The device need
not return STALL for class-specific and vendor-specific requests.
9.4.6 Set Address
This request sets the device address for all future device accesses.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
00000000B
SET_ADDRESS
Device
Address
Zero
Zero
None
The wValue field specifies the device address to use for all subsequent accesses.
As noted elsewhere, requests actually may result in up to three stages. In the first stage, the Setup packet is
sent to the device. In the optional second stage, data is transferred between the host and the device. In the
final stage, status is transferred between the host and the device. The direction of data and status transfer
depends on whether the host is sending data to the device or the device is sending data to the host. The
Status stage transfer is always in the opposite direction of the Data stage. If there is no Data stage, the
Status stage is from the device to the host.
Stages after the initial Setup packet assume the same device address as the Setup packet. The USB device
does not change its device address until after the Status stage of this request is completed successfully. Note
that this is a difference between this request and all other requests. For all other requests, the operation
indicated must be completed before the Status stage.
If the specified device address is greater than 127, or if wIndex or wLength are non-zero, then the behavior
of the device is not specified.
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Device response to SetAddress() with a value of 0 is undefined.
Default state:
If the address specified is non-zero, then the device shall enter the Address state;
otherwise, the device remains in the Default state (this is not an error condition).
Address state:
If the address specified is zero, then the device shall enter the Default state;
otherwise, the device remains in the Address state but uses the newly-specified
address.
Configured state:
Device behavior when this request is received while the device is in the Configured
state is not specified.
9.4.7 Set Configuration
This request sets the device configuration.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
00000000B
SET_CONFIGURATION
Configuration Value
Zero
Zero
None
The lower byte of the wValue field specifies the desired configuration. This configuration value must be
zero or match a configuration value from a configuration descriptor. If the configuration value is zero, the
device is placed in its Address state. The upper byte of the wValue field is reserved.
If wIndex, wLength, or the upper byte of wValue is non-zero, then the behavior of this request is not
specified.
Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
If the specified configuration value is zero, then the device remains in the Address
state. If the specified configuration value matches the configuration value from a
configuration descriptor, then that configuration is selected and the device enters the
Configured state. Otherwise, the device responds with a Request Error.
Configured state:
If the specified configuration value is zero, then the device enters the Address state.
If the specified configuration value matches the configuration value from a
configuration descriptor, then that configuration is selected and the device remains in
the Configured state. Otherwise, the device responds with a Request Error.
9.4.8 Set Descriptor
This request is optional and may be used to update existing descriptors or new descriptors may be added.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
00000000B
SET_DESCRIPTOR
Descriptor
Type and
Descriptor
Index
Language ID
(refer to
Section 9.6.7)
or zero
Descriptor
Length
Descriptor
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The wValue field specifies the descriptor type in the high byte (refer to Table 9-5) and the descriptor index
in the low byte. The descriptor index is used to select a specific descriptor (only for configuration and string
descriptors) when several descriptors of the same type are implemented in a device. For example, a device
can implement several configuration descriptors. For other standard descriptors that can be set via a
SetDescriptor() request, a descriptor index of zero must be used. The range of values used for a descriptor
index is from 0 to one less than the number of descriptors of that type implemented by the device.
The wIndex field specifies the Language ID for string descriptors or is reset to zero for other descriptors.
The wLength field specifies the number of bytes to transfer from the host to the device.
The only allowed values for descriptor type are device, configuration, and string descriptor types.
If this request is not supported, the device will respond with a Request Error.
Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
If supported, this is a valid request when the device is in the Address state.
Configured state:
If supported, this is a valid request when the device is in the Configured state.
9.4.9 Set Feature
This request is used to set or enable a specific feature.
bmRequestType
bRequest
wValue
00000000B
00000001B
00000010B
SET_FEATURE
Feature
Selector
wIndex
Test Selector
Zero
Interface
Endpoint
wLength
Data
Zero
None
Feature selector values in wValue must be appropriate to the recipient. Only device feature selector values
may be used when the recipient is a device; only interface feature selector values may be used when the
recipient is an interface, and only endpoint feature selector values may be used when the recipient is an
endpoint.
Refer to Table 9-6 for a definition of which feature selector values are defined for which recipients.
The TEST_MODE feature is only defined for a device recipient (i.e., bmRequestType = 0) and the lower
byte of wIndex must be zero. Setting the TEST_MODE feature puts the device upstream facing port into
test mode. The device will respond with a request error if the request contains an invalid test selector. The
transition to test mode must be complete no later than 3 ms after the completion of the status stage of the
request. The transition to test mode of an upstream facing port must not happen until after the status stage
of the request. The power to the device must be cycled to exit test mode of an upstream facing port of a
device. See Section 7.1.20 for definitions of each test mode. A device must support the TEST_MODE
feature when in the Default, Address or Configured high-speed device states.
A SetFeature() request that references a feature that cannot be set or that does not exist causes a STALL to
be returned in the Status stage of the request.
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Table 9-7. Test Mode Selectors
Value
Description
00H
Reserved
01H
Test_J
02H
Test_K
03H
Test_SE0_NAK
04H
Test_Packet
05H
Test_Force_Enable
06H-3FH
Reserved for standard test selectors
3FH-BFH
Reserved
C0H-FFH
Reserved for vendor-specific test modes.
If the feature selector is TEST_MODE, then the most significant byte of wIndex is used to specify the
specific test mode. The recipient of a SetFeature(TEST_MODE…) must be the device; i.e., the lower byte
of wIndex must be zero and the bmRequestType must be set to zero. The device must have its power cycled
to exit test mode. The valid test mode selectors are listed in Table 9-7. See Section 7.1.20 for more
information about the specific test modes.
If wLength is non-zero, then the behavior of the device is not specified.
If an endpoint or interface is specified that does not exist, then the device responds with a Request Error.
Default state:
A device must be able to accept a SetFeature(TEST_MODE, TEST_SELECTOR)
request when in the Default State. Device behavior for other SetFeature requests
while the device is in the Default state is not specified.
Address state:
If an interface or an endpoint other than endpoint zero is specified, then the device
responds with a Request Error.
Configured state:
This is a valid request when the device is in the Configured state.
9.4.10 Set Interface
This request allows the host to select an alternate setting for the specified interface.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
00000001B
SET_INTERFACE
Alternate
Setting
Interface
Zero
None
Some USB devices have configurations with interfaces that have mutually exclusive settings. This request
allows the host to select the desired alternate setting. If a device only supports a default setting for the
specified interface, then a STALL may be returned in the Status stage of the request. This request cannot be
used to change the set of configured interfaces (the SetConfiguration() request must be used instead).
If the interface or the alternate setting does not exist, then the device responds with a Request Error. If
wLength is non-zero, then the behavior of the device is not specified.
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Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
The device must respond with a Request Error.
Configured state:
This is a valid request when the device is in the Configured state.
9.4.11 Synch Frame
This request is used to set and then report an endpoint’s synchronization frame.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
10000010B
SYNCH_FRAME
Zero
Endpoint
Two
Frame
Number
When an endpoint supports isochronous transfers, the endpoint may also require per-frame transfers to vary
in size according to a specific pattern. The host and the endpoint must agree on which frame the repeating
pattern begins. The number of the frame in which the pattern began is returned to the host.
If a high-speed device supports the Synch Frame request, it must internally synchronize itself to the zeroth
microframe and have a time notion of classic frame. Only the frame number is used to synchronize and
reported by the device endpoint (i.e., no microframe number). The endpoint must synchronize to the zeroth
microframe.
This value is only used for isochronous data transfers using implicit pattern synchronization. If wValue is
non-zero or wLength is not two, then the behavior of the device is not specified.
If the specified endpoint does not support this request, then the device will respond with a Request Error.
9.5
Default state:
Device behavior when this request is received while the device is in the Default state
is not specified.
Address state:
The device shall respond with a Request Error.
Configured state:
This is a valid request when the device is in the Configured state.
Descriptors
USB devices report their attributes using descriptors. A descriptor is a data structure with a defined format.
Each descriptor begins with a byte-wide field that contains the total number of bytes in the descriptor
followed by a byte-wide field that identifies the descriptor type.
Using descriptors allows concise storage of the attributes of individual configurations because each
configuration may reuse descriptors or portions of descriptors from other configurations that have the same
characteristics. In this manner, the descriptors resemble individual data records in a relational database.
Where appropriate, descriptors contain references to string descriptors that provide displayable information
describing a descriptor in human-readable form. The inclusion of string descriptors is optional. However,
the reference fields within descriptors are mandatory. If a device does not support string descriptors, string
reference fields must be reset to zero to indicate no string descriptor is available.
If a descriptor returns with a value in its length field that is less than defined by this specification, the
descriptor is invalid and should be rejected by the host. If the descriptor returns with a value in its length
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field that is greater than defined by this specification, the extra bytes are ignored by the host, but the next
descriptor is located using the length returned rather than the length expected.
A device may return class- or vendor-specific descriptors in two ways:
9.6
1.
If the class or vendor specific descriptors use the same format as standard descriptors (e.g., start with a
length byte and followed by a type byte), they must be returned interleaved with standard descriptors in
the configuration information returned by a GetDescriptor(Configuration) request. In this case, the
class or vendor-specific descriptors must follow a related standard descriptor they modify or extend.
2.
If the class or vendor specific descriptors are independent of configuration information or use a nonstandard format, a GetDescriptor() request specifying the class or vendor specific descriptor type and
index may be used to retrieve the descriptor from the device. A class or vendor specification will
define the appropriate way to retrieve these descriptors.
Standard USB Descriptor Definitions
The standard descriptors defined in this specification may only be modified or extended by revision of the
Universal Serial Bus Specification.
Note: An extension to the USB 1.0 standard endpoint descriptor has been published in Device Class
Specification for Audio Devices Revision 1.0. This is the only extension defined outside USB Specification
that is allowed. Future revisions of the USB Specification that extend the standard endpoint descriptor will
do so as to not conflict with the extension defined in the Audio Device Class Specification Revision 1.0.
9.6.1 Device
A device descriptor describes general information about a USB device. It includes information that applies
globally to the device and all of the device’s configurations. A USB device has only one device descriptor.
A high-speed capable device that has different device information for full-speed and high-speed must also
have a device_qualifier descriptor (see Section 9.6.2).
The DEVICE descriptor of a high-speed capable device has a version number of 2.0 (0200H). If the device
is full-speed only or low-speed only, this version number indicates that it will respond correctly to a request
for the device_qualifier desciptor (i.e., it will respond with a request error).
The bcdUSB field contains a BCD version number. The value of the bcdUSB field is 0xJJMN for version
JJ.M.N (JJ – major version number, M – minor version number, N – sub-minor version number), e.g.,
version 2.1.3 is represented with value 0x0213 and version 2.0 is represented with a value of 0x0200.
The bNumConfigurations field indicates the number of configurations at the current operating speed.
Configurations for the other operating speed are not included in the count. If there are specific
configurations of the device for specific speeds, the bNumConfigurations field only reflects the number of
configurations for a single speed, not the total number of configurations for both speeds.
If the device is operating at high-speed, the bMaxPacketSize0 field must be 64 indicating a 64 byte
maximum packet. High-speed operation does not allow other maximum packet sizes for the control
endpoint (endpoint 0).
All USB devices have a Default Control Pipe. The maximum packet size of a device’s Default Control Pipe
is described in the device descriptor. Endpoints specific to a configuration and its interface(s) are described
in the configuration descriptor. A configuration and its interface(s) do not include an endpoint descriptor
for the Default Control Pipe. Other than the maximum packet size, the characteristics of the Default
Control Pipe are defined by this specification and are the same for all USB devices.
The bNumConfigurations field identifies the number of configurations the device supports. Table 9-8 shows
the standard device descriptor.
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Table 9-8. Standard Device Descriptor
Offset
Field
Size
Value
Description
0
bLength
1
Number
Size of this descriptor in bytes
1
bDescriptorType
1
Constant
DEVICE Descriptor Type
2
bcdUSB
2
BCD
USB Specification Release Number in
Binary-Coded Decimal (i.e., 2.10 is 210H).
This field identifies the release of the USB
Specification with which the device and its
descriptors are compliant.
4
bDeviceClass
1
Class
Class code (assigned by the USB-IF).
If this field is reset to zero, each interface
within a configuration specifies its own
class information and the various
interfaces operate independently.
If this field is set to a value between 1 and
FEH, the device supports different class
specifications on different interfaces and
the interfaces may not operate
independently. This value identifies the
class definition used for the aggregate
interfaces.
If this field is set to FFH, the device class
is vendor-specific.
5
bDeviceSubClass
1
SubClass
Subclass code (assigned by the USB-IF).
These codes are qualified by the value of
the bDeviceClass field.
If the bDeviceClass field is reset to zero,
this field must also be reset to zero.
If the bDeviceClass field is not set to FFH,
all values are reserved for assignment by
the USB-IF.
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Table 9-8. Standard Device Descriptor (Continued)
Offset
6
Field
bDeviceProtocol
Size
1
Value
Protocol
Description
Protocol code (assigned by the USB-IF).
These codes are qualified by the value of
the bDeviceClass and the
bDeviceSubClass fields. If a device
supports class-specific protocols on a
device basis as opposed to an interface
basis, this code identifies the protocols
that the device uses as defined by the
specification of the device class.
If this field is reset to zero, the device
does not use class-specific protocols on a
device basis. However, it may use classspecific protocols on an interface basis.
If this field is set to FFH, the device uses a
vendor-specific protocol on a device basis.
7
bMaxPacketSize0
1
Number
Maximum packet size for endpoint zero
(only 8, 16, 32, or 64 are valid)
8
idVendor
2
ID
Vendor ID (assigned by the USB-IF)
10
idProduct
2
ID
Product ID (assigned by the manufacturer)
12
bcdDevice
2
BCD
Device release number in binary-coded
decimal
14
iManufacturer
1
Index
Index of string descriptor describing
manufacturer
15
iProduct
1
Index
Index of string descriptor describing
product
16
iSerialNumber
1
Index
Index of string descriptor describing the
device’s serial number
17
bNumConfigurations
1
Number
Number of possible configurations
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9.6.2 Device_Qualifier
The device_qualifier descriptor describes information about a high-speed capable device that would
change if the device were operating at the other speed. For example, if the device is currently operating
at full-speed, the device_qualifier returns information about how it would operate at high-speed and
vice-versa. Table 9-9 shows the fields of the device_qualifier descriptor.
Table 9-9. Device_Qualifier Descriptor
Offset
Field
Size
Value
Description
0
bLength
1
Number
Size of descriptor
1
bDescriptorType
1
Constant
Device Qualifier Type
2
bcdUSB
2
BCD
USB specification version number (e.g.,
0200H for V2.00 )
4
bDeviceClass
1
Class
Class Code
5
bDeviceSubClass
1
SubClass
SubClass Code
6
bDeviceProtocol
1
Protocol
Protocol Code
7
bMaxPacketSize0
1
Number
Maximum packet size for other speed
8
bNumConfigurations
1
Number
Number of Other-speed Configurations
9
bReserved
1
Zero
Reserved for future use, must be zero
The vendor, product, device, manufacturer, product, and serialnumber fields of the standard device
descriptor are not included in this descriptor since that information is constant for a device for all supported
speeds. The version number for this descriptor must be at least 2.0 (0200H).
The host accesses this descriptor using the GetDescriptor() request. The descriptor type in the
GetDescriptor() request is set to device_qualifier (see Table 9-5).
If a full-speed only device (with a device descriptor version number equal to 0200H) receives a
GetDescriptor() request for a device_qualifier, it must respond with a request error. The host must not make
a request for an other_speed_configuration descriptor unless it first successfully retrieves the
device_qualifier descriptor.
9.6.3 Configuration
The configuration descriptor describes information about a specific device configuration. The descriptor
contains a bConfigurationValue field with a value that, when used as a parameter to the SetConfiguration()
request, causes the device to assume the described configuration.
The descriptor describes the number of interfaces provided by the configuration. Each interface may
operate independently. For example, an ISDN device might be configured with two interfaces, each
providing 64 Kb/s bi-directional channels that have separate data sources or sinks on the host. Another
configuration might present the ISDN device as a single interface, bonding the two channels into one
128 Kb/s bi-directional channel.
When the host requests the configuration descriptor, all related interface and endpoint descriptors are
returned (refer to Section 9.4.3).
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A USB device has one or more configuration descriptors. Each configuration has one or more interfaces
and each interface has zero or more endpoints. An endpoint is not shared among interfaces within a single
configuration unless the endpoint is used by alternate settings of the same interface. Endpoints may be
shared among interfaces that are part of different configurations without this restriction.
Once configured, devices may support limited adjustments to the configuration. If a particular interface has
alternate settings, an alternate may be selected after configuration. Table 9-10 shows the standard
configuration descriptor.
Table 9-10. Standard Configuration Descriptor
Offset
Field
Size
Value
Description
0
bLength
1
Number
Size of this descriptor in bytes
1
bDescriptorType
1
Constant
CONFIGURATION Descriptor Type
2
wTotalLength
2
Number
Total length of data returned for this
configuration. Includes the combined length
of all descriptors (configuration, interface,
endpoint, and class- or vendor-specific)
returned for this configuration.
4
bNumInterfaces
1
Number
Number of interfaces supported by this
configuration
5
bConfigurationValue
1
Number
Value to use as an argument to the
SetConfiguration() request to select this
configuration
6
iConfiguration
1
Index
Index of string descriptor describing this
configuration
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Table 9-10. Standard Configuration Descriptor (Continued)
Offset
7
Field
bmAttributes
Size
1
Value
Bitmap
Description
Configuration characteristics
D7:
D6:
D5:
D4...0:
Reserved (set to one)
Self-powered
Remote Wakeup
Reserved (reset to zero)
D7 is reserved and must be set to one for
historical reasons.
A device configuration that uses power from
the bus and a local source reports a non-zero
value in bMaxPower to indicate the amount of
bus power required and sets D6. The actual
power source at runtime may be determined
using the GetStatus(DEVICE) request (see
Section 9.4.5).
If a device configuration supports remote
wakeup, D5 is set to one.
8
bMaxPower
1
mA
Maximum power consumption of the USB
device from the bus in this specific
configuration when the device is fully
operational. Expressed in 2 mA units
(i.e., 50 = 100 mA).
Note: A device configuration reports whether
the configuration is bus-powered or selfpowered. Device status reports whether the
device is currently self-powered. If a device is
disconnected from its external power source, it
updates device status to indicate that it is no
longer self-powered.
A device may not increase its power draw
from the bus, when it loses its external power
source, beyond the amount reported by its
configuration.
If a device can continue to operate when
disconnected from its external power source, it
continues to do so. If the device cannot
continue to operate, it fails operations it can
no longer support. The USB System Software
may determine the cause of the failure by
checking the status and noting the loss of the
device’s power source.
9.6.4 Other_Speed_Configuration
The other_speed_configuration descriptor shown in Table 9-11 describes a configuration of a highspeed capable device if it were operating at its other possible speed. The structure of the
other_speed_configuration is identical to a configuration descriptor.
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Table 9-11. Other_Speed_Configuration Descriptor
Offset
Field
Size
Value
Description
0
bLength
1
Number
Size of descriptor
1
bDescriptorType
1
Constant
Other_speed_Configuration Type
2
wTotalLength
2
Number
Total length of data returned
4
bNumInterfaces
1
Number
Number of interfaces supported by this speed
configuration
5
bConfigurationValue
1
Number
Value to use to select configuration
6
iConfiguration
1
Index
Index of string descriptor
7
bmAttributes
1
Bitmap
Same as Configuration descriptor
8
bMaxPower
1
mA
Same as Configuration descriptor
The host accesses this descriptor using the GetDescriptor() request. The descriptor type in the
GetDescriptor() request is set to other_speed_configuration (see Table 9-5).
9.6.5 Interface
The interface descriptor describes a specific interface within a configuration. A configuration provides one
or more interfaces, each with zero or more endpoint descriptors describing a unique set of endpoints within
the configuration. When a configuration supports more than one interface, the endpoint descriptors for a
particular interface follow the interface descriptor in the data returned by the GetConfiguration() request.
An interface descriptor is always returned as part of a configuration descriptor. Interface descriptors cannot
be directly accessed with a GetDescriptor() or SetDescriptor() request.
An interface may include alternate settings that allow the endpoints and/or their characteristics to be varied
after the device has been configured. The default setting for an interface is always alternate setting zero.
The SetInterface() request is used to select an alternate setting or to return to the default setting. The
GetInterface() request returns the selected alternate setting.
Alternate settings allow a portion of the device configuration to be varied while other interfaces remain in
operation. If a configuration has alternate settings for one or more of its interfaces, a separate interface
descriptor and its associated endpoints are included for each setting.
If a device configuration supported a single interface with two alternate settings, the configuration
descriptor would be followed by an interface descriptor with the bInterfaceNumber and bAlternateSetting
fields set to zero and then the endpoint descriptors for that setting, followed by another interface descriptor
and its associated endpoint descriptors. The second interface descriptor’s bInterfaceNumber field would
also be set to zero, but the bAlternateSetting field of the second interface descriptor would be set to one.
If an interface uses only endpoint zero, no endpoint descriptors follow the interface descriptor. In this case,
the bNumEndpoints field must be set to zero.
An interface descriptor never includes endpoint zero in the number of endpoints. Table 9-12 shows the
standard interface descriptor.
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Table 9-12. Standard Interface Descriptor
Offset
Field
Size
Value
Description
0
bLength
1
Number
Size of this descriptor in bytes
1
bDescriptorType
1
Constant
INTERFACE Descriptor Type
2
bInterfaceNumber
1
Number
Number of this interface. Zero-based
value identifying the index in the array of
concurrent interfaces supported by this
configuration.
3
bAlternateSetting
1
Number
Value used to select this alternate setting
for the interface identified in the prior field
4
bNumEndpoints
1
Number
Number of endpoints used by this
interface (excluding endpoint zero). If this
value is zero, this interface only uses the
Default Control Pipe.
5
bInterfaceClass
1
Class
Class code (assigned by the USB-IF).
A value of zero is reserved for future
standardization.
If this field is set to FFH, the interface
class is vendor-specific.
All other values are reserved for
assignment by the USB-IF.
6
bInterfaceSubClass
1
SubClass
Subclass code (assigned by the USB-IF).
These codes are qualified by the value of
the bInterfaceClass field.
If the bInterfaceClass field is reset to zero,
this field must also be reset to zero.
If the bInterfaceClass field is not set to
FFH, all values are reserved for
assignment by the USB-IF.
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Table 9-12. Standard Interface Descriptor (Continued)
Offset
7
Field
Size
bInterfaceProtocol
1
Value
Protocol
Description
Protocol code (assigned by the USB).
These codes are qualified by the value of
the bInterfaceClass and the
bInterfaceSubClass fields. If an interface
supports class-specific requests, this code
identifies the protocols that the device
uses as defined by the specification of the
device class.
If this field is reset to zero, the device
does not use a class-specific protocol on
this interface.
If this field is set to FFH, the device uses
a vendor-specific protocol for this
interface.
8
iInterface
1
Index
Index of string descriptor describing this
interface
9.6.6 Endpoint
Each endpoint used for an interface has its own descriptor. This descriptor contains the information
required by the host to determine the bandwidth requirements of each endpoint. An endpoint descriptor is
always returned as part of the configuration information returned by a GetDescriptor(Configuration)
request. An endpoint descriptor cannot be directly accessed with a GetDescriptor() or SetDescriptor()
request. There is never an endpoint descriptor for endpoint zero. Table 9-13 shows the standard endpoint
descriptor.
Table 9-13. Standard Endpoint Descriptor
Offset
Field
Size
Value
Description
0
bLength
1
Number
Size of this descriptor in bytes
1
bDescriptorType
1
Constant
ENDPOINT Descriptor Type
2
bEndpointAddress
1
Endpoint
The address of the endpoint on the USB device
described by this descriptor. The address is
encoded as follows:
Bit 3...0: The endpoint number
Bit 6...4: Reserved, reset to zero
Bit 7:
Direction, ignored for
control endpoints
0 = OUT endpoint
1 = IN endpoint
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Universal Serial Bus Specification Revision 2.0
Table 9-13. Standard Endpoint Descriptor (Continued)
Offset
3
Field
bmAttributes
Size
1
Value
Bitmap
Description
This field describes the endpoint’s attributes when it is
configured using the bConfigurationValue.
Bits 1..0: Transfer Type
00 = Control
01 = Isochronous
10 = Bulk
11 = Interrupt
If not an isochronous endpoint, bits 5..2 are reserved
and must be set to zero. If isochronous, they are
defined as follows:
Bits 3..2: Synchronization Type
00 = No Synchronization
01 = Asynchronous
10 = Adaptive
11 = Synchronous
Bits 5..4: Usage Type
00 = Data endpoint
01 = Feedback endpoint
10 = Implicit feedback Data endpoint
11 = Reserved
Refer to Chapter 5 for more information.
All other bits are reserved and must be reset to zero.
Reserved bits must be ignored by the host.
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Table 9-13. Standard Endpoint Descriptor (Continued)
Offset
4
Field
wMaxPacketSize
Size
Value
2
Number
Description
Maximum packet size this endpoint is capable of
sending or receiving when this configuration is
selected.
For isochronous endpoints, this value is used to
reserve the bus time in the schedule, required for the
per-(micro)frame data payloads. The pipe may, on an
ongoing basis, actually use less bandwidth than that
reserved. The device reports, if necessary, the actual
bandwidth used via its normal, non-USB defined
mechanisms.
For all endpoints, bits 10..0 specify the maximum
packet size (in bytes).
For high-speed isochronous and interrupt endpoints:
Bits 12..11 specify the number of additional transaction
opportunities per microframe:
00 = None (1 transaction per microframe)
01 = 1 additional (2 per microframe)
10 = 2 additional (3 per microframe)
11 = Reserved
Bits 15..13 are reserved and must be set to zero.
Refer to Chapter 5 for more information.
6
bInterval
1
Number
Interval for polling endpoint for data transfers.
Expressed in frames or microframes depending on the
device operating speed (i.e., either 1 millisecond or
125 µs units).
For full-/high-speed isochronous endpoints, this value
must be in the range from 1 to 16. The bInterval value
bInterval-1
is used as the exponent for a 2
value; e.g., a
4-1
bInterval of 4 means a period of 8 (2 ).
For full-/low-speed interrupt endpoints, the value of
this field may be from 1 to 255.
For high-speed interrupt endpoints, the bInterval value
bInterval-1
is used as the exponent for a 2
value; e.g., a
4-1
bInterval of 4 means a period of 8 (2 ). This value
must be from 1 to 16.
For high-speed bulk/control OUT endpoints, the
bInterval must specify the maximum NAK rate of the
endpoint. A value of 0 indicates the endpoint never
NAKs. Other values indicate at most 1 NAK each
bInterval number of microframes. This value must be
in the range from 0 to 255.
See Chapter 5 description of periods for more detail.
The bmAttributes field provides information about the endpoint’s Transfer Type (bits 1..0) and
Synchronization Type (bits 3..2). In addition, the Usage Type bit (bits 5..4) indicate whether this is an
endpoint used for normal data transfers (bits 5..4=00B), whether it is used to convey explicit feedback
information for one or more data endpoints (bits 5..4=01B) or whether it is a data endpoint that also serves
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Universal Serial Bus Specification Revision 2.0
as an implicit feedback endpoint for one or more data endpoints (bits 5..4=10B). Bits 5..2 are only
meaningful for isochronous endpoints and must be reset to zero for all other transfer types.
If the endpoint is used as an explicit feedback endpoint (bits 5..4=01B), then the Transfer Type must be set
to isochronous (bits1..0 = 01B) and the Synchronization Type must be set to No Synchronization
(bits 3..2=00B).
A feedback endpoint (explicit or implicit) needs to be associated with one (or more) isochronous data
endpoints to which it provides feedback service. The association is based on endpoint number matching. A
feedback endpoint always has the opposite direction from the data endpoint(s) it services. If multiple data
endpoints are to be serviced by the same feedback endpoint, the data endpoints must have ascending
ordered–but not necessarily consecutive–endpoint numbers. The first data endpoint and the feedback
endpoint must have the same endpoint number (and opposite direction). This ensures that a data endpoint
can uniquely identify its feedback endpoint by searching for the first feedback endpoint that has an endpoint
number equal or less than its own endpoint number.
Example: Consider the extreme case where there is a need for five groups of OUT asynchronous
isochronous endpoints and at the same time four groups of IN adaptive isochronous endpoints. Each group
needs a separate feedback endpoint and the groups are composed as shown in Figure 9-7.
OUT
Group
Nr of OUT
Endpoints
IN
Group
Nr of IN
Endpoints
1
1
6
1
2
2
7
2
3
2
8
3
4
3
9
4
5
3
Figure 9-7. Example of Feedback Endpoint Numbers
The endpoint numbers can be intertwined as illustrated in Figure 9-8.
1
2
3
4
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
2
3
Data Endpoint
4
Feedback Endpoint
Figure 9-8. Example of Feedback Endpoint Relationships
272
OUT
5
IN
Universal Serial Bus Specification Revision 2.0
High-speed isochronous and interrupt endpoints use bits 12..11 of wMaxPacketSize to specify multiple
transactions for each microframe specified by bInterval. If bits 12..11 of wMaxPacketSize are zero, the
maximum packet size for the endpoint can be any allowed value (as defined in Chapter 5). If bits 12..11 of
wMaxPacketSize are not zero (0), the allowed values for wMaxPacketSize bits 10..0 are limited as shown in
Table 9-14.
Table 9-14. Allowed wMaxPacketSize Values for Different Numbers of Transactions per Microframe
wMaxPacketSize
bits 12..11
wMaxPacketSize
bits 10..0 Values
Allowed
00
1 – 1024
01
513 – 1024
10
683 – 1024
11
N/A; reserved
For high-speed bulk and control OUT endpoints, the bInterval field is only used for compliance purposes;
the host controller is not required to change its behavior based on the value in this field.
9.6.7 String
String descriptors are optional. As noted previously, if a device does not support string descriptors, all
references to string descriptors within device, configuration, and interface descriptors must be reset to zero.
String descriptors use UNICODE encodings as defined by The Unicode Standard, Worldwide Character
Encoding, Version 3.0, The Unicode Consortium, Addison-Wesley Publishing Company, Reading,
Massachusetts (URL: http://www.unicode.com). The strings in a USB device may support multiple
languages. When requesting a string descriptor, the requester specifies the desired language using a sixteenbit language ID (LANGID) defined by the USB-IF. The list of currently defined USB LANGIDs can be
found at http://www.usb.org/developers/docs.html. String index zero for all languages returns a string
descriptor that contains an array of two-byte LANGID codes supported by the device. Table 9-15 shows the
LANGID code array. A USB device may omit all string descriptors. USB devices that omit all string
descriptors must not return an array of LANGID codes.
The array of LANGID codes is not NULL-terminated. The size of the array (in bytes) is computed by
subtracting two from the value of the first byte of the descriptor.
Table 9-15. String Descriptor Zero, Specifying Languages Supported by the Device
Offset
Field
Size
Value
Description
0
bLength
1
N+2
Size of this descriptor in bytes
1
bDescriptorType
1
Constant
STRING Descriptor Type
2
wLANGID[0]
2
Number
LANGID code zero
...
...
...
...
...
N
wLANGID[x]
2
Number
LANGID code x
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Universal Serial Bus Specification Revision 2.0
The UNICODE string descriptor (shown in Table 9-16) is not NULL-terminated. The string length is
computed by subtracting two from the value of the first byte of the descriptor.
Table 9-16. UNICODE String Descriptor
Offset
9.7
Field
Size
Value
Description
0
bLength
1
Number
Size of this descriptor in bytes
1
bDescriptorType
1
Constant
STRING Descriptor Type
2
bString
N
Number
UNICODE encoded string
Device Class Definitions
All devices must support the requests and descriptor definitions described in this chapter. Most devices
provide additional requests and, possibly, descriptors for device-specific extensions. In addition, devices
may provide extended services that are common to a group of devices. In order to define a class of devices,
the following information must be provided to completely define the appearance and behavior of the device
class.
9.7.1 Descriptors
If the class requires any specific definition of the standard descriptors, the class definition must include
those requirements as part of the class definition. In addition, if the class defines a standard extended set of
descriptors, they must also be fully defined in the class definition. Any extended descriptor definitions must
follow the approach used for standard descriptors; for example, all descriptors must begin with a length
field.
9.7.2 Interface(s) and Endpoint Usage
When a class of devices is standardized, the interfaces used by the devices, including how endpoints are
used, must be included in the device class definition. Devices may further extend a class definition with
proprietary features as long as they meet the base definition of the class.
9.7.3 Requests
All of the requests specific to the class must be defined.
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Chapter 10
USB Host: Hardware and Software
The USB interconnect supports data traffic between a host and a USB device. This chapter describes the
host interfaces necessary to facilitate USB communication between a software client, resident on the host,
and a function implemented on a device. The implementation described in this chapter is not required.
This implementation is provided as an example to illustrate the host system behavior expected by a USB
device. A host system may provide a different host software implementation as long as a USB device
experiences the same host behavior.
10.1 Overview of the USB Host
10.1.1 Overview
The basic flow and interrelationships of the USB communications model are shown in Figure 10-1.
Host
Client
USB System
USB Bus
Interface
Interconnect
Device
Function
USB Device
USB Bus
Interface
Actual communications flow
Logical communications flow
Figure 10-1. Interlayer Communications Model
The host and the device are divided into the distinct layers depicted in Figure 10-1. Vertical arrows
indicate the actual communication on the host. The corresponding interfaces on the device are
implementation-specific. All communications between the host and device ultimately occur on the
physical USB wire. However, there are logical host-device interfaces between each horizontal layer.
These communications, between client software resident on the host and the function provided by the
device, are typified by a contract based on the needs of the application currently using the device and the
capabilities provided by the device.
This client-function interaction creates the requirements for all of the underlying layers and their interfaces.
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This chapter describes this model from the point of view of the host and its layers. Figure 10-2 illustrates,
based on the overall view introduced in Chapter 5, the host’s view of its communication with the device.
Host
Interconnect
Client
manages interfaces
Pipe Bundle
to an interface
Configuration
IRPs
Host
Software
USB Driver
Default Pipe
HC Driver
to Endpoint Zero
USB System
manages pipes
HW-Defined
Host
Controller
HCDefined
SIE
USB Wire
USB Bus
Interface
Pipe: Represents connection
abstraction between two horizontal
layers
Optional
Component
Interprocess Communication
Figure 10-2. Host Communications
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There is only one host for each USB. The major layers of a host consist of the following:
•
USB bus interface
•
USB System
•
Client
The USB bus interface handles interactions for the electrical and protocol layers (refer to Chapter 7 and
Chapter 8). From the interconnect point of view, a similar USB bus interface is provided by both the USB
device and the host, as exemplified by the Serial Interface Engine (SIE). On the host, however, the USB
bus interface has additional responsibilities due to the unique role of the host on the USB and is
implemented as the Host Controller. The Host Controller has an integrated root hub providing attachment
points to the USB wire.
The USB System uses the Host Controller to manage data transfers between the host and USB devices.
The interface between the USB System and the Host Controller is dependent on the hardware definition of
the Host Controller. The USB System, in concert with the Host Controller, performs the translation
between the client’s view of data transfers and the USB transactions appearing on the interconnect. This
includes the addition of any USB feature support such as protocol wrappers. The USB System is also
responsible for managing USB resources, such as bandwidth and bus power, so that client access to the
USB is possible.
The USB System has three basic components:
•
Host Controller Driver
•
USB Driver
•
Host Software
The Host Controller Driver (HCD) exists to more easily map the various Host Controller implementations
into the USB System, such that a client can interact with its device without knowing to which Host
Controller the device is connected. The USB Driver (USBD) provides the basic host interface (USBDI) for
clients to USB devices. The interface between the HCD and the USBD is known as the Host Controller
Driver Interface (HCDI). This interface is never available directly to clients and thus is not defined by the
USB Specification. A particular HCDI is, however, defined by each operating system that supports various
Host Controller implementations.
The USBD provides data transfer mechanisms in the form of I/O Request Packets (IRPs), which consist of
a request to transport data across a specific pipe. In addition to providing data transfer mechanisms, the
USBD is responsible for presenting to its clients an abstraction of a USB device that can be manipulated for
configuration and state management. As part of this abstraction, the USBD owns the default pipe (see
Chapter 5 and Chapter 9) through which all USB devices are accessed for the purposes of standard USB
control. This default pipe represents a logical communication between the USBD and the abstraction of a
USB device as shown in Figure 10-2.
In some operating systems, additional non-USB System Software is available that provides configuration
and loading mechanisms to device drivers. In such operating systems, the device driver shall use the
provided interfaces instead of directly accessing the USBDI mechanisms.
The client layer describes all the software entities that are responsible for directly interacting with USB
devices. When each device is attached to the system, these clients might interact directly with the
peripheral hardware. The shared characteristics of the USB place USB System Software between the client
and its device; that is, a client cannot directly access the device’s hardware.
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Overall, the host layers provide the following capabilities:
•
Detecting the attachment and removal of USB devices
•
Managing USB standard control flow between the host and USB devices
•
Managing data flow between the host and USB devices
•
Collecting status and activity statistics
•
Controlling the electrical interface between the Host Controller and USB devices, including the
provision of a limited amount of power
The following sections describe these responsibilities and the requirements placed on the USBDI in greater
detail. The actual interfaces used for a specific combination of host platform and operating system are
described in the appropriate operating system environment guide.
All hubs (see Chapter 11) report internal status changes and their port change status via the status change
pipe. This includes a notification of when a USB device is attached to or removed from one of their ports.
A USBD client generically known as the hub driver receives these notifications as owner of the hub’s
Status Change pipe. For device attachments, the hub driver then initiates the device configuration process.
In some systems, this hub driver is a part of the host software provided by the operating system for
managing devices.
10.1.2 Control Mechanisms
Control information may be passed between the host and a USB device using in-band or out-of-band
signaling. In-band signaling mixes control information with data in a pipe outside the awareness of the
host. Out-of-band signaling places control information in a separate pipe.
There is a message pipe called the default pipe for each attached USB device. This logical association
between a host and a USB device is used for USB standard control flow such as device enumeration and
configuration. The default pipe provides a standard interface to all USB devices. The default pipe may
also be used for device-specific communications, as mediated by the USBD, which owns the default pipes
of all of the USB devices.
A particular USB device may allow the use of additional message pipes to transfer device-specific control
information. These pipes use the same communications protocol as the default pipe, but the information
transferred is specific to the USB device and is not standardized by the USB Specification.
The USBD supports the sharing of the default pipe, which it owns and uses, with its clients. It also
provides access to any other control pipes associated with the device.
10.1.3 Data Flow
The Host Controller is responsible for transferring streams of data between the host and USB devices.
These data transfers are treated as a continuous stream of bytes. The USB supports four basic types of data
transfers:
•
Control transfers
•
Isochronous transfers
•
Interrupt transfers
•
Bulk transfers
For additional information on transfer types, refer to Chapter 5.
Each device presents one or more interfaces that a client may use to communicate with the device. Each
interface is composed of zero or more pipes that individually transfer data between the client and a
particular endpoint on the device. The USBD establishes interfaces and pipes at the explicit request of the
Host Software. The Host Controller provides service based on parameters provided by the Host Software
when the configuration request is made.
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A pipe has several characteristics based on the delivery requirements of the data to be transferred.
Examples of these characteristics include the following:
•
The rate at which data needs to be transferred
•
Whether data is provided at a steady rate or sporadically
•
How long data may be delayed before delivery
•
Whether the loss of data being transferred is catastrophic
A USB device endpoint describes the characteristics required for a specific pipe. Endpoints are described
as part of a USB device’s characterization information. For additional details, refer to Chapter 9.
10.1.4 Collecting Status and Activity Statistics
As a common communicant for all control and data transfers between the host and USB devices, the USB
System and the Host Controller are well-positioned to track status and activity information. Such
information is provided upon request to the Host Software, allowing that software to manage status and
activity information. This specification does not identify any specific information that should be tracked or
require any particular format for reporting activity and status information.
10.1.5 Electrical Interface Considerations
The host provides power to USB devices attached to the root hub. The amount of power provided by a port
is specified in Chapter 7.
10.2 Host Controller Requirements
In all implementations, Host Controllers perform the same basic duties with regard to the USB and its
attached devices. These basic duties are described below.
The Host Controller has requirements from both the host and the USB. The following is a brief overview
of the functionality provided. Each capability is discussed in detail in subsequent sections.
State Handling
As a component of the host, the Host Controller reports and manages
its states.
Serializer/Deserializer
For data transmitted from the host, the Host Controller converts
protocol and data information from its native format to a bit stream
transmitted on the USB. For data being received into the host, the
reverse operation is performed.
(micro)frame Generation
The Host Controller produces SOF tokens at a period of 1 ms when
operating with full-speed devices, and at a period of 125 µs when
operating with high-speed devices.
Data Processing
The Host Controller processes requests for data transmission to and
from the host.
Protocol Engine
The Host Controller supports the protocol specified by the USB.
Transmission Error
Handling
All Host Controllers exhibit the same behavior when detecting and
reacting to the defined error categories.
Remote Wakeup
All Host Controllers must have the ability to place the bus into the
Suspended state and to respond to bus wakeup events.
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Root Hub
The root hub provides standard hub function to link the Host
Controller to one or more USB ports.
Host System Interface
Provides a high-speed data path between the Host Controller and host
system.
The following sections present a more detailed discussion of the required capabilities of the Host
Controller.
10.2.1 State Handling
The Host Controller has a series of states that the USB System manages. Additionally, the Host Controller
provides the interface to the following two areas of USB-relevant state:
•
State change propagation
•
Root hub
The root hub presents to the hub driver the same standard states as other USB devices. The Host Controller
supports these states and their transitions for the hub. For detailed discussions of USB states, including
their interrelations and transitions, refer to Chapter 9.
The overall state of the Host Controller is inextricably linked with that of the root hub and of the overall
USB. Any Host Controller state changes that are visible to attached devices must be reflected in the
corresponding device state change information such that the resulting Host Controller and device states are
consistent.
USB devices request a wakeup through the use of resume signaling (refer to Chapter 7). The Host
Controller must notify the rest of the host of a resume event through a mechanism or mechanisms specific
to that system’s implementation. The Host Controller itself may cause a resume event through the same
signaling method.
10.2.2 Serializer/Deserializer
The actual transmission of data across the physical USB takes places as a serial bit stream. A Serial
Interface Engine (SIE), whether implemented as part of the host or a USB device, handles the serialization
and deserialization of USB transmissions. On the host, this SIE is part of the Host Controller.
10.2.3 Frame and Microframe Generation
It is the Host Controller’s responsibility to partition USB time into quantities called “frames” when
operating with full-speed devices, and "microframes" when operating with high-speed devices. Frames and
microframes are created by the Host Controller through issuing Start-of-Frame (SOF) tokens as shown in
Figure 10-3. The SOF token is the first transmission in the (micro)frame period. Host controllers operating
with high-speed devices generate SOF tokens at 125 µs intervals. Host controllers operating with fullspeed devices generate SOF tokens at 1.00 ms intervals. After issuing an SOF token, the Host Controller is
free to transmit other transactions for the remainder of the (micro)frame period. When the Host Controller
is in its normal operating state, SOF tokens must be continuously generated at appropriate periodic rate,
regardless of other bus activity or lack thereof. If the Host Controller enters a state where it is not
providing power on the bus, it must not generate SOFs. When the Host Controller is not generating SOFs,
it may enter a power-reduced state.
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(micro)frame N-1
SOF
(micro)frame N
SOF
EOF Interval (micro)frame N-1)
(micro)frame N+1
SOF
EOF Interval (micro)frame N)
SOF
EOF Interval (micro)frame N+1)
Figure 10-3. Frame and Microframe Creation
The SOF token holds the highest priority access to the bus. Babble circuitry in hubs electrically isolates
any active transmitters during the End-of-microframe or End-of-Frame (EOF) interval, providing an idle
bus for the SOF transmission.
The Host Controller maintains the current (micro)frame number that may be read by the USB System.
The following apply to the current (micro)frame number maintained by the host:
•
Used to uniquely identify one (micro)frame from another
•
Incremented at the end of every (micro)frame period
•
Valid through the subsequent (micro)frame
Host controllers operating with full-speed devices maintain a current frame number (at least 11 bits) that
increments at a 1 ms period. The host transmits the lower 11 bits of the current frame number in each SOF
token transmission.
Host controllers operating with high-speed devices maintain a current microframe number (at least 14 bits)
that increments at a 125 µs period. The host transmits bits 3 through 13 of the current microframe number
in each SOF token transmission. This results in the same SOF packet value being transmitted for eight
consecutive microframes before the SOF packet value increments.
When requested from the Host Controller, the current (micro)frame number is the (micro)frame number in
existence at the time the request was fulfilled. The current (micro)frame number as returned by the host
(Host Controller or HCD) is at least 32 bits, although the Host Controller itself is not required to maintain
more than 11 bits when operating with full-speed devices or 14 bits when operating with high-speed
devices.
The Host Controller shall cease transmission during the EOF interval. When the EOF interval begins, any
transactions scheduled specifically for the (micro)frame that has just passed are retired. If the Host
Controller is executing a transaction at the time the EOF interval is encountered, the Host Controller
terminates the transaction.
10.2.4 Data Processing
The Host Controller is responsible for receiving data from the USB System and sending it to the USB and
for receiving data from the USB and sending it to the USB System. The particular format used for the data
communications between the USB System and the Host Controller is implementation specific, within the
rules for transfer behavior described in Chapter 5.
10.2.5 Protocol Engine
The Host Controller manages the USB protocol level interface. It inserts the appropriate protocol
information for outgoing transmissions. It also strips and interprets, as appropriate, the incoming protocol
information.
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10.2.6 Transmission Error Handling
The Host Controller must be capable of detecting the following transmission error conditions, which are
defined from the host’s point of view:
•
Timeout conditions after a host-transmitted token or packet. These errors occur when the addressed
endpoint is unresponsive or when the structure of the transmission is so badly damaged that the
targeted endpoint does not recognize it.
•
Data errors resulting in missing or invalid transmissions:
•
−
The Host Controller is unable to completely send or receive a packet for host specific reasons, for
example, a transmission extending beyond EOF or a lack of resources available to the Host
Controller.
−
An invalid CRC field on a received data packet.
Protocol errors:
−
An invalid handshake PID, such as a malformed or inappropriate handshake
−
A false EOP
−
A bit stuffing error
For each bulk, control, and interrupt transaction, the host must maintain an error count tally. Errors result
from the conditions described above, not as a result of an endpoint NAKing a request. This value reflects
the number of times the transaction has encountered a transmission error. It is recommended that the error
count not be incremented when there was an error due to host specific reasons (buffer underrun or overrun),
and that whenever a transaction does not encounter a transmission error, the error count is reset to zero.
If the error count for a given transaction reaches three, the host retires the transfer. When a transfer is
retired due to excessive errors, the last error type must be indicated. Isochronous transactions are attempted
only once, regardless of outcome, and, therefore, no error count is maintained for this type.
10.2.7 Remote Wakeup
If USB System wishes to place the bus in the Suspended state, it commands the Host Controller to stop all
bus traffic, including SOFs. This causes all USB devices to enter the Suspended state. In this state, the
USB System may enable the Host Controller to respond to bus wakeup events. This allows the Host
Controller to respond to bus wakeup signaling to restart the host system.
10.2.8 Root Hub
The root hub provides the connection between the Host Controller and one or more USB ports. The root
hub provides the same functionality for dealing with USB topology as other hubs (see Chapter 11), except
that the hardware and software interface between the root hub and the Host Controller is defined by the
specific hardware implementation.
10.2.8.1 Port Resets
Section 7.1.7.5 describes the requirements of a hub to ensure all upstream resume attempts are
overpowered with a long reset downstream. Root hubs must provide an aggregate reset period of at least
50 ms. If the reset duration is controlled in hardware and the hardware timer is <50 ms, the USB System
can issue several consecutive resets to accumulate the specified reset duration as described in
Section 7.1.7.5.
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10.2.9 Host System Interface
The Host Controller provides a high-speed bus-mastering interface to and from main system memory. The
physical transfer between memory and the USB wire is performed automatically by the Host Controller.
When data buffers need to be filled or emptied, the Host Controller informs the USB System.
10.3 Overview of Software Mechanisms
The HCD and the USBD present software interfaces based on different levels of abstraction. They are,
however, expected to operate together in a specified manner to satisfy the overall requirements of the USB
System (see Figure 10-2). The requirements for the USB System are expressed primarily as requirements
for the USBDI. The division of duties between the USBD and the HCD is not defined. However, the one
requirement of the HCDI that must be met is that it supports, in the specified operating system context,
multiple Host Controller implementations.
The HCD provides an abstraction of the Host Controller and an abstraction of the Host Controller’s view of
data transfer across the USB. The USBD provides an abstraction of the USB device and of the data
transfers between the client of the USBD and the function on the USB device. Overall, the USB System
acts as a facilitator for transmitting data between the client and the function and as a control point for the
USB-specific interfaces of the USB device. As part of facilitating data transfer, the USB System provides
buffer management capabilities and allows the synchronization of the data transmittal to the needs of the
client and the function.
The specific requirements for the USBDI are described later in this chapter. The exact functions that fulfill
these requirements are described in the relevant operating system environment guide for the HCDI and the
USBDI. The procedures involved in accomplishing data transfers via the USBDI are described in the
following sections.
10.3.1 Device Configuration
Different operating system environments perform device configuration using different software
components and different sequences of events. The USB System does not assume a specific operating
system method. However, there are some basic requirements that must be fulfilled by any USB System
implementation. In some operating systems, existing host software provides these requirements. In others,
the USB System provides the capabilities.
The USB System assumes a specialized client of the USBD, called a hub driver, that acts as a
clearinghouse for the addition and removal of devices from a particular hub. Once the hub driver receives
such notifications, it will employ additional host software and other USBD clients, in an operating system
specific manner, to recognize and configure the device. This model, shown in Figure 10-4, is the basis of
the following discussion.
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Device
Driver
Host Software
Configuration
Support
Hub
Driver
Optional
Component
Configuration
Control
USBD
Optional
Configuration
Control
HCD
Figure 10-4. Configuration Interactions
When a device is attached, the hub driver receives a notification from the hub detecting the change. The
hub driver, using the information provided by the hub, requests a device identifier from the USBD. The
USBD in turn sets up the default pipe for that device and returns a device identifier to the hub driver.
The device is now ready to be configured for use. For each device, there are three configurations that must
be complete before that device is ready for use:
284
1.
Device Configuration: This includes setting up all of the device’s USB parameters and allocating all
USB host resources that are visible to the device. This is accomplished by setting the configuration
value on the device. A limited set of configuration changes, such as alternate settings, is allowed
without totally reconfiguring the device. Once the device is configured, it is, from its point of view,
ready for use.
2.
USB Configuration: In order to actually create a USBD pipe ready for use by a client, additional USB
information, not visible to the device, must be specified by the client. This information, known as the
Policy for the pipe, describes how the client will use the pipe. This includes such items as the
maximum amount of data the client will transfer with one IRP, the maximum service interval the client
will use, the client’s notification identification, and so on.
3.
Function Configuration: Once configuration types 1 and 2 have been accomplished, the pipe is
completely ready for use from the USB’s point of view. However, additional vendor- or class-specific
setup may be required before the client can actually use the pipe. This configuration is a private matter
between the device and the client and is not standardized by the USBD.
Universal Serial Bus Specification Revision 2.0
The following paragraphs describe the device and USB configuration requirements.
The responsible configuring software performs the actual device configuration. Depending on the
particular operating system implementation, the software responsible for configuration can include the
following:
•
The hub driver
•
Other host software
•
A device driver
The configuring software first reads the device descriptor, then requests the description for each possible
configuration. It may use the information provided to load a particular client, such as a device driver,
which initially interacts with the device. The configuring software, perhaps with input from that device
driver, chooses a configuration for the device. Setting the device configuration sets up all of the endpoints
on the device and returns a collection of interfaces to be used for data transfer by USBD clients. Each
interface is a collection of pipes owned by a single client.
This initial configuration uses the default settings for interfaces and the default bandwidth for each
endpoint. A USBD implementation may additionally allow the client to specify alternate interfaces when
selecting the initial configuration. The USB System will verify that the resources required for the support
of the endpoint are available and, if so, will allocate the bandwidth required. Refer to Section 10.3.2 for a
discussion of resource management.
The device is now configured, but the created pipes are not yet ready for use. The USB configuration is
accomplished when the client initializes each pipe by setting a Policy to specify how it will interact with
the pipe. Among the information specified is the client’s maximum service interval and notification
information. Among the actions taken by the USB System, as a result of setting the Policy, is determining
the amount of buffer working space required beyond the data buffer space provided by the client. The size
of the buffers required is based upon the usage chosen by the client and upon the per-transfer needs of the
USB System.
The client receives notifications when IRPs complete, successfully or due to errors. The client may also
wake up independently of USB notification to check the status of pending IRPs.
The client may also choose to make configuration modifications, such as enabling an alternate setting for
an interface or changing the bandwidth allocated to a particular pipe. In order to perform these changes,
the interface or pipe, respectively, must be idle.
10.3.2 Resource Management
Whenever a pipe is setup by the USBD for a given endpoint, the USB System must determine if it can
support the pipe. The USB System makes this determination based on the requirements stated in the
endpoint descriptor. One of the endpoint requirements, which must be supported in order to create a pipe
for an endpoint, is the bandwidth necessary for that endpoint’s transfers. There are two stages to check for
available bandwidth. First the maximum execution time for a transaction is calculated. Then the
(micro)frame schedule is consulted to determine if the indicated transaction will fit.
The allocation of the guaranteed bandwidth for isochronous and interrupt pipes, and the determination of
whether a particular control or bulk transaction will fit into a given (micro)frame, can be determined by a
software heuristic in the USB System. If the actual transaction execution time in the Host Controller
exceeds the heuristically determined value, the Host Controller is responsible for ensuring that
(micro)frame integrity is maintained (refer to Section 10.2.3). The following discussion describes the
requirements for the USB System heuristic.
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In order to determine if bandwidth can be allocated, or if a transaction can be fit into a particular
(micro)frame, the maximum transaction execution time must be calculated. The calculation of the
maximum transaction execution time requires that the following information be provided: (Note that an
agent other than the client may provide some of this information.)
•
Number of data bytes (wMaxPacketSize) to be transmitted.
•
Transfer type.
•
Depth in the topology. If less precision is allowed, the maximum topology depth may be assumed.
This calculation must include the bit transmission time, the signal propagation delay through the topology,
and any implementation-specific delays, such as preparation or recovery time required by the Host
Controller itself. Refer to Chapter 5 for examples of formulas that can be used for such calculations.
10.3.3 Data Transfers
The basis for all client-function communication is the interface: a bundle of related pipes associated with a
particular USB device.
Exactly one client on the host manages a given interface. The client initializes each pipe of an interface by
setting the Policy for that pipe. This includes the maximum amount of data to be transmitted per IRP and
the maximum service interval for the pipe. A service interval is stated in milliseconds and describes the
interval over which an IRP’s data will be transmitted for an isochronous pipe. It describes the polling
interval for an interrupt pipe. The client is notified when a specified request is completed. The client
manages the size of each IRP such that its duty cycle and latency constraints are maintained. Additional
Policy information includes the notification information for the client.
The client provides the buffer space required to hold the transmitted data. The USB System uses the Policy
to determine the additional working space it will require.
The client views its data as a contiguous serial stream, which it manages in a similar manner to those
streams provided over other types of bus technologies. Internally, the USB System may, depending on its
own Policy and any Host Controller constraints, break the client request down into smaller requests to be
sent across the USB. However, two requirements must be met whenever the USB System chooses to
undertake such division:
•
The division of the data stream into smaller chunks is not visible to the client.
•
USB samples are not split across bus transactions.
When a client wishes to transfer data, it will send an IRP to the USBD. Depending on the direction of data
transfer, a full or empty data buffer will be provided. When the request is complete (successfully or due to
an error condition), the IRP and its status is returned to the client. Where relevant, this status is also
provided on a per-transaction basis.
10.3.4 Common Data Definitions
In order to allow the client to receive request results as directly as possible from its device, it is desirable to
minimize the amount of processing and copying required between the device and the client. To facilitate
this, some control aspects of the IRP are standardized such that different layers in the stack may directly
use the information provided by the client. The particular format for this data is dependent on the
actualization of the USBDI in the operating system. Some data elements may in fact not be directly visible
to the client at all but are generated as a result of the client request.
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The following data elements define the relevant information for a request:
•
Identification of the pipe associated with the request. Identifying this pipe also describes information
such as transfer type for this request.
•
Notification identification for the particular client.
•
Location and length of data buffer that is to be transmitted or received.
•
Completion status for the request. Both the summary status and, as required, detailed per-transaction
status must be provided.
•
Location and length of working space. This is implementation-dependent.
The actual mechanisms used to communicate requests to the USBD are operating system-specific.
However, beyond the requirements stated above for what request-related information must be available,
there are also requirements on how requests will be processed. The basic requirements are described in
Chapter 5. Additionally, the USBD provides a mechanism to designate a group of isochronous IRPs for
which the transmission of the first transaction of each IRP will occur in the same (micro)frame. The USBD
also provides a mechanism for designating an uninterruptable set of vendor- or class-specific requests to a
default pipe. No other requests to that default pipe, including standard, class, or vendor request may be
inserted in the execution flow for such an uninterruptable set. If any request in this set fails, the entire set is
retired.
10.4 Host Controller Driver
The Host Controller Driver (HCD) is an abstraction of Host Controller hardware and the Host Controller’s
view of data transmission over the USB. The HCDI meets the following requirements:
•
Provides an abstraction of the Host Controller hardware.
•
Provides an abstraction for data transfers by the Host Controller across the USB interconnect.
•
Provides an abstraction for the allocation (and de-allocation) of Host Controller resources to support
guaranteed service to USB devices.
•
Presents the root hub and its behavior according to the hub class definition. This includes supporting
the root hub such that the hub driver interacts with the root hub exactly as it would for any hub. In
particular, even though a root hub can be implemented in a combination of hardware and software, the
root hub responds initially to the default device address (from a client perspective), returns descriptor
information, supports having its device address set, and supports the other hub class requests.
However, bus transactions may or may not need to be generated to accomplish this behavior given the
close integration possible between the Host Controller and the root hub.
The HCD provides a software interface (HCDI) that implements the required abstractions. The function of
the HCD is to provide an abstraction, which hides the details of the Host Controller hardware. Below the
Host Controller hardware is the physical USB and all the attached USB devices.
The HCD is the lowest tier in the USB software stack. The HCD has only one client: the Universal Serial
Bus Driver (USBD). The USBD maps requests from many clients to the appropriate HCD. A given HCD
may manage many Host Controllers.
The HCDI is not directly accessible from a client. Therefore, the specific interface requirements for the
HCDI are not discussed here.
10.5 Universal Serial Bus Driver
The USBD provides a collection of mechanisms that operating system components, typically device
drivers, use to access USB devices. The only access to a USB device is that provided by the USBD. The
USBD implementations are operating system-specific. The mechanisms provided by the USBD are
implemented, using as appropriate and augmenting as necessary, the mechanisms provided by the operating
system environment in which the USB runs. The following discussion centers on the basic capabilities
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required for all USBD implementations. For specifics of the USBD operation within a specific
environment, see the relevant operating system environment guide for the USBD. A single instance of the
USBD directs accesses to one or more HCDs that in turn connect to one or more Host Controllers. If
allowed, how USBD instancing is managed is dependent upon the operating system environment.
However, from the client’s point of view, the USBD with which the client communicates manages all of
the attached USB devices.
10.5.1 USBD Overview
Clients of USBD direct commands to devices or move streams of data to or from pipes. The USBD
presents two groups of software mechanisms to clients: command mechanisms and pipe mechanisms.
Command mechanisms allow clients to configure and control USBD operation as well as to configure and
generically control a USB device. In particular, command mechanisms provide all access to the device’s
default pipe.
Pipe mechanisms allow a USBD client to manage device specific data and control transfers. Pipe
mechanisms do not allow a client to directly address the device’s default pipe.
Pipe Interfaces
Power Control
Bus and Device
Management
Device Data
Access
(Hub)
Interrupt Transfer
Message
and
Stream
Pipe
Access
Configuration
Management
Figure 10-5 presents an overview of the USBD structure.
Command Interfaces
Services
Universal Serial Bus Driver
Host
Controller
Driver
USB Host
Controller
Host
Controller
Driver
USB Host
Controller
Figure 10-5. Universal Serial Bus Driver Structure
10.5.1.1 USBD Initialization
Specific USBD initialization is operating system-dependent. When a particular USB managed by USBD is
initialized, the management information for that USB is also created. Part of this management information
is the default address device and its default pipe used to communicate to a newly reset device.
When a device is attached to a USB, it responds to a special address known as the default address (refer to
Chapter 9) until its unique address is assigned by the bus enumerator. In order for the USB System to
interact with the new device, the default device address and the device’s default pipe must be available to
the hub driver when a device is attached. During device initialization, the default address is changed to a
unique address.
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10.5.1.2 USBD Pipe Usage
Pipes are the method by which a device endpoint is associated with a Host Software entity. Pipes are
owned by exactly one such entity on the host. Although the basic concept of a pipe is the same no matter
who the owner, some distinction of capabilities provided to the USBD client occurs between two groups of
pipes:
•
Default pipes, which are owned and managed by the USBD
•
All other pipes, which are owned and managed by clients of the USBD
Default pipes are never directly accessed by clients, although they are often used to fulfill some part of
client requests relayed via command mechanisms.
10.5.1.2.1 Default Pipes
The USBD is responsible for allocating and managing appropriate buffering to support transfers on the
default pipe that are not directly visible to the client such as setting a device address. For those transfers
that are directly visible to the client, such as sending vendor and class commands or reading a device
descriptor, the client must provide the required buffering.
10.5.1.2.2 Client Pipes
Any pipe not owned and managed by the USBD can be owned and managed by a USBD client. From the
USBD viewpoint, a single client owns the pipe. In fact, a cooperative group of clients can manage the pipe,
provided they behave as a single coordinated entity when using the pipe.
The client is responsible for providing the amount of buffering it needs to service the data transfer rate of
the pipe within a service interval attainable by the client. Additional buffering requirements for working
space are specified by the USB System.
10.5.1.3 USBD Service Capabilities
The USBD provides services in the following categories:
•
Configuration via command mechanisms
•
Transfer services via both command and pipe mechanisms
•
Event notification
•
Status reporting and error recovery
10.5.2 USBD Command Mechanism Requirements
USBD command mechanisms allow a client generic access to a USB device. Generally, these commands
allow the client to make read or write accesses to one of potentially several device data and control spaces.
The client provides as little as a device identifier and the relevant data or empty buffer pointer.
USBD command transfers do not require that the USB device be configured. Many of the device
configuration facilities provided by the USBD are command transfers.
Following are the specific requirements on the command mechanisms provided.
10.5.2.1 Interface State Control
USBD clients must be able to set a specified interface to any settable pipe state. Setting an interface state
results in all of the pipes in that interface moving to that state. Additionally, all of the pipes in an interface
may be reset or aborted.
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10.5.2.2 Pipe State Control
USBD pipe state has two components:
•
Host status
•
Reflected endpoint status
Whenever the pipe status is reported, the value for both components will be identified. The pipe status
reflected from the endpoint is the result of the endpoint being in a particular state. The USBD client
manages the pipe state as reported by the USBD. For any pipe state reflected from the endpoint, the client
must also interact with the endpoint to change the state.
A USBD pipe is in exactly one of the following states:
•
Active: The pipe’s Policy has been set and the pipe is able to transmit data. The client can query as to
whether any IRPs are outstanding for a particular pipe. Pipes for which there are no outstanding IRPs
are still considered to be in the Active state as long as they are able to accept new IRPs.
•
Halted: An error has occurred on the pipe. This state may also be a reflection of the corresponding
Halted endpoint on the device.
A pipe and endpoint are considered active when the device is configured and the pipe and/or endpoint is
not stalled. Clients may manipulate pipe state in the following ways:
•
Aborting a Pipe: All of the IRPs scheduled for a pipe are retired immediately and returned to the client
with a status indicating they have been aborted. Neither the host state nor the reflected endpoint state
of the pipe is affected.
•
Resetting a Pipe: The pipe’s IRPs are aborted. The host state is moved to Active. If the reflected
endpoint state needs to be changed, that must be commanded explicitly by the USBD client.
•
Clearing a Halted pipe: The pipe's state is cleared from Halted to Active.
•
Halting a Pipe: The pipe's state is set to Halted.
10.5.2.3 Getting Descriptors
The USBDI must provide a mechanism to retrieve standard device, configuration, and string descriptors, as
well as any class- or vendor-specific descriptors.
10.5.2.4 Getting Current Configuration Settings
The USBDI must provide a facility to return, for any specified device, the current configuration descriptor.
If the device is not configured, no configuration descriptor is returned. This action is equivalent to
returning the configuration descriptor for the current configuration by requesting the specific configuration
descriptor. It does not, however, require the client to know the identifier for the current configuration.
This will return all of the configuration information, including the following:
•
All of the configuration descriptor information as stored on the device, including all of the alternate
settings for all of the interfaces
•
Indicators for which of the alternate settings for interfaces are active
•
Pipe handles for endpoints in the active alternate settings for interfaces
•
Actual wMaxPacketSize values for endpoints in the active alternate settings for interfaces
Additionally, for any specified pipe, the USBDI must provide a facility to return the wMaxPacketSize that
is currently being used by the pipe.
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10.5.2.5 Adding Devices
The USBDI must provide a mechanism for the hub driver to inform USBD of the addition of a new device
to a specified USB and to retrieve the USBD ID of the new USB device. The USBD tasks include
assigning the device address and preparing the device’s default pipe for use.
10.5.2.6 Removing Devices
The USBDI must provide a facility for the hub driver to inform the USBD that a specific device has been
removed.
10.5.2.7 Managing Status
The USBDI must provide a mechanism for obtaining and clearing device-based status on a device,
interface, or pipe basis.
10.5.2.8 Sending Class Commands
This USBDI mechanism is used by a client, typically a class-specific or adaptive driver, to send one or
more class-specific commands to a device.
10.5.2.9 Sending Vendor Commands
This USBDI mechanism is used by a client to send one or more vendor-specific commands to a device.
10.5.2.10 Establishing Alternate Settings
The USBDI must provide a mechanism to change the alternate setting for a specified interface. As a result,
the pipe handles for the previous setting are released and new pipe handles for the interface are returned.
For this request to succeed, the interface must be idle; i.e., no data buffers may be queued for any pipes in
the interface.
10.5.2.11 Establishing a Configuration
Configuring software requests a configuration by passing a buffer containing the configuration information
to the USBD. The USBD requests resources for the endpoints in the configuration, and if all resource
requests succeed, the USBD sets the device configuration and returns interface handles with corresponding
pipe handles for all of the active endpoints. The default values are used for all alternate settings for
interfaces.
Note: The interface implementing the configuration may require specific alternate settings to be identified.
10.5.2.12 Setting Descriptors
For devices supporting this behavior, the USBDI allows existing descriptors to be updated or new
descriptors to be added.
10.5.3 USBD Pipe Mechanisms
This part of the USBDI offers clients the highest-speed, lowest overhead data transfer services possible.
Higher performance is achieved by shifting some pipe management responsibilities from the USBD to the
client. As a result, the pipe mechanisms are implemented at a more primitive level than the data transfer
services provided by the USBD command mechanisms. Pipe mechanisms do not allow access to a device’s
default pipe.
USBD pipe transfers are available only after both the device and USB configuration have completed
successfully. At the time the device is configured, the USBD requests the resources required to support all
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device pipes in the configuration. Clients are allowed to modify the configuration, constrained by whether
the specified interface or pipe is idle.
Clients provide full buffers to outgoing pipes and retrieve transfer status information following the
completion of a request. The transfer status returned for an outgoing pipe allows the client to determine the
success or failure of the transfer.
Clients provide empty buffers to incoming pipes and retrieve the filled buffers and transfer status
information from incoming pipes following the completion of a request. The transfer status returned for an
incoming pipe allows a client to determine the amount and the quality of the data received.
10.5.3.1 Supported Pipe Types
The four types of pipes supported, based on the four transfer types, are described in the following sections.
10.5.3.1.1 Isochronous Data Transfers
Each buffer queued for an isochronous pipe is required to be viewable as a stream of samples. As with all
pipe transfers, the client establishes a Policy for using this isochronous pipe, including the relevant service
interval for this client. Lost or missing bytes, which are detected on input, and transmission problems,
which are noted on output, are indicated to the client.
The client queues a first buffer, starting the pipe streaming service. To maintain the continuous streaming
transfer model used in all isochronous transfers, the client queues an additional buffer before the current
buffer is retired.
The USBD is required to be able to provide a sample stream view of the client’s data stream. In other
words, using the client’s specified method of synchronization, the precise packetization of the data is
hidden from the client. Additionally, a given transaction is always contained completely within some client
data buffer.
For an output pipe, the client provides a buffer of data. The USBD allocates the data across the
(micro)frames for the service period using the client’s chosen method of synchronization.
For an input pipe, the client must provide an empty buffer large enough to hold the maximum number of
bytes the client’s device will deliver in the service period. Where missing or invalid bytes are indicated,
the USBD may leave the space that the bytes would have occupied in place in the buffer and identify the
error. One of the consequences of using no synchronization method is that this reserved space is assumed
to be the maximum packet size. The buffer-retired notification occurs when the IRP completes. Note that
the input buffer need not be full when returned to the client.
The USBD may optionally provide additional views of isochronous data streams. The USBD is also
required to be able to provide a packet stream view of the client’s data stream.
10.5.3.1.2 Interrupt Transfers
The Interrupt out transfer originates in the client of the USBD and is delivered to the USB device. The
Interrupt in transfer originates in a USB device and is delivered to a client of the USBD. The USB System
guarantees that the transfers meet the maximum latency specified by the USB endpoint descriptor.
The client queues a buffer large enough to hold the interrupt transfer data (typically a single USB
transaction). When all of the data is transferred, or if the error threshold is exceeded, the IRP is returned to
the client.
10.5.3.1.3 Bulk Transfers
Bulk transfers may originate either from the device or the client. No periodicity or guaranteed latency is
assumed. When all of the data is transferred, or if the error threshold is exceeded, the IRP is returned to the
client.
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10.5.3.1.4 Control Transfers
All message pipes transfer data in both directions. In all cases, the client outputs a setup stage to the device
endpoint. The optional data stage may be either input or output and the final status is always logically
presented to the host. For details of the defined message protocol, refer to Chapter 8.
The client prepares a buffer specifying the command phase and any optional data or empty buffer space.
The client receives a buffer-retired notification when all phases of the control transfer are complete, or an
error notification, if the transfer is aborted due to transmission error.
10.5.3.2 USBD Pipe Mechanism Requirements
The following pipe mechanisms are provided.
10.5.3.2.1 Aborting IRPs
The USBDI must allow IRPs for a particular pipe to be aborted.
10.5.3.2.2 Managing Pipe Policy
The USBDI must allow a client to set and clear the Policy for an individual pipe or for an entire interface.
Any IRPs made by the client prior to successfully setting a Policy are rejected by the USBD.
10.5.3.2.3 Queuing IRPs
The USBDI must allow clients to queue IRPs for a given pipe. When IRPs are returned to the client, the
request status is also returned. A mechanism is provided by the USBD to identify a group of isochronous
IRPs whose first transactions will all occur in the same (micro)frame.
10.5.4 Managing the USB via the USBD Mechanisms
Using the provided USBD mechanisms, the following general capabilities are supported by any USB
System.
10.5.4.1 Configuration Services
Configuration services operate on a per-device basis. The configuring software tells the USBD when to
perform device configuration. A hub driver has a special role in device management and provides at least
the following capabilities:
•
Device attach/detach recognition, driven by an interrupt pipe owned by the hub driver
•
Device reset, accomplished by the hub driver by resetting the hub port upstream of the device
•
Tells the USBD to perform device address assignment
•
Power control
The USBDI additionally provides the following configuration facilities, which may be used by the hub
driver or other configuring software available on the host:
•
Device identification and access to configuration information (via access to descriptors on the device)
•
Device configuration via command mechanisms
When the hub driver informs the USBD of a device attachment, the USBD establishes the default pipe for
the new device.
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10.5.4.1.1 Configuration Management
Configuration management services are provided primarily as a set of specific interface commands that
generate USB transactions on the default pipe. The notable exception is the use of an additional interrupt
pipe that delivers hub status directly to the hub driver.
Every hub initiates an interrupt transfer when there is a change in the state of one of the hub ports.
Generally, the port state change will be the connection or removal of a downstream USB device. (Refer to
Chapter 11 for more information.)
10.5.4.1.2 Initial Device Configuration
The device configuration process begins when a hub reports, via its status change pipe, the connection of a
new USB device.
Configuration management services allow configuring software to select a USB device configuration from
the set of configurations listed in the device. The USBD verifies that adequate power is available and the
data transfer rates given for all endpoints in the configuration do not exceed the capabilities of the USB
with the current schedule before setting the device configuration.
10.5.4.1.3 Modifying a Device Configuration
Configuration management services allow configuring software to replace a USB device configuration with
another configuration from the set of configurations listed in the device. The operation succeeds if
adequate power is available and the data transfer rates given for all endpoints in the new configuration fit
within the capabilities of the USB with the current schedule. If the new configuration is rejected, the
previous configuration remains.
Configuration management services allow configuring software to return a USB device to a Not
Configured state.
10.5.4.1.4 Device Removal
Error recovery and/or device removal processing begins when a hub reports via its status change pipe that
the USB device has been removed.
10.5.4.2 Power Control
There are two cooperating levels of power management for the USB: bus and device level management.
This specification provides mechanisms for managing power on the USB bus. Device classes may define
class-specific power control capabilities.
All USB devices must support the Suspended state (refer to Chapter 9). The device is placed into the
Suspended state via control of the hub port to which the device is attached. Normal device operation ceases
in the Suspend State; however, if the device is capable of wakeup signaling and the device is enabled for
remote wakeup, it may generate resume signaling in response to external events.
The power management system may transition a device to the Suspended state or power-off the device in
order to control and conserve power. The USB provides neither requirements nor commands for the device
state to be saved and restored across these transitions. Device classes may define class-specific device state
save-and-restore capabilities.
The USB System coordinates the interaction between device power states and the Suspended state.
It is recommended that while a device is not being used by the system (i.e., no transactions are being
transmitted to or from the device besides SOF tokens), the device be suspended as soon as possible by
selectively suspending the port to which the device is attached. Suspending inactive devices reduces
reliability issues due to high currents passing through a transceiver operating in high-speed mode in the
presence of short circuit conditions described in Section 7.1.1. Some of these short circuit conditions are
not detectable in the absence of transactions to the device. Suspending the unused device will place the
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transceiver interface into full-speed mode which has a greater reliability in the presence of short circuit
conditions.
10.5.4.3 Event Notifications
USBD clients receive several kinds of event notifications through a number of sources:
•
Completion of an action initiated by a client.
•
Interrupt transfers over stream pipes can deliver notice of device events directly to USBD clients. For
example, hubs use an interrupt pipe to deliver events corresponding to changes in hub status.
•
Event data can be embedded by devices in streams.
•
Standard device interface commands, device class commands, vendor-specific commands, and even
general control transfers over message pipes can all be used to poll devices for event conditions.
10.5.4.4 Status Reporting and Error Recovery Services
The command and pipe mechanisms both provide status reporting on individual requests as they are
invoked and completed.
Additionally, USB device status is available to USBD clients using the command mechanisms.
The USBD provides clients with pipe error recovery mechanisms by allowing pipes to be reset or aborted.
10.5.4.5 Managing Remote Wakeup Devices
The USB System can minimize the resume power consumption of a suspended USB tree. This is
accomplished by explicitly enabling devices capable of resume signaling and controlling propagation of
resume signaling via selectively suspending and/or disabling hub ports between the device and the nearest
self-powered, awake hub.
In some error-recovery scenarios, the USB System will need to re-enumerate sub-trees. The sub-tree may
be partially or completely suspended. During error-recovery, the USB System must avoid contention
between a device issuing resume signaling and simultaneously driving reset down the port. Avoidance is
accomplished via management of the devices’ remote wakeup feature and the hubs’ port features. The
rules are as follows:
•
Issue a SetDeviceFeature(DEVICE_REMOTE_WAKEUP) request to the leaf device, only just prior to
selectively suspending any port between where the device is connected and the root port (via a
SetPortFeature(PORT_SUSPEND) request).
•
Do not reset a suspended port that has had a device enabled for remote wakeup without first enabling
that port.
•
Verify that after a remote wakeup, the devices in the subtree affected by the remote wakeup are still
present. This will typically be done as part of determining which potential remote wakeup device was
the source of the wakeup. This should be done to ensure that a suspended device is not disconnected
(and possibly reconnected) or reset (e.g., by noise) during a suspend/resume process.
10.5.5 Passing USB Preboot Control to the Operating System
A single software driver owns the Host Controller. If the host system implements USB services before the
operating system loads, the Host Controller must provide a mechanism that disables access by the preboot
software and allows the operating system to gain control. Preboot USB configuration is not passed to the
operating system. Once the operating system gains control, it is responsible to fully configure the bus. If
the operating system provides a mechanism to pass control back to the preboot environment, the bus will be
in an unknown state. The preboot software should treat this event as a powerup.
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10.6 Operating System Environment Guides
As noted previously, the actual interfaces between the USB System and host software are specific to the host
platform and operating system. A companion specification is required for each combination of platform and
operating system with USB support. These specifications describe the specific interfaces used to integrate the
USB into the host. Each operating system provider for the USB System identifies a compatible Universal USB
Specification revision.
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Chapter 11
Hub Specification
This chapter describes the architectural requirements for the USB hub. It contains a description of the three
principal sub-blocks: the Hub Repeater, the Hub Controller, and the Transaction Translator. The chapter
also describes the hub’s operation for error recovery, reset, and suspend/resume. The second half of the
chapter defines hub request behavior and hub descriptors.
The hub specification supplies sufficient additional information to permit an implementer to design a hub
that conforms to the USB specification.
11.1 Overview
Hubs provide the electrical interface between USB devices and the host. Hubs are directly responsible for
supporting many of the attributes that make USB user friendly and hide its complexity from the user. Listed
below are the major aspects of USB functionality that hubs must support:
•
Connectivity behavior
•
Power management
•
Device connect/disconnect detection
•
Bus fault detection and recovery
•
High-, full-, and low-speed device support
A hub consists of three components: the Hub Repeater, the Hub Controller, and the Transaction Translator.
The Hub Repeater is responsible for connectivity setup and tear-down. It also supports exception handling,
such as bus fault detection and recovery and connect/disconnect detect. The Hub Controller provides the
mechanism for host-to-hub communication. Hub-specific status and control commands permit the host to
configure a hub and to monitor and control its individual downstream facing ports. The Transaction
Translator responds to high-speed split transactions and translates them to full-/low-speed transactions with
full-/low-speed devices attached on downstream facing ports.
11.1.1 Hub Architecture
Figure 11-1 shows a hub and the locations of its upstream and downstream facing ports. A hub consists of a
Hub Repeater section, a Hub Controller section, and a Transaction Translator section. The hub must
operate at high-speed when its upstream facing port is connected at high-speed. The hub must operate at
full-speed when its upstream facing port is connected at full-speed.
The Hub Repeater is responsible for managing connectivity between upstream and downstream facing ports
which are operating at the same speed. The Hub Repeater supports full-/low-speed connectivity and highspeed connectivity. The Hub Controller provides status and control and permits host access to the hub. The
Transaction Translator takes high-speed split transactions and translates them to full-/low-speed transactions
when the hub is operating at high-speed and has full-/low-speed devices attached. The operating speed of a
device attached on a downstream facing port determines whether the Routing Logic connects a port to the
Transaction Translator or hub repeater sections.
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Port 0
Upstream Facing Port State Machines
Upstream Facing Port
Transaction
Translator
Hub State
Hub
Repeater Machines
Hub
Controller
Routing Logic
Downstream
Facing Port
State Machine(s)
...
Port 1
Port 2
Port N
Downstream Facing Ports
Figure 11-1. Hub Architecture
When a hub’s upstream facing port is attached to an electrical environment that is operating at full-/lowspeed, the hub’s high-speed functionality is disallowed. This means that the hub will only operate at full/low-speed and the transaction translator and high-speed repeater will not operate. In this electrical
environment, the hub repeater must operate as a full-/low-speed repeater and the routing logic connects
ports to the hub repeater.
When the hub upstream facing port is attached to an electrical environment that is operating at high-speed,
the full-/low-speed hub repeater is not operational. In this electrical environment when a high-speed device
is attached on downstream facing port, the routing logic will connect the port to the hub repeater and the
hub repeater must operate as a high-speed repeater. In this case, when a full-/low-speed device is attached
on a downstream facing port, the routing logic must connect the port to the transaction translator.
11.1.2 Hub Connectivity
Hubs exhibit different connectivity behavior depending on whether they are propagating packet traffic, or
resume signaling, or are in the Idle state.
11.1.2.1 Packet Signaling Connectivity
The Hub Repeater contains one port that must always connect in the upstream direction (referred to as the
upstream facing port) and one or more downstream facing ports. Upstream connectivity is defined as being
towards the host, and downstream connectivity is defined as being towards a device. Figure 11-2 shows the
packet signaling connectivity behavior for hubs in the upstream and downstream directions. A hub also has
an Idle state, during which the hub makes no connectivity. When in the Idle state, all of the hub’s ports are
in the receive mode waiting for the start of the next packet.
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Upstream
Port
ownstream
Ports
Downstream
Connectivity
Upstream
Connectivity
Idle
(No Connectivity)
Enabled Port
Port not Enabled
Figure 11-2. Hub Signaling Connectivity
If a downstream facing port is enabled (i.e., in a state where it can propagate signaling through the hub), and
the hub detects the start of a packet on that port, connectivity is established in an upstream direction to the
upstream facing port of that hub, but not to any other downstream facing ports. This means that when a
device or a hub transmits a packet upstream, only those hubs in line between the transmitting device and the
host will see the packet. Refer to Section 11.8.3 for optional behavior when a hub detects simultaneous
upstream signaling on more than one port.
In the downstream direction, hubs operate in a broadcast mode. When a hub detects the start of a packet on
its upstream facing port, it establishes connectivity to all enabled downstream facing ports. If a port is not
enabled, it does not propagate packet signaling downstream.
11.1.2.2 Resume Connectivity
Hubs exhibit different connectivity behaviors for upstream- and downstream-directed resume signaling. A
hub that is suspended reflects resume signaling from its upstream facing port to all of its enabled
downstream facing ports. Figure 11-3 illustrates hub upstream and downstream resume connectivity.
Upstream
Port
Upstream
Port
Enabled Port
Disabled or
Suspended
Port
Enabled or
Suspended
Port
Downstream
Ports
Downstream Connectivity
Source of resume
signaling
Upstream Connectivity
Figure 11-3. Resume Connectivity
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If a hub is suspended and detects resume signaling from a selectively suspended or an enabled downstream
facing port, the hub reflects that signaling upstream and to all of its enabled downstream facing ports,
including the port that initiated the resume sequence. Resume signaling is not reflected to disabled or
suspended ports. A detailed discussion of resume connectivity appears in Section 11.9.
11.1.2.3 Hub Fault Recovery Mechanisms
Hubs are the essential USB component for establishing connectivity between the host and other devices. It
is vital that any connectivity faults, especially those that might result in a deadlock, be detected and
prevented from occurring. Hubs need to handle connectivity faults only when they are in the repeater mode.
Hubs must also be able to detect and recover from lost or corrupted packets that are addressed to the Hub
Controller. Because the Hub Controller is, in fact, another USB device, it must adhere to the same timeout
rules as other USB devices, as described in Chapter 8.
11.2 Hub Frame/Microframe Timer
Each hub has a (micro)frame timer whose timing is derived from the hub’s local clock and is synchronized
to the host (micro)frame period by the host-generated Start-of-(micro)frame (SOF). The (micro)frame
timer provides timing references that are used to allow the hub to detect a babbling device and prevent the
hub from being disabled by the upstream hub. The hub (micro)frame timer must track the host
(micro)frame period and be capable of remaining synchronized with the host even if two consecutive SOF
tokens are missed by the hub.
The (micro)frame timer must lock to the host’s (micro)frame timing for worst case clock accuracies and
timing offsets between the host and hub. There are specific requirements for hubs when their upstream
facing port is operating at high-speed and full-speed.
11.2.1 High-speed Microframe Timer Range
The range for a microframe timer must be from 59904 to 60096 high-speed bits.
The nominal microframe interval is 60000 high-speed bit times. The hub microframe timer range specified
above is 60000 +/- 96 high-speed bit times in order to accommodate host accuracy, hub accuracy, repeater
jittter, and hub quantization. The +/-96 full-speed bit time variation is calculated in Table 11-2.
Table 11-1. High-speed Microframe Timer Range Contributions
Source of Variation
300
Variation (ppm)
Variation (bits) Over
One Microframe Interval
Host accuracy
+/- 500
+/- 30
Hub accuracy
+/- 500
+/- 30
Comment
Host jitter
+/- 2
Hub chain jitter
+/- 20
Four hubs in series
upstream of hub; 0 to 5
bits of jitter per hub
Quantization
+/-14
Bits need to round total
variation to multiple of 16
Universal Serial Bus Specification Revision 2.0
11.2.2 Full-speed Frame Timer Range
The range of the frame timer must be from 11958 to 12042 full-speed bits.
The nominal frame interval is 12000 full-speed bit times. The hub frame timer range specified above is
12000 +/- 42 full-speed bit times in order to accommodate host accuracy and hub accuracy. The +/-42 fullspeed bit time variation is calculated in Table 11-2.
Table 11-2. Full-speed Frame Timer Range Contributions
Source of Variation
Variation (ppm)
Variation (bits) Over
One Frame Interval
Host accuracy
+/- 500
+/- 6
Hub accuracy
+/- 3000
+/- 36
Comment
+/-6 bits due to hub
accuracy (500 ppm)
+/-30 bits due to 1.x
parent hub accuracy
(2500 ppm)
11.2.3 Frame/Microframe Timer Synchronization
A hub’s (micro)frame timer is clocked by the hub’s clock source and is synchronized to SOF packets that
are derived from the host’s (micro)frame timer. After a reset or resume, the hub’s (micro)frame timer is not
synchronized. Whenever the hub receives two consecutive SOF packets, its (micro)frame timer must be
synchronized. Synchronized is synonymous with lock(ed). An example for a method of constructing a
timer that properly synchronizes is as follows.
11.2.3.1 Example (Micro)frame Timer Synchronization Method
The hub maintains three timer values: (micro)frame timer (down counter), current (micro)frame (up
counter), and next (micro)frame (register). After a reset or resume, a flag is set to indicate that the
(micro)frame timer is not synchronized.
When the first SOF token is detected, the current (micro)frame timer resets and starts counting once per hub
bit time. On the next SOF, if the timer has not rolled over, the value in the current (micro)frame timer is
loaded into the next (micro)frame register and into the (micro)frame timer. The current (micro)frame timer
is reset to zero and continues to count and the flag is set to indicate that the (micro)frame timer is locked.
The (micro)frame timer rolls over when the count exceeds 60096 for high-speed or 12042 for full-speed (a
test at 65535 for high-speed or 16383 for full-speed is adequate). If the current (micro)frame timer has
rolled over, then an SOF was missed and the (micro)frame timer and next (micro)frame values are not
loaded. When an SOF is missed, the flag indicating that the timer is not synchronized remains set.
Whenever the (micro)frame timer counts down to zero, the current value of the next (micro)frame register is
loaded into the (micro)frame timer. When an SOF is detected, and the current (micro)frame timer has not
rolled over, the value of the current (micro)frame timer is loaded into the (micro)frame timer and the next
(micro)frame registers. The current (micro)frame timer is then reset to zero and continues to count. If the
current (micro)frame timer has rolled over, then the value in the next (micro)frame register is loaded into
the (micro)frame timer. This process can cause the (micro)frame timer to be updated twice in a single
(micro)frame: once when the (micro)frame timer reaches zero and once when the SOF is detected.
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11.2.3.2 EOF Advancement
The hub must advance its EOF points based on its SOF decode time in order to ensure that in the tiered
topology, hubs farther away from the host will always have later EOF points than hubs nearer to the host.
The magnitude of advance is implementation-dependent; the possible range of advance is derived below.
The synchronization circuit described above depends on successfully decoding an SOF packet identifier
(PID). This means that the (micro)frame timer will be synchronized to a time that is later than the
synchronization point in the SOF packet: later by at least 40 bit times for high-speed or 16 bit times for fullspeed. Each implementation also takes some time to react to the SOF decode and set the appropriate
timer/counter values. This reaction time is implementation-dependent but is assumed to be less than 192 bit
times for high-speed and four bit times for full-speed. Subsequent sections describe the actions that are
controlled by the (micro)frame timer. These actions are defined at the EOF1, EOF2, and EOF. EOF1 and
EOF2 are defined in later sections. These sections assume that the hub’s (micro)frame timer will count to
zero at the end of the (micro)frame (EOF). The circuitry described above will have the (micro)frame timer
counting to zero after 40 to 192 for high-speed bit times or 16-20 full-speed bit times after the start of a
(micro)frame (or end of previous (micro)frame). The timings and bit offsets in the later sections must be
advanced to account for this delay (i.e., add 40-192 for high-speed or 16-20 bit times for full-speed to the
EOF1 and EOF2 points).
Advancing the EOF points by the processing delay ensures that the spread between the EOFs is only due to
the propagation delay. For example, for high-speed, the maximum spread between 2 EOF points anywhere
on the USB is less than 216 bits (144 + 72). 144 bit times are due to 36 bit times of max latency through
4 repeaters. 72 bit times are due to five maximum cable and interconnect delays of 30 ns each. As can be
seen in Figure 11-4 without EOF advancement, a hub with a larger tier number could have an EOF occuring
earlier than a hub with a smaller tier number. In Figure 11-5 with EOF advancement ensures that in the
tiered topology, hubs with larger tier numbers always have later EOF points than hubs with smaller tier
numbers. Note: 13 bit times in the figures is an example maximum cable delay (approximately 30 ns).
Time
Tier 1
13+192 bits delay
Tier N
Tier
Depth
13+13+36+40 bits delay
Tier N+1
Figure 11-4. Example High-speed EOF Offsets Due to Propagation Delay Without EOF
Advancement
Time
Tier 1
13 bits delay
Tier N
Tier
Depth
13+13+36 bits delay
Tier N+1
Figure 11-5. Example High-speed EOF Offsets Due to Propagation Delay With EOF Advancement
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11.2.3.3 Effect of Synchronization on Repeater Behavior
The (micro)frame timer provides an indication to the hub Repeater state machine that the (micro)frame
timer has synchronized to SOF and that the (micro)frame timer is capable of generating the EOF1 and
EOF2 timing points. This signal is important after a global resume because of the possibility that a full/low-speed device may have been detached, and a low-/full-speed device attached while the host was
generating a long resume (several seconds) and the disconnect cannot be detected. The new device will bias
D+ and D- to appear like a K on the hub which would then be treated as an SOP and, unless inhibited, this
SOP would propagate though the resumed hubs. Since the hubs would not have seen any SOFs at this point,
the hubs would not be synchronized and, thus, unable to generate the EOF1 and EOF2 timing points. The
only recovery from this would be for the host to reset and re-enumerate the section of the bus containing the
changed device. This scenario is prevented by inhibiting any downstream facing port from establishing
connectivity until the hub is locked after a resume.
11.2.4 Microframe Jitter Related to Frame Jitter
The period between the SOFs from the Transaction Translator must not vary by more than +/- 42 ns. The
microframe timer count must be used by the Transaction Translator to generate SOFs to full-speed devices
(and keepalives to low-speed devices) connected to it.
The SOF received at the upstream facing port of the hub is repeated with a local clock. The frequency of
this clock may be a divided version of the bit rate. This could result in a quantization error and microframeto-microframe jitter. The microframe-to-microframe jitter of a hub repeater must be between 0 and 5 bit
times. This means that the latency through the repeater of consecutive SOFs must differ by less than 5 bits.
A hub may register the SOF for internal use, e.g., microframe synchronization. This requires SOF PID
detection. The circuitry used for internal registering of the SOF must have a jitter which is less than or
equal to 16 bits. This means that the microframe timer count values between consecutive equally spaced
SOFs must differ by less than or equal to 16 bits. The host controller frequency may drift over the period of
a microframe resulting in microframe period jitter. The host controller source jitter for SOFs must be less
than 4 bits. This means that the consecutive periods between SOFs must differ by less than 4 bits. These
requirements ensure that the microframe period at the end of five hub tiers will have a jitter of less than
40 bits (4 from host controller + 4*5 from hub repeaters + 16 from the internal SOF registering). This
means that the consecutive periods between SOFs as measured at any microframe timer will differ by less
than 40 bits (83.3 ns at 480 Mbs). This is less than the +/- 42 ns variation allowed.
11.2.5 EOF1 and EOF2 Timing Points
The EOF1 and EOF2 are timing points that are derived from the hub’s (micro)frame timer. Table 11-3
specifies the required host and hub EOF timing points for high-speed and full-speed operation.
Table 11-3. Hub and Host EOF1/EOF2 Timing Points
Bit Times Before EOF
for High-speed
Bit Times Before EOF
for Full-speed
Label
Notes
EOF1
560
32
End-of-(micro)frame point #1
EOF2
64
10
End-of-(micro)frame point #2
These timing points are used to ensure that devices and hubs do not interfere with the proper transmission of
the SOF packet from the host. These timing points have meaning only when the (micro)frame timer has
been synchronized to the SOF.
The host and hub (micro)frame markers, while all synchronized to the host’s SOF, are subject to certain
skews that dictate the placement of the EOF points. Figure 11-6 illustrates EOF2 timing point for high303
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speed operation. Figure 11-7 illustrates the EOF1 high-speed timing point. The numbers in the figures are
in high-speed bit times.
ime
EOF1
EOF=0
tier m
tier depth
EOF2=64
quantization=16
tier n
skew=38
Figure 11-6. High-speed EOF2 Timing Point
EOF2
time
EOF1=560
tier depth
EOF=0
tier 0
SOF propagation=216
skew=2
EOP propagation=216 +
quiescent time = 8
tier 5
skew=38
Figure 11-7. High-speed EOF1 Timing Point
At the EOF2 point, any port that has upstream connectivity will be disabled as a babbler. Hubs operating as
a full-/low-speed repeater prevent becoming disabled by sending an end of packet to the upstream hub
before that hub reaches its EOF2 point (i.e., at EOF1).
Figure 11-8 illustrates EOF timing points for full-/low-speed repeater operation.
EOF1
EOF2
SOF
Bit times
50
40
30
EOF1 range
20
10
0
EOF2 range
Figure 11-8. Full-speed EOF Timing Points
The hub operating as a full-/low-speed repeater is permitted to send the EOP if upstream connectivity is not
established at EOF1 time. A full-speed repeater must send the EOP if connectivity is established from any
downstream facing port at the EOF1 point.
A high-speed repeater must tear down upstream connectivity at the EOF1 point.
A high-speed repeater must tear down connectivity after the bus returns to the Idle state and the Elasticity
buffer is emptied (as described in Section 11.7.2) rather than on decoding an EOP pattern as in full-/lowspeed. Therefore, abrupt end of signaling (i.e, without a high-speed EOP) may cause malformed packets,
and this must not affect repeater operation. The host controller design must be capable of processing such
packets correctly.
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11.2.5.1 High-speed EOF1 and EOF2 Timing Points
The EOF2 point is 64 bit times before EOF as shown in Figure 11-6, and the EOF1 point is 560 bit times
before EOF as shown in Figure 11-7.
Although the hub is synchronized to the SOF, timing skew can accumulate between the host and a hub or
between hubs. This timing skew represents the difference between different microframe timers on different
hubs and the host. The total accumulated skew can be as much as 38 bit times. This is composed of ±2 bit
times of (micro)frame host source jitter and 0 to 36 bit times of repeater jitter as derived earlier. This skew
timing affects the placement of the EOF1 and EOF2 points.
Note: The hub skew timing assumes that the microframe interval will not be changed by the host after the
microframe timers have synchronized.
EOF skew can be from –2 to + 38 bits, so all EOFs are within 256 bits (216 bits of EOF propagation delay +
40 bits of EOF skew) of each other.
Note: The EOF2 point is based on 16 bit times for quantization + 38 bit times of skew; therefore, the EOF2
point needs to located at least 54 bit times before EOF. The EOF2 point is set at 64 bit times to allow
babble detection to be done with a divided (by 16) version of the bit clock. An upstream-directed packet
ending before EOF1 must reach every upstream hub/host before it gets to its EOF2 point. This is achieved
if the EOF1 point is located at least 544 bits before any upstream EOF (64 bits of EOF2 offset + 216 bits of
EOP propagation delay + 8 bits of idle time + 216 bits of SOF propagation delay + 38 bits of EOF1 skew +
2 bits of EOF2 skew). The EOF1 point is set at 560 bit times to allow using a divided (by 16) version of the
bit clock.
11.2.5.2 Full-speed EOF1 and EOF2 Timing Points
When the hub operates as a full-/low-speed repeater, the EOF1 point is 10 bit times before EOF and EOF1
is 32 bit times before EOF as shown in Figure 11-8.
The EOF2 point is defined to occur at least one bit time before the first bit of the SYNC for an SOP. The
period allowed for an EOP is four full-speed bit times (the upstream facing port on a hub is always fullspeed).
Although the hub is synchronized to the SOF, timing skew can accumulate between the host and a hub or
between hubs. This timing skew represents the difference between different frame timers on different hubs
and the host. The total accumulated skew can be as large as ±9 bit times. This is composed of ±1 bit times
per frame of quantization error and ±1 bit per frame of wander. The quantization error occurs when the hub
times the interval between SOFs and arrives at a value that is off by a fraction of a bit time but, due to
quantization, is rounded to a full bit. Frame wander occurs when the host's frame timer is adjusted by the
USB System Software so that the value sampled by the hub in a previous frame differs from the frame
interval being used by the host. (Note: Such adjustment was permitted in the USB 1.0 and 1.1 specification
but is no longer permitted.) These values accumulate over multiple frames because SOF packets can be lost
and the hub cannot resynchronize its frame timer. This specification allows for the loss of two consecutive
SOFs. During this interval, the quantization error accumulates to ±3 bit times, and the wander accumulates
to ±1 ± 2 ± 3 = ±6 for a total of ±9 bit times of accumulated skew in three frames. This skew timing affects
the placement of the EOF1 and EOF2 points as follows.
A hub must reach its EOF2 point one bit time before the end of the frame. In order to ensure this, a 9-bit
time guard-band must be added so that the EOF2 point is set to occur when the hub's local frame timer
reaches 10. A hub must complete its EOP before the hub to which it is attached reaches its EOF2 point. A
hub may reach its EOF2 point nine bit times before bit time 10 (at bit time 19 before the SOF). To ensure
that the EOP is completed by bit time 19, it must start before bit time 23. To ensure that the hub starts at bit
time 23 with respect to another hub, a hub must set its EOF1 point nine bit times ahead of bit time 23 (at bit
time 32). If a hub sets its timer to generate an EOP at bit time 32, that EOP may start as much as 9 bit times
early (at bit time 41).
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11.3 Host Behavior at End-of-Frame
It is the responsibility of the USB host controller (the host) to not provoke a response from a device if the
response would cause the device to be sending a packet at the EOF2 point. Furthermore, because a hub will
terminate an upstream directed packet when the hub reaches its EOF1 point, the host should not start a
transaction if a response from the device (data or handshake) would be pending or in process when a hub
reaches its EOF1 point. The implications of these limitations are described in the following sections.
Note: The above requirements can be met if the host controller ensures that the last transaction will
complete by its EOF1. The time consumed by a transaction (and consequently the latest start time of the
transaction) can be evaluated by accumulating the various delay components in the transaction. The packet
lengths should include all fields and account for bit-stuffing overhead as described in Chapter 7 and
Chapter 8. Formulae for calculating transaction times are located in Section 5.11.3.
In defining the timing points below, the last bit interval in a (micro)frame is designated as bit time zero. Bit
times in a (micro)frame that occur before the last have values that increase the further they are from bit time
zero (earlier bit times have higher numbers). These bit time designations are used for convenience only and
are not intended to imply a particular implementation. The only requirement of an implementation is that
the relative time relationships be preserved.
Host controllers issuing high-speed transactions on a high-speed bus must meet the above requirements.
Host controllers issuing full-/low-speed transactions on a full-/low-speed bus may also use the following
three behaviors near EOF.
11.3.1 Full-/low-speed Latest Host Packet
Hubs are allowed to send an EOP on their upstream facing ports at the EOF1 point if there is no
downstream-directed traffic in progress at that time. To prevent potential contention, the host is not allowed
to start a packet if connectivity will not be established on all connections before a hub reaches its EOF1
point. This means that the host must not start a packet after bit time 42.
Note: Although there is as much as a six-bit time delay between the time the host starts a packet and all
connections are established, this time need not be added to the packet start time as this phase delay exists for
the SOF packet as well, causing all hub frame timers to be phase delayed with respect to the host by the
propagation delay. There is only one bit time of phase delay between any two adjacent hubs and this has
been accounted for in the skew calculations.
11.3.2 Full-/low-speed Packet Nullification
If a device is sending a packet (data or handshake) when a hub in the device’s upstream path reaches its
EOF1 point, the hub will send a full-speed EOP. Any packet that is truncated by a hub must be discarded.
A host implementation may discard any packet that is being received at bit time 41. Alternatively, a host
implementation may attempt to maximize bus utilization by accepting a packet if the packet is predicted to
start at or before bit time 41.
11.3.3 Full-/low-speed Transaction Completion Prediction
A device can send two types of packets: data and handshake. A handshake packet is always exactly 16 bit
times long (sync byte plus PID byte.) The time from the end of a packet from the host until the first bit of
the handshake must be seen at the host is 17 bit times. This gives a total allocation of 35 bit times from the
end of data packet from the root (start of EOP) until it is predicted that the handshake will be completed
(start of EOP) from the device. Therefore, if the host is sending a data packet for which the device can
return a handshake (anything other than an isochronous packet), then if the host completes the data packet
and starts sending EOP before bit time 76, then the host can predict that the device will complete the
handshake and start the EOP for the handshake on or before bit time 41. For a low-speed device, the 36 bit
times from start of EOP from root to start of EOP from the device are low-speed bit times, which convert 1
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to 8 into full-speed bit times. Therefore, if the host completes the low-speed data packet by bit time 329,
then the low-speed device can be predicted to complete the handshake before bit time 41.
Note: If the host cannot accept a full-speed EOP as a valid end of a low-speed packet, then the low-speed
EOP will need to complete before bit time 41, which will add 13 full-speed bit times to the low-speed
handshake time.
As the host approaches the end of the frame, it must ensure that it does not require a device to send a
handshake if that handshake cannot be completed before bit time 41. The host expects to receive a
handshake after any valid, non-isochronous data packet. Therefore, if the host is sending a non-isochronous
data packet when it reaches bit time 76 (329 for low-speed), then the host should start an abnormal
termination sequence to ensure that the device will not try to respond. This abnormal termination sequence
consists of 7 consecutive (non-bitstuffed) bits of 1 followed by an EOP. The abnormal termination
sequence is sent at the speed of the current packet. Note: The intent of this sequence is to force a
bitstuffing violation (and possibly other errors) at the receiver.
If the host is preparing to send an IN token, it may not send the token if the predicted packet from the device
would not complete by bit time 41. The maximum valid length of the response from the device is known by
the host and should be used in the prediction calculation. For a full-speed packet, the maximum interval
between the start of the IN token and the end of a data packet is:
token_length + (packet_length + header + CRC) * 7/6 + 18
Where token_length is 34 bit times, packet_length is the maximum number of data bits in the packet,
header is eight bits of sync and eight bits of PID, and CRC is 16 bits. The 7/6 multiplier accounts for the
absolute worst case bit-stuff on the packet, and the 18 extra bits allow for worst case turn-around delay. For
a low-speed device, the same calculation applies, but the result must be multiplied by 8 to convert to fullspeed bit times, and an additional 20 full-speed bit times must be added to account for the low-speed prefix.
This gives the maximum number of bit times between the start of the IN token and the end of the data
packet, so the token cannot be sent if this number of bit times does not exist before the earliest EOF1 point
(bit time 41). (For example, take the results of the above calculation and add 41. If the number of bits left
in the frame is less than this value, the token may not be sent.)
The host is allowed to use a more conservative algorithm than the one given above for deciding whether or
not to start a transaction. The calculation might also include the time required for the host to send the
handshake when one is required, as there is no benefit in starting a transfer if the handshake cannot be
completed.
11.4 Internal Port
The internal port is the connection between the Hub Controller and the Hub Repeater. Besides conveying
the serial data to/from the Hub Controller, the internal port is the source of certain resume signals.
Figure 11-9 illustrates the internal port state machine; Table 11-4 defines the internal port signals and
events.
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!Rx_Suspend
Inactive
! = Logical NOT
Rx_Suspend
Suspend Delay
EOI
Fsus
Resume_Event
GResume
Figure 11-9. Internal Port State Machine
Table 11-4. Internal Port Signal/Event Definitions
Signal/Event Name
Event/Signal
Source
Description
EOI
Internal
End of timed interval
Rx_Suspend
Receiver
Receiver is in the Suspend state
Resume_Event
Hub Controller
A resume condition exists in the Hub Controller
11.4.1 Inactive
This state is entered whenever the Receiver is not in the Suspend state.
11.4.2 Suspend Delay
This state is entered from the Inactive state when the Receiver transitions to the Suspend state.
This is a timed state with a 2 ms interval.
11.4.3 Full Suspend (Fsus)
This state is entered when the Suspend Delay interval expires.
11.4.4 Generate Resume (GResume)
This state is entered from the Fsus state when a resume condition exists in the Hub Controller. A resume
condition exists if the C_PORT_SUSPEND bit is set in any port, or if the hub is enabled as a wakeup source
and any bit is set in a Port Change field or the Hub Change field (as described in Figures 11-22 and 11-20,
respectively).
In this state, the internal port generates signaling to emulate an SOP_FD to the Hub Repeater.
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11.5 Downstream Facing Ports
The following sections provide a functional description of a state machine that exhibits the correct behavior
for a downstream facing port.
Figure 11-10 is an illustration of the downstream facing port state machine. The events and signals are
defined in Table 11-5. Each of the states is described in Section 11.5.1. In the diagram below, some of the
entry conditions into states are shown without origin. These conditions have multiple origin states and the
individual transitions lines are not shown so that the diagram can be simplified. The description of the
entered state indicates from which states the transition is applicable.
Note: For the root hub, the signals from the upstream facing port state machines are implementation
dependent.
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Port Outputs in States
# = Logical OR
Configuration = 0
& = Logical AND
Not
Configured
ClearPortFeature(PORT_POWER) #
SetConfiguration(non-zero) #
Power_Source_Off #
Over-current
!
= Logical NOT
The hub is not configured.
SetConfiguration(non-zero)
Powered_off: Port requires explicit
request to transition.
Powered-off
SetPortFeature(PORT_POWER)
Disconnect_Detect
Disconnected: Port does not propagate
any traffic in either direction. All ports
are HiZ. Port is timing length of J/K
(2.5µS to 2mS).
Disconnected
EOI
ClearPortFeature(PORT_ENABLE)
Disabled
Disabled: Port cannot propagate any
traffic. All ports are HiZ.
Resetting
Resetting: Drive SE0 through the port for
10mS.
SetPortFeature(PORT_RESET)
SetTest
Testing
EOI
Rx_Suspend & (SE0 # K)
Port_Error
Enabled
Rptr_Enter_WFEOPFU
Transmit
LS & SOF
Enabled: Port can propagate both
upstream and downstream traffic.
Rx_Resume
TransmitR
Transmit: Port propagates downstream
directed traffic.
SetPortFeature(PORT_SUSPEND)
Rptr_Exit_WFEOPFU
Rx_Suspend & (SE0 # K)
Suspended
Rptr_Exit_WFEOPFU
Suspended: No traffic is propagated
downstream or upstream.
(!Rx_Suspend & PK) #
ClearPortFeature(PORT_SUSPEND)
Resuming: Drive ’K’ for 20mS.
Resuming
EOI
EOI
TransmitR: Port propagates downstream
directed resume signaling.
SendEOR
!(PK#PS)&EOI
Restart_S
PK /TrueRWU
RestartS and Restart_E: Port enters one of
these states to wait through timing
iintervals or for clocks to restart. Delay
iinterval is implementation dependent.
PS
!(PK#PS)&EOI
Restart_E
PK /TrueRWU
PS
Figure 11-10. Downstream Facing Hub Port State Machine
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State machine exports:
TrueRWU signal
(“/TrueRWU” indicates signal is
generated on transition from state)
Universal Serial Bus Specification Revision 2.0
Table 11-5. Downstream Facing Port Signal/Event Definitions
Signal/Event Name
Event/Signal
Source
Description
Power_source_off
Implementationdependent
Power to the port not available due to over-current or
termination of source power (e.g., external power
removed)
Over-current
Hub Controller
Over-current condition exists on the hub or the port
EOI
Internal
End of a timed interval or sequence
SE0
Internal
SE0 received on port
Disconnect_Detect
Internal
Disconnect seen at port
LS
Hub Controller
Low-speed device attached to this port
SOF
Hub Controller
SOF token received
TrueRWU
Internal
K lasting for at least TDDIS (see Table 7-13)
PK
Internal
K lasting for at least TDDIS
PS
Internal
SE0 lasting for at least TDDIS
K
Internal
‘K’ received on port
Rx_Resume
Receiver
Upstream Receiver in Resume state
Rx_Suspend
Receiver
Upstream Receiver in Suspend state
Rptr_Exit_WFEOPFU
Hub Repeater
Hub Repeater exits the WFEOPFU state
Rptr_Enter_WFEOPFU
Hub Repeater
Hub Repeater enters the WFEOPFU state
Port_Error
Internal
Error condition detected (see Section 11.8.1)
SetTest
Hub Controller
Logical OR of SetPortFeature(Test_SE0_NAK),
SetPortFeature(Test_J), SetPortFeature(Test_K),
SetPortFeature(Test_PRBS),
SetPortFeature(Test_Force_Enable)
Configuration = 0
Hub Controller
Hub controller's configuration value is zero
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11.5.1 Downstream Facing Port State Descriptions
11.5.1.1 Not Configured
A port transitions to and remains in this state whenever the value of the hub configuration is zero. While the
port is in this state, the hub will drive an SE0 on the port (this behavior is optional on root hubs). No other
active signaling takes place on the port when it is in this state.
11.5.1.2 Powered-off
This state is supported for all hubs.
A port transitions to this state in any of the following situations:
•
From any state except Not Configured when the hub receives a ClearPortFeature(PORT_POWER)
request for this port
•
From any state when the hub receives a SetConfiguration() request with a configuration value other
than zero
•
From any state except Not Configured when power is lost to the port or an over-current condition exists
A port will enter this state due to an over-current condition on another port if that over-current condition
may have caused the power supplied to this port to drop below specified limits for port power (see
Section 7.2.1.2.1 and Section 7.2.4.1).
If a hub was configured while the hub was self-powered, and then if external power is lost, the hub must
place all ports in the Powered-off state. If the hub is configured while bus powered, then the hub need not
change port status if the hub switched to externally applied power. However, if external power is
subsequently lost, the hub must place ports in the Powered-off state.
In this state, the port’s differential and single-ended transmitters and receivers are disabled.
Control of power to the port is covered in Section 11.11.
11.5.1.3 Disconnected
A port transitions to this state in any of the following situations:
• From the Powered-off state when the hub receives a SetPortFeature(PORT_POWER) request
• From any state except the Not Configured and Powered-off states when the port’s disconnect timer times
out
• From the Restart_S or Restart_E state at the end of the restart interval
In the Disconnected state, the port’s differential transmitter and receiver are disabled and only connection
detection is possible.
This is a timed state. While in this state, the timer is reset as long as the port’s signal lines are in the SE0 or
SE1 state. If another signaling state is detected, the timer starts. Unless the hub is suspended with clocks
stopped, this timer's duration is 2.5 µs to 2 ms.
If the hub is suspended with its remote wakeup feature enabled, then on a transition to any state other than
the SE0 state or SE1 state on a Disconnected port, the hub will start its clocks and time this event. The hub
must be able to start its clocks and time this event within 12 ms of the transition. If a hub does not have its
remote wakeup feature enabled, then transitions on a port that is in the Disconnected state are ignored until
the hub is resumed.
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11.5.1.4 Disabled
A port transitions to this state in any of the following situations:
•
From the Disconnected state when the timer expires indicating a connection is detected on the port
•
From any but the Powered-off, Disconnected, or Not Configured states on receipt of a
ClearPortFeature(PORT_ENABLE) request
•
From the Enabled state when an error condition is detected on the port
A port in the Disabled state will not propagate signaling in either the upstream or the downstream direction.
While in this state, the duration of any SE0 received on the port is timed. If the port is using high-speed
terminations when it enters this state, it switches to full-speed terminations. The port must not perform
normal disconnect detection until at least 4 ms after entering this state.
11.5.1.5 Resetting
Unless it is in the Powered-off or Disconnected states, a port transitions to the Resetting state upon receipt
of a SetPortFeature(PORT_RESET) request. The hub drives SE0 on the port during this timed interval.
The duration of the Resetting state is nominally 10 ms to 20 ms (10 ms is preferred).
A hub in high-speed operation will use the high-speed terminations of the port when in this state.
11.5.1.6 Enabled
A port transitions to this state in any of the following situations:
•
At the end of the Resetting state
•
From the Transmit state or the TransmitR state when the Hub Repeater exits the WFEOPFU state
•
From the Suspended state if the upstream Receiver is in the Suspend state when a ’K’ is detected on the
port
•
At the end of the SendEOR state
•
From the Restart_E state when a persistent K or persistent SE0 has not been seen within 900 µs of
entering that state
While in this state, the output of the port’s differential receiver is available to the Hub Repeater so that
appropriate signaling transitions can establish upstream connectivity.
A port which is using high-speed terminations in this state switches to full-speed terminations on
Rx_Suspend (i.e., when the hub is suspended). The port must not perform normal disconnect detection until
at least 1 ms after Rx_Suspend becomes active.
11.5.1.7 Transmit
This state is entered from the Enabled state on the transition of the Hub Repeater to the WFEOPFU state.
While in this state, the port will transmit the data that is received on the upstream facing port.
For a low-speed port, this state is entered from the Enabled state if a full-speed PRE PID is received on the
upstream facing port. While in this state, the port will retransmit the data that is received on the upstream
facing port (after proper inversion).
In high-speed, this state is used for testing for disconnect at the port. The disconnect detection circuit is
enabled after 32 bits of the same signaling level (‘J’ or ‘K’) have been transmitted down the port.
Note: Because of the timing skew in the repeater path to the downstream facing ports, all downstream
facing ports may not be enabled for disconnect detection at the same instant in time.
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11.5.1.8 TransmitR
This state is entered in either of the following situations:
•
From the Enabled state if the upstream Receiver is in the Resume state
•
From the Restart_S or Restart_E state if a PK is detected on the port
When in this state, the port repeats the resume ‘K’ at the upstream facing port to the downstream facing
port. Depending on the speed of the port, two behaviors are possible on the K->SE0 transition at the
upstream facing port at the end of the resume.
•
Upstream facing port high-speed and downstream facing port full-/low-speed: After the K->SE0
transition, the port drives SE0 for 16 to 18 full-speed bit times followed by driving J for at least one
full-speed bit time. Note: The timer in the Resume state of the upstream port receiver state machine
which generates EOITR can be used to time this requirement at the downstream facing port(s). The
pullup resistor and the latency of the Transaction Translator(TT) results in this Idle state being
maintained for at least one low-speed bit time ensuring that a device sees the same end of resume
behavior below the TT as it would below a USB 1.x hub.
•
Upstream facing port and downstream facing port are the same speed: port continues to repeat the
signaling which follows the K->SE0 transition.
A port operating in high-speed reverts to its high-speed terminations within 18 full-speed bit times after the
K->SE0 transition as described in Section 7.1.7.7.
11.5.1.9 Suspended
A port enters the Suspended state:
•
From the Enabled state when it receives a SetPortFeature(PORT_SUSPEND) request
•
From the Restart_S state when a persistent K or persistent SE0 has not been seen within 900 µs of
entering that state
While a port is in the Suspended state, the port's differential transmitter is disabled. A high-speed port
reverts from high-speed to full-speed terminations but its speed status continues to be high-speed. The port
must not perform normal disconnect detection until at least 4 ms after entering this state.
An implementation must have a K/SE0 ‘noise’ filter for a port that is in the suspended state. This filter can
time the length of K/SE0 and, if the length of the K/SE0 is shorter than TDDIS, the port must remain in this
state. If the hub is suspended with its clocks stopped, a transition to K/SE0 on a suspended port must cause
the port to immediately transition to the Restart_S state.
11.5.1.10 Resuming
A port enters this state from the Suspended state in either of the following situations:
•
If a 'K' is detected on the port and persists for at least 2.5 µs and the Receiver is not in the Suspended
state. The transition from the Suspended state must happen within 900 µs of the J->K transition.
•
When a ClearPortFeature(PORT_SUSPEND) request is received.
This is a timed state with a nominal duration of 20 ms (the interval may be longer under the conditions
described in the note below). While in this state, the hub drives a 'K' on the port.
Note: A single timer is allowed to be used to time both the Resetting interval and the Resuming interval and
that timer may be shared among multiple ports. When shared, the timer is reset when a port enters the
Resuming state or the Resetting state. If shared, it may not be shared among more than ten ports as the
cumulative delay could exceed the amount of time required to replace a device and a disconnect could be
missed.
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11.5.1.11 SendEOR
This state is entered from the Resuming state if the 20 ms timer expires. It is also entered from the Enabled
state when an SOF (or other FS token) is received and a low-speed device is attached to this port.
This is a timed state which lasts for three low-speed bit times.
In this state, if the port is high-speed it will drive the bus to the Idle state for three low-speed bit times and
then exit from this state to the Enabled state. It must also revert to its high-speed terminations within
18 full-speed bit times after the K->SE0 transition as described in Section 7.1.7.7.
If the port is full-speed or low-speed, the port must drive two low-speed bit times of SE0 followed by one
low-speed bit time of Idle state and then exit from this state to the Enabled state.
Since the driven SE0 period should be of fixed length, the SendEOR timer, if shared, should not be reset. If
the hub implementation shares the SendEOR timing circuits between ports, then for a port with a low-speed
device attached, the Resuming state should not end until an SOF (or other FS token) has been received (see
Section 11.8.4.1 for Keep-alive generation rules).
11.5.1.12 Restart_S
A port enters the Restart_S state from the Suspended state when an SE0 or ‘K’ is seen at the port and the
Receiver is in the Suspended state.
In this state, the port continuously monitors the bus state. If the bus is in the ‘K’ state for at least TDDIS, the
port sets the C_PORT_SUSPEND bit, exits to the TransmitR, and generates a signal to the repeater called
‘TrueRWU’. If the bus is in the ‘SE0’ state for at least TDDIS, the port exits to the Disconnected state.
Either of these transitions must happen within 900 µs after entering the Restart_S state; otherwise, the port
must transition back to the Suspended state.
11.5.1.13 Restart_E
A port enters the Restart_E state from the Enabled state when an ‘SE0’ or ‘K’ is seen at the port and the
Receiver is in the Suspended state.
In this state, the port continuously monitors the bus state. If the bus is in the ‘K’ state for at least TDDIS, the
port exits to the TransmitR state and generates a signal to the repeater called ‘TrueRWU’. If the bus is in the
‘SE0’ state for at least TDDIS, the port exits to the Disconnected state. Either of these transitions must
happen within 900 µs after entering the Restart_E state; otherwise the port must transition back to the
Enabled state.
11.5.1.14 Testing
A port transitions to this state from any state when the port sees SetTest.
While in this state, the port executes the host command as decoded by the hub controller. If the command
was a SetPortFeature(PORT_TEST, Test_Force_Enable), the port supports packet connectivity in the
downstream direction in a manner identical to that when the port is in the Enabled state.
11.5.2 Disconnect Detect Timer
11.5.2.1 High-speed Disconnect Detection
High-speed disconnect detection is described in Section 7.1.7.3.
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11.5.2.2 Full-/low-speed Disconnect Detection
Each port is required to have a timer used for detecting disconnect when a full-/low-speed device is attached
to the port. This timer is used to constantly monitor the port’s single-ended receivers to detect a disconnect
event. The reason for constant monitoring is that a noise event on the bus can cause the attached device to
detect a reset condition on the bus after 2.5 µs of SE0 or SE1 on the bus. If the hub does not place the port in
the disconnect state before the device resets, then the device can be at the Default Address state with the port
enabled. This can cause systems errors that are very difficult to isolate and correct.
This timer must be reset whenever the D+ and D- lines on the port are not in the SE0 or SE1 state or when
the port is not in the Enabled, Suspended, Disabled, Restart-E, or Restart_S states. This timer must be reset
for 4ms upon entry to the Suspended and Disabled states. This timer times an interval TDDIS. The range of
TDDIS is 2.0 µs to 2.5 as defined in Table 7-13. When this timer expires, it generates the
Disconnect_Detect signal to the port state machine.
This timer can also be used for filtering the K/SE0 signal in the Suspended, Restart_E, or Restart_S states as
described in Section 11.5.1.
11.5.3 Port Indicator
Each downstream facing port of a hub can support an optional status indicator. The presence of indicators
for downstream facing ports is specified by bit 7 of the wHubCharacteristics field of the hub class
descriptor. Each port’s indicator must be located in a position that obviously associates the indicator with
the port. The indicator provides two colors: green and amber. This can be implemented as physically one
LED with two color capability or two separate LEDs. A combination of hardware and software control is
used to inform the user of the current status of the port or the device attached to the port and to guide the
user through problem resolution. Colors and blinking are used to provide information to the user.
An external hub must automatically control the color of the indicator as specified in Figure 11-11.
Automatic port indicator setting support for root hubs may be implemented with either hardware or
software. The port indicator color selector value is zero (indicating automatic control) when the hub
transitions to the configured device state. When the hub is suspended or not configured, port indicators
must be off.
Table 11-6 identifies the mapping of color to port state when the port indicators are automatically
controlled.
Table 11-6. Automatic Port State to Port Indicator Color Mapping
Power
Switching
With
Without
316
Downstream Facing Hub Port State
Powered-off
Disconnected, Disabled, Not
Configured, Resetting,
Testing
Enabled,
Transmit, or
TransmitR
Suspended,
Resuming,
SendEOR,
Restart_E, or
Restart_S
Off or amber if due
to an over-current
condition
Off
Green
Off
Off
Off or amber if due to an overcurrent condition
Green
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Universal Serial Bus Specification Revision 2.0
Automatic
Mode
SetPortFeature
(PORT_INDICATOR,
indicator_selector != 0)
Enabled or Transmit or TransmitR
Off
Green
Manual Mode
! (Enabled or Transmit or TransmitR)
and PORT_OVER_CURRENT != 1
PORT_OVER_CURRENT = 1
PORT_OVER_CURRENT = 1
SetPortFeature
(PORT_POWER)
SetPortFeature
(PORT_INDICATOR,
indicator_selector = 0)
Amber
Figure 11-11. Port Indicator State Diagram
In Manual Mode the color of a port indicator (Amber, Green, or Off) is set by a system software USB Hub
class request. In Automatic Mode the color of a port indicator is set by the port state information.
Table 11-7 defines port state as understood by the user.
Table 11-7. Port Indicator Color Definitions
Color
Definition
Off
Not operational
Amber
Error condition
Green
Fully operational
Blinking
Software attention
Off/Green
Blinking
Hardware attention
Off/Amber
Blinking
Reserved
Green/Amber
Note that the indicators reflect the status of the port, not necessarily the device attached to it. Blinking of
the indicator is used to draw the user’s attention to the port, irrespective of its color.
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Port indicators allow control by software. Host software forces the state of the indicator to draw attention to
the port or to indicate the current state of the port.
See Section 11.24.2.7.1.10 for the specification of indicator requests.
11.5.3.1 Labeling
USB system software uses port numbers to reference an individual port with a ClearPortFeature or
SetPortFeature request. If a vendor provides a labeling to identify individual downstream facing ports, then
each port connector must be labeled with their respective port number.
11.6 Upstream Facing Port
The upstream facing port has four components: transmitter, transmitter state machine, receiver, and receiver
state machine. The transmitter and its state machine are the Transmitter, while the receiver and its state
machine are the Receiver. The Transmitter and Receiver operate in high-speed and full-speed depending on
the current hub configuration.
11.6.1 Full-speed
Both the transmitter and receiver have differential and single-ended components. The differential
transmitter and receiver can send/receive ’J’ or ’K’ to/from the bus while the single-ended components are
used to send/receive SE0, suspend, and resume signaling. The single-ended components are also used to
receive SE1. In this section, when it is necessary to differentiate the signals sent/received by the differential
component of the transmitter/receiver from those of the single-ended components, DJ and DK will be used
to denote the differential signal, while SJ, SK, SE0, and SE1 will be used for the single-ended signals.
When the Hub Repeater has connectivity in the upstream direction, the transmitter must not send or
propagate SE1 signaling. Instead, the SE1 must be propagated as a DJ.
11.6.2 High-speed
Both the transmitter and receiver have differential components only. These signals are called HJ and HK.
The HS_Idle state is the idle state of the bus in high-speed.
It is assumed that the differential transmitter and receiver are turned off during suspend to minimize power
consumption. The single-ended components are left on at all times, as they will take minimal power.
11.6.3 Receiver
The receiver state machine is responsible for monitoring the signaling state of the upstream connection to
detect long-term signaling events such as bus reset, resume, and suspend. This state machine details the
operation of the device state diagram shown in Figure 9-1 in the Default, Address, Configured, and
Suspended state. The Suspend, Resume, and ReceivingSE0 states are only used when the upstream facing
port is operating in full-speed mode with full-speed terminations. The ReceivingIS, ReceivingHJ, and
ReceivingHK states are only used when the upstream facing port is operating in high-speed mode with highspeed terminations; so these states are categorized as the HS (high-speed) states, and all other states are
categorized as nonHS in the description below.
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Figure 11-12 illustrates the state transition diagram.
Tx_active
HJ
State Machine Exports:
J
ReceivingHJ
Rx_Bus_Reset(Bus_Reset)
Rx_Suspend(Suspend)
Rx_Resume(Resume)
EOITR
ReceivingJ
EOI
Suspend
HK
K
ReceivingHK
ReceivingK
# = Logical OR
& = Logical AND
! = Logical NOT
EOI
Tx_resume # K
Resume
SE0
EOITR
ReceivingSE0
POR
EOI
Bus_Reset
HS_Idle
HS &EOR
EOI & HS_Idle
ReceivingIS
EOI & !HS_Idle
Figure 11-12. Upstream Facing Port Receiver State Machine
Table 11-8 defines the signals and events referenced in the figures.
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Table 11-8. Upstream Facing Port Receiver Signal/Event Definitions
Signal/Event
Name
Event/Signal
Source
Description
HS
Internal
Port is operating in high-speed
Tx_active
Transmitter
Transmitter in the Active state
J
Internal
Receiving a 'J' (IDLE) or an ‘SE1’ on the upstream facing port
HJ
Internal
Receiving an HJ on the upstream facing port
EOI
Internal
End of timed interval
EOITR
Internal
Generated 24 full-speed bit times after the K->SE0 transition
at the end of resume
HK, K
Internal
Receiving an HK, 'K' on the upstream facing port
Tx_resume
Transmitter
Transmitter is in the Sresume state
HS_Idle
Internal
Receiving an Idle state on the high-speed upstream facing
port
SE0
Internal
Receiving an SE0 on the full-speed upstream facing port
EOR
Internal
End of Reset signaling from upstream
POR
Implementationdependent
Power_On_Reset
11.6.3.1 ReceivingIS
This state is entered
•
From the ReceivingHJ or ReceivingHK state when a SE0 is seen at the port and the port is in highspeed operation
•
From the Resume state when a EOITR is seen and the port is in high-speed operation
•
From the Bus Reset state at the End of Reset signaling from upstream when the port is in high-speed
operation
This is a timed state with an interval of 3 ms. The timer is reset each time this state is entered.
11.6.3.2 ReceivingHJ
This state is entered from an HS state when a HJ is seen on the bus.
11.6.3.3 ReceivingJ
This state is entered from a nonHS state except the Suspend state if the receiver detects an SJ (or Idle) or
SE1 condition on the bus or while the Transmitter is in the Active state.
This is a timed state with an interval of 3 ms. The timer is reset each time this state is entered.
The timer only advances if the Transmitter is in the Inactive state.
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11.6.3.4 Suspend
This state is entered when:
•
The 3 ms timer expires in the ReceivingJ
•
The 3 ms timer expires in the ReceivingIS state and the port has removed its high-speed
terminations and connected its D+ pull-up resistor and the resulting bus state is not SE0.
When the Receiver enters this state, the Hub Controller starts a 2 ms timer. If that timer expires while the
Receiver is still in this state, then the Hub Controller is suspended. When the Hub Controller is suspended,
it may generate resume signaling.
11.6.3.5 ReceivingHK
This state is entered from an HS state when a HK is seen on the bus.
11.6.3.6 ReceivingK
This state is entered from any nonHS state except the Resume state when the receiver detects an SK
condition on the bus and the Hub Repeater is in the WFSOP or WFSOPFU state.
This is a timed state with a duration of 2.5 µs to 100 µs. The timer is reset each time this state starts.
11.6.3.7 Resume
This state is entered:
•
From the ReceivingK state when the timer expires
•
From the Suspend state while the Transmitter is in the Sresume state or if there is a transition to the
K state on the upstream facing port
If the hub enters this state when its timing reference is not available, the hub may remain in this state until
the hub’s timing reference becomes stable (timing references must stabilize in less than 10 ms). If this state
is being held pending stabilization of the hub’s clock, the Receiver must provide a K to the repeater for
propagation to the downstream facing ports. When clocks are stable, the Receiver must repeat the incoming
signals.
Note: Hub timing references will be stable in less than 10 ms since reset requirements already specify that
they be stable in less than 10 ms and a hub must support reset from suspend.
11.6.3.8 ReceivingSE0
This state is entered from any nonHS state except Bus_Reset when the receiver detects an SE0 condition
and the Hub Repeater is in the WFSOP or WFSOPFU state.
This is a timed state. The minimum interval for this state is 2.5 µs. The maximum depends on the hub but
this interval must timeout early enough such that if the width of the SE0 on the upstream facing port is only
10 ms, the Receiver will enter the Bus_Reset state with sufficient time remaining in the 10 ms interval for
the hub to complete its reset processing. Furthermore, if the hub is suspended when the Receiver enters this
state, the hub must be able to start its clocks, time this interval, and complete its reset (chirp) protocol and
processing in the Bus_Reset state within 10 ms. It is preferred that this interval be as long as possible given
the constraints listed here. This will provide for the maximum immunity to noise on the upstream facing
port and reduce the probability that the device will reset in the presence of noise before the upstream hub
disables the port.
The timer is reset each time this state starts.
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11.6.3.9 Bus_Reset
This state is entered:
•
From the ReceivingSE0 state when the timer expires. As long as the port continues to receive SE0, the
Receiver will remain in this state.
•
This state is also entered while power-on-reset (POR) is being generated by the hub’s local circuitry.
The state machine cannot exit this state while POR is active.
•
The 3 ms timer expires in the ReceivingIS state and the port has removed its high-speed terminations
and connected its D+ pull-up resistor and the resulting bus state is still SE0.
In this state, a high-speed capable port will implement the chirp signaling, handshake, and timing protocol
as described in Section 7.1.7.5.
11.6.4 Transmitter
This state machine is used to monitor the upstream facing port while the Hub Repeater has connectivity in
the upstream direction. The purpose of this monitoring activity is to prevent propagation of erroneous
indications in the upstream direction. In particular, this machine prevents babble and disconnect events on
the downstream facing ports of this hub from propagating and causing this hub to be disabled or
disconnected by the hub to which it is attached. Figure 11-13 is the transmitter state transition diagram.
Table 11-9 defines the signals and events referenced in Figure 11-13.
Rx_Bus_Reset
HS&(EOF1#
HEOP)
EOF1&!HS
State Machine Exports:
Inactive
WFEOP & !Rx_Suspend
Tx_Active(Active)
Tx_Resume(Sresume)
Active
SE0sent
EOF1&!HS
RepeatingSE0
K
# = Logical OR
EOI # J
SendJ
EOI
& = Logical AND
! = Logical NOT
EOI
GEOPTU
Rx_Suspend &
Rptr_WFEOP
Sresume
EOI
Figure 11-13. Upstream Facing Port Transmitter State Machine
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Universal Serial Bus Specification Revision 2.0
Table 11-9. Upstream Facing Port Transmit Signal/Event Definitions
Signal/Event
Name
Event/Signal
Source
Description
Rx_Bus_Reset
Receiver
Receiver is in the Bus_Reset state
EOF1
(micro)frame
Timer
Hub (micro)frame time has reached the EOF1 point or is
between EOF1 and the end of the (micro)frame
J
Internal
Transmitter transitions to sending a ’J’ and transmits a ’J’
Rptr_WFEOP
Hub Repeater
Hub Repeater is in the WFOEP state
K
Internal
Transmitter transmits a ’K’
SE0sent
Internal
At least one bit time of SE0 has been sent through the
transmitter
Rx_Suspend
Receiver
Receiver is in Suspend state
HEOP
Repeater
Completion of packet transmission in upstream direction
HS
Internal
Upstream facing port is operating as high-speed port
EOI
Internal
End of timed interval
11.6.4.1 Inactive
This state is entered at the end of the SendJ state or while the Receiver is in the Bus_Reset state. This state
is also entered at the end of the Sresume state. While the transmitter is in this state, both the differential and
single-ended transmit circuits are disabled and placed in their high-impedance state.
When port is operating as a high-speed port, this state is entered from the Active state at EOF1 or after an
HEOP from downstream.
11.6.4.2 Active
This state is entered from the Inactive state when the Hub Repeater transitions to the WFEOP state. This
state is entered from the RepeatingSE0 state if the first transition after the SE0 is not to the J state. In this
state, the data from a downstream facing port is repeated and transmitted on the upstream facing port.
11.6.4.3 RepeatingSE0
The port enters this state from the Active state when one bit time of SE0 has been sent on the upstream
facing port. While in this state, the transmitter is still active and downstream signaling is repeated on the
port. This is a timed state with a duration of 23 full-speed bit times.
11.6.4.4 SendJ
The port enters this state from the RepeatingSE0 state if either the bit timer reaches 23 or the repeated
signaling changes from SE0 to 'J' or ‘SE1’. This state is also entered at the end of the GEOPTU state. This
state lasts for one full-speed bit time. During this state, the hub drives an SJ on the port.
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11.6.4.5 Generate End of Packet Towards Upstream Port (GEOPTU)
The port enters this state from the Active or RepeatingSEO state if the frame timer reaches the EOF1 point.
In this state, the port transmits SE0 for two full-speed bit times.
11.6.4.6 Send Resume (Sresume)
The port enters this state from the Inactive state if the Receiver is in the Suspend state and the Hub Repeater
transitions to the WFEOP state. This indicates that a downstream device (or the port to the Hub Controller)
has generated resume signaling causing upstream connectivity to be established.
On entering this state, the hub will restart clocks if they had been turned off during the Suspend state.
While in this state, the Transmitter will drive a ’K’ on the upstream facing port. While the Transmitter is in
this state, the Receiver is held in the Resume state. While the Receiver is in the Resume state, all
downstream facing ports that are in the Enabled state are placed in the TransmitR state and the resume on
this port is transmitted to those downstream facing ports.
The port stays in this state for at least 1 ms but for no more than 15 ms.
11.7 Hub Repeater
The Hub Repeater provides the following functions:
•
Sets up and tears down connectivity on packet boundaries
•
Ensures orderly entry into and out of the Suspend state, including proper handling of remote wakeups
11.7.1 High-speed Packet Connectivity
High-speed packet repeaters must reclock the packets in both directions. Reclocking means that the
repeater extracts the data from the received stream and retransmits the stream using its own local clock.
This is necessary in order to keep the jitter seen at a receiver within acceptable limits (see Chapter 7 for
definition and limits on jitter).
Reclocking creates several requirements which can be best understood with the example repeater signal path
shown in Figure 11-14.
Squelch
Port Selector state
machine
Xmt_stream
Rcv_stream
Data
Recovery
Elasticity
Buffer
Rcv_Clk
Xmt_Clk
Figure 11-14. Example Hub Repeater Organization
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11.7.1.1 Squelch Circuit
Because of squelch detection, the initial bits of the SYNC field may not be seen in the rest of the repeater.
At most, 4 bits of the SYNC field may be sacrificed in the entire repeater path.
The squelch circuit may take at most 4 bit times to disable the repeater after the bus returns to the Idle state.
This results in bits being added after the end of the packet. This is also known as EOP dribble and up to
4 random bits may get added after the packet by the entire repeater path.
11.7.1.2 Data Recovery Unit
The data recovery unit extracts the receive clock and receive data from this stream. Note that this is a
conceptual model only; actual implementations (e.g., DLL) may achieve the reclocking by the local clock
without separation of the receive clock and data.
11.7.1.3 Elasticity Buffer
The half-depth of the elasticity buffer in the repeater must be at least 12 bits.
The total latency of a packet through a repeater must be less than 36 bit times. This includes the latency
through the elasticity buffer.
The elasticity buffer is used to handle the difference in frequency between the receive clock and the local
clock and works as follows. The elasticity buffer is primed (filled with at least 12 bits) by the receive clock
before the data is clocked out of it by the transmit clock. If the transmit clock is faster than the receive
clock, the buffer will get emptied more quickly than it gets filled. If the transmit clock is slower, the buffer
will get emptied slower than it gets filled. If the half-depth of the buffer is chosen to be equal to the
maximum difference in clock rate over the length of a packet, bits will not be lost or added to the packet.
The half-depth is calculated as follows.
The clock tolerance allowed is 500 ppm. This takes into account the effect of voltage, temperature, aging,
etc. So the received clock and the local clock could be different by 1000 ppm. The longest packet has a
data payload of 1 Kbytes. The maximum length of a packet is computed by adding the length of all the
fields and assuming maximum bit-stuffing. This maximum length is 9644 bits (9624 bits of packet + 20 bits
of EOP dribble). This means that when the repeater is clocking out a packet with its local clock, it could get
ahead of or fall behind the receive clock by 9.644 bits (1000 ppm*9644). This calculation yields 10 bits.
The half-depth of the elasticity buffer in the repeater must be at least 12 bits to provide system timing
margin.
11.7.1.4 High-Speed Port Selector State Machine
This state machine is used to establish connectivity on a valid packet and to keep the repeater from
establishing connectivity from a port which is seeing noise. This state machine must implement the
behavior shown in Figure 11-15. (Note: This state machine may be implemented on a per-port or per-hub
basis.)
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Rx_Bus_Reset
EBEmptied
Inactive
Enable Transmit
!Squelch
Squelch&EOI&!SORP
Priming
EOI&SORP
!Squelch&EOI&!SORP
Squelch
! = Logical NOT
&=Logical AND
Not Packet
#=Logical OR
Figure 11-15. High-speed Port Selector State Machine
Table 11-10. High-speed Port Selector Signal/Event Definitions
Signal/Event Name
Description
Event/Signal
Source
Rx_Bus_Reset
Internal
Receiver is in the Bus_reset state.
EBEmptied
Internal
All bits accumulated in the elasticity buffer have been
transmitted.
EOI
Internal
End of interval of time needed for priming elasticity buffer
Squelch
Internal
Bus is in squelch state
SORP
Internal
Start Of Repeating Pattern; a ‘JKJK’ or ‘KJKJ’ pattern has
been seen in data in elasticity buffer.
11.7.1.4.1 Inactive
This state is entered
•
From the Enable Transmit state when all the bits accumulated in the elasticity buffer have been
transmitted
•
From the Priming state if squelch is seen and the elasticity buffer is primed without a SORP being seen
•
From the Not Packet state when the squelch circuit indicates a squelch state on the port
•
From on any state on Rx_Bus_Reset
11.7.1.4.2 Priming
This state is entered from the Inactive state when the squelch circuit indicates that valid signal levels have
been observed at the port. This is a timed state and the priming interval is the time needed for the
implementation to fill the elasticity buffer with at least 12 bits.
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11.7.1.4.3 Enable Transmit
This state is entered from the Priming state when the Elasticity buffer priming interval has elapsed and the
bits in the elasticity buffer include the SORP pattern.
In this state, the state machine generates a signal “start of high-speed packet” (SOHP) to the repeater state
machine which allows the repeater to establish connectivity from this port to the upstream facing port (or
downstream facing ports).
11.7.1.4.4 Not Packet
This state is entered from the Priming state when the Elasticity buffer priming interval has elapsed, and the
bits in the elasticity buffer do not include the SORP pattern, and the squelch signal is not active.
11.7.2 Hub Repeater State Machine
The Hub repeater state machine in Figure 11-16 shows the states and transitions needed to implement the
Hub Repeater. Table 11-11 defines the Hub Repeater signals and events. The following sections describe
the states and the transitions.
11.7.2.1 High-speed Repeater Operation
Connectivity is setup on SOHP and torn down on HEOP. (HEOP is either the EBemptied signal from the
port selector state machine ‘OR’ the EOI signal which causes the transition out of the SendEOR state in
downstream facing port state machine.) Several of the state transitions below will occur when the HEOP is
seen. When such a transition is indicated, the transition does not occur until after the hub has repeated the
last bit in the elasticity buffer. Some of the transitions are triggered by an SOHP. Transitions of this type
occur as soon as the hub detects the SOHP from the port selector state machine ensuring that a valid packet
start has been seen.
11.7.2.2 Full-/low-speed Repeater Operation
Connectivity is setup on SOP and torn down on EOP. Several of the state transitions below will occur when
the EOP is seen. When such a transition is indicated, the transition does not occur until after the hub has
repeated the SE0-to-'J' transition and has driven 'J' for at least one bit time (bit time is determined by the
speed of the port.) Some of the transitions are triggered by an SOP. Transitions of this type occur as soon
as the hub detects the 'J'-to-'K' transition, ensuring that the initial edge of the SYNC field is preserved.
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11.7.2.3 Repeater State Machine
Rx_Bus_Reset
State Machine Exports:
WFSOPFU
UEOP & !Lock
SOP_FU
Rptr_WFEOP(WFEOP)
Rptr_WFSOPFU(WFSOPFU)
Rptr_Enter_WFEOPFU
Rptr_Exit_WFEOPFU
Rx_Resume
WFEOPFU
UEOP & Lock
SOP_FU
Rx_Suspend
# = Logical OR
& = Logical AND
! = Logical NOT
WFSOP
EOF1
SOP_FD
DEOP
WFEOP
EOF2
Figure 11-16. Hub Repeater State Machine
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Table 11-11. Hub Repeater Signal/Event Definitions
Signal/Event
Name
Rx_Bus_Reset
Event/Signal
Source
Receiver
Description
Receiver is in the Bus_Reset state
Three sources of HEOP:
HEOP
Internal (Port selector,
EBEmptied signal from port selector state machine OR
Downstream port,
transition at EOI from SendEOR state in downstream facing
port state machine OR
Upstream port
receiver)
EOITR from upstream facing port receiver state machine
UEOP
Internal
(HEOP)EOP received from the upstream facing port
DEOP
Internal
Generated when the Transmitter enters the (Inactive) SendJ
state
EOF1
(Micro)frame Timer
(micro)frame timer is at the EOF1 point or between EOF1
and End-of-(micro)frame
EOF2
(Micro)frame Timer
(micro)frame timer is at the EOF2 point or between EOF2
and End-of-(micro)frame
Lock
(Micro)frame Timer
(micro)frame timer is locked
Rx_Suspend
Receiver
Receiver is in the Suspend state
Rx_Resume
Receiver
Receiver is in the Resume state
SOP_FD
Internal
(SOHP)SOP received from downstream facing port or Hub
Controller. Generated (after SOHP identified) on the
transition from the Idle to K state on a port.
SOP_FU
Internal
(SOHP)SOP received from upstream facing port.
Generated (after SOHP identified) on the transition from the
Idle to K state on the upstream facing port.
11.7.3 Wait for Start of Packet from Upstream Port (WFSOPFU)
This state is entered in either of the following situations:
•
From any other state when the upstream Receiver is in the Bus_Reset state
•
From the WFSOP state if the (micro)frame timer is at or has passed the EOF1 point
•
From the WFEOP state at the EOF2 point
•
From the WFEOPFU if the (micro)frame timer is not synchronized (locked) when an (HEOP)EOP is
received on the upstream facing port
In this state, the hub is waiting for an (SOHP)SOP on the upstream facing port, and transitions on
downstream facing ports are ignored by the Hub Repeater. While the Hub Repeater is in this state,
connectivity is not established.
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This state is used during the End-of-(micro)frame (past the EOF1 point) to ensure that the hub will be able
to receive the SOF when it is sent by the host.
11.7.4 Wait for End of Packet from Upstream Port (WFEOPFU)
The hub enters this state if the hub is in the WFSOP or WFSOPFU state and an (SOHP)SOP is detected on
the upstream facing port. The hub also enters this state from the WFSOP, WFSOPFU, or WFEOP states
when the Receiver enters the Resume state.
While in this state, connectivity is established from the upstream facing port to all enabled downstream
facing ports. Downstream facing ports that are in the Enabled state are placed in the Transmit state on the
transition to this state.
11.7.5 Wait for Start of Packet (WFSOP)
This state is entered in any of the following situations:
•
From the WFEOP state when an (HEOP)EOP is detected from the downstream facing port
•
From the WFEOPFU state if the (micro)frame timer is synchronized (locked) when an (HEOP)EOP is
received from upstream
•
From the WFSOPFU or WFEOPFU states when the upstream Receiver transitions to the Suspend state
A hub in this state is waiting for an (SOHP)SOP on the upstream facing port or any downstream facing port
that is in the Enabled state. While the Hub Repeater is in this state, connectivity is not established.
11.7.6 Wait for End of Packet (WFEOP)
This state is entered from the WFSOP state when an (SOHP)SOP is received from a downstream facing
port in the Enabled state.
In this state, the hub has connectivity established in the upstream direction and the signaling received on an
enabled downstream facing port is repeated and driven on the upstream facing port. The upstream
Transmitter is placed in the Active state on the transition to this state.
If the Hub Repeater is in this state when the EOF2 point is reached, the downstream facing port for which
connectivity is established is disabled as a babble port.
Note: The full-speed Transmitter will send an EOP at EOF1, but the Repeater stays in this state until the
device sends an (HEOP)EOP or the EOF2 point is reached.
11.8 Bus State Evaluation
A hub is required to evaluate the state of the connection on a port in order to make appropriate port state
transitions. This section describes the appropriate times and means for several of these evaluations.
11.8.1 Port Error
A Port Error can occur on a downstream facing port that is in the Enabled state. A Port Error condition
exists when:
330
•
The hub is in the WFEOP state with connectivity established upstream from the port when the
(micro)frame timer reaches the EOF2 point.
•
At the EOF2 point, the Hub Repeater is in the WFSOPFU state, and there is other than Idle state on the
port.
Universal Serial Bus Specification Revision 2.0
If upstream-directed connectivity is established when the (micro)frame timer reaches the EOF1 point, the
upstream Transmitter will (return to Inactive state) generate a full-speed EOP to prevent the hub from being
disabled by the upstream hub. The connected port is then disabled if it has not ended the packet and
returned to the Idle state before the (micro)frame timer reaches the EOF2 point.
11.8.2 Speed Detection
At the end of reset, the bus is in the Idle state for the speed recorded in the port status register. Speed
detection is described in Section 7.1.7.5.
If the device connected at the downstream facing port is high-speed, the repeater (rather than the
Transaction Translator) is used to signal between this port and the upstream facing port.
Due to connect and start-up transients, the hub may not be able to reliably determine the speed of the device
until the transients have ended. The USB System Software is required to "debounce" the connection and
provide a delay between the time a connection is detected and the device is used (see Section 7.1.7.3). At
the end of the debounce interval, the device is expected to have placed its upstream facing port in the Idle
state and be able to react to reset signaling. The USB System Software must send a
SetPortFeature(PORT_RESET) request to the port to enable the port and make the attached device ready for
use.
The downstream facing port monitors the state of the D+ and D- lines to determine if the connected device
is low-speed. If so, the PORT_LOW_SPEED status bit is set to one to indicate a low-speed device. If not,
the PORT_LOW_SPEED status bit is set to zero to indicate a full-/high-speed device. Upon exit from the
reset process, the hub must set the PORT_HIGH_SPEED status bit according to the detected speed. The
downstream facing port performs the required reset processing as defined in Section 7.1.7.5. At the end of
the Resetting state, the hub will return the bus to the Idle state that is appropriate for the speed of the
attached device and transition to the Enabled state.
11.8.3 Collision
If the Hub Repeater is in the WFEOP state and an (SOHP)SOP is detected on another enabled port, a
Collision condition exists. There are two allowed behaviors for the hub in this instance. In either case,
connectivity teardown at EOF1 and babble detection at EOF2 is required.
The first, and preferred, behavior is to ‘garble’ the message so that the host can detect the problem. The hub
garbles the message by transmitting a (‘J’ or) 'K' on the upstream facing port. This (‘J’ or) 'K' should persist
until packet traffic from all downstream facing ports ends. The hub should use the last (‘J’ or ‘K’) EOP to
terminate the garbled packet. Babble detection is enabled during this garbled message.
A second behavior is to block the second packet and, when the first message ends, return the hub to the
WFSOPFU or WFSOP state as appropriate. If the second stream is still active, the hub may reestablish
connectivity upstream. This method is not preferred, as it does not convey the problem to the host.
Additionally, if the second stream causes the hub to reestablish upstream connectivity as the host is trying to
establish downstream connectivity, additional packets can be lost and the host cannot properly associate the
problem.
Note: In high-speed repeaters, use of the SOHP to detect collisions would need replication of the datapath
shown in Figure 11-14 at every port. The unsquelch signal at a port can be used instead of the SOHP to
detect collisions; in this case, the second behavior (blocking) described above must be used.
11.8.4 Low-speed Port Behavior
When a hub is configured for full-/low-speed operation, low-speed data is sent or received through the hub’s
upstream facing port at full-speed signaling even though the bit times are low-speed.
Full-speed signaling must not be transmitted to low-speed ports.
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If a port is detected to be attached to a low-speed device, the hub port’s output buffers are configured to
operate at the slow slew rate (75-300 ns), and the port will not propagate downstream-directed packets
unless they are prefaced with a PRE PID. When a PRE PID is received, the ‘J’ state must be driven on
enabled low-speed ports within four bit times of receiving the last bit of the PRE PID.
Low-speed data follows the PID and is propagated to both low- and full-speed devices. Hubs continue to
propagate downstream signaling to all enabled ports until a downstream EOP is detected, at which time all
output drivers are turned off.
Full-speed devices will not misinterpret low-speed traffic because no low-speed data pattern can generate a
valid full-speed PID.
When a low-speed device transmits, it does not preface its data packet with a PRE PID. Hubs will
propagate upstream-directed packets of full-/low-speed using full-speed signaling polarity and edge rates.
For both upstream and downstream low-speed data, the hub is responsible for inverting the polarity of the
data before transmitting to/from a low-speed port.
Although a low-speed device will send a low-speed EOP to properly terminate a packet, a hub may truncate
a low-speed packet at the EOF1 point with a full-speed EOP. Thus, hubs must always be able to tear down
connectivity in response to a full-speed EOP regardless of the data rate of the packet.
Because of the slow transitions on low-speed ports, when the D+ and D- signal lines are switching between
the 'J' and 'K', they may both be below 2.0 V for a period of time that is longer than a full-speed bit time. A
hub must ensure that these slow transitions do not result in termination of connectivity and must not result in
an SE0 being sent upstream.
11.8.4.1 Low-speed Keep-alive
All hub ports to which low-speed devices are connected must generate a low-speed keep-alive strobe,
generated at the beginning of the frame, which consists of a valid low-speed EOP (described in
Section 7.1.13.2). The strobe must be generated at least once in each frame in which an SOF is received.
This strobe is used to prevent low-speed devices from suspending if there is no other low-speed traffic on the
bus. The hub can generate the keep-alive on any valid full-speed token packet. The following rules for
generation of a low-speed keep-alive must be adhered to:
•
A keep-alive must minimally be derived from each SOF. It is recommended that a keep-alive be
generated on any valid full-speed token.
•
The keep-alive must start by the eighth bit after the PID of the full-speed token.
11.9 Suspend and Resume
Hubs must support suspend and resume both as a USB device and in terms of propagating suspend and
resume signaling. Hubs support both global and selective suspend and resume. Global and selective
suspend are defined in Section 7.1.7.6. Global suspend/resume refers to the entire bus being suspended or
resumed without affecting any hub’s downstream facing port states; selective suspend/resume refers to a
downstream facing port of a hub being suspended or resumed without affecting the hub state. Global
suspend/resume is implemented through the root port(s) at the host. Selective suspend/resume is
implemented via requests to a hub. Device-initiated resume is called remote-wakeup (see Section 7.1.7.7).
If the hub upstream facing port is in (high-speed) full-speed, the required behavior is the same as that for a
function with upstream facing port in (high-speed) full-speed and is described in Chapter 7.
When a downstream facing port operating at high-speed goes into the Suspended state, it switches to fullspeed terminations but continues to have high-speed port status. In response to a remote wakeup or
selective resume, this port will drive full-speed ‘K’ throughout its Resuming state. The requirements and
timings are the same as for full-speed ports and described below. At the end of this signaling, the bus will
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be returned to the high-speed Idle state (using the SendEOR state). After this, the port will return to the
Enabled state. The high-speed status of the port is maintained throughout the suspend-resume cycle.
Figure 11-17 and Figure 11-18 show the timing relationships for an example remote-wakeup sequence.
This example illustrates a device initiating resume signaling through a suspended hub (‘B’) to an awake hub
(‘A’). Hub ‘A’ in this example times and completes the resume sequence and is the "Controlling Hub".
The timings and events are defined in Section 7.1.7.7.
Full/low speed Bus driving
Full/low speed Bus driving –
repeat
Full/low speed Bus Idle or
driven at other end
High speed idle state
Everything
below Hub ‘A’
in Suspend
state
Hub ‘A’
(Controlling Hub)
Controlling Hub
suspended DS
Port
Controlling Hub
sends EOR ending
resume
Controlling Hub Drives Resume (DS)
20ms (nominal)
Idle (‘J’)
Resume (‘K’)
idle
Controlling Hub Reflects Resume
(DS) 900µs
Hub
Upstream
Port
Hub ‘B’ generates
EOP ending resume
Hub ‘B’
Enabled DS
Idle (‘J’)
Idle (‘J’)
Resume (‘K’)
Hub Ports
Hub ‘B’ Drives Resume (US and DS)
[e.g., 10ms]
Device
Hub Port
Device
Device
Hub ‘B’ Reflects Resume (US and DS)
900µs
Idle (‘J’)
Device
Remote
Wakeup
Idle (‘J’)
Resume (‘K’)
t0
t1
t2
t3
Device Drives Resume
[e.g., 10ms]
t4
t5
Figure 11-17. Example Remote-wakeup Resume Signaling With Full-/low-speed Device
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Full/low speed Bus driving
Full/low speed Bus driving –
repeat
Full/low speed Bus Idle or
driven at other end
High speed idle state
Everything
below Hub ‘A’
in Suspend
state
Hub ‘A’
(Controlling Hub)
Controlling Hub
suspended DS
Port
Controlling Hub
sends EOR ending
resume
Controlling Hub Drives Resume (DS)
20ms (nominal)
Idle (‘J’)
Resume (‘K’)
idle
Controlling Hub Reflects Resume
(DS) 900µs
Hub
Upstream
Port
Hub ‘B’
Enabled DS
Idle (‘J’)
Resume (‘K’)
idle
Hub Ports
Hub ‘B’ Drives Resume (US and DS)
[e.g., 10ms]
Device
Hub Port
Device
Device
Hub ‘B’ Reflects Resume (US and DS)
900µs
Idle (‘J’)
Device
Remote
Wakeup
Resume (‘K’)
t0
t1
t2
idle
t3
Device Drives Resume
[e.g., 10ms]
t4
t5
Figure 11-18. Example Remote-wakeup Resume Signaling With High-speed Device
Here is an explanation of what happens at each tn:
t0 Suspended device initiates remote-wakeup by driving a ’K’ on the data lines.
t1 Suspended hub ‘B’ detects the ‘K’ on its downstream facing port and wakes up enough within 900 µs
to filter and then reflect the resume upstream and down through all enabled ports.
t2 Hub ‘A’ is not suspended (implication is that the port at which ‘B’ is attached is selectively
suspended), detects the ‘K’ on the selectively suspended port where ‘B’ is attached, and filters and
then reflects the resume signal back to ‘B’ within 900 µs.
t3 Device ceases driving ‘K’ upstream.
t4 Hub ‘B’ ceases driving ‘K’ upstream and down all enabled ports and begins repeating upstream
signaling to all enabled downstream facing ports.
t5 Hub ‘A’ completes resume sequence, after appropriate timing interval, by driving a speed-appropriate
end of resume downstream. (End of resume will be an Idle state for a high-speed device or a lowspeed EOP for a full-/low-speed device.)
The hub reflection time is much smaller than the minimum duration a USB device will drive resume
upstream. This relationship guarantees that resume will be propagated upstream and downstream without
any gaps.
11.10 Hub Reset Behavior
Reset signaling to a hub is defined only in the downstream direction, which is at the hub's upstream facing
port. Reset signaling required of the hub is described in Section 7.1.7.5.
A suspended hub must interpret the start of reset as a wakeup event; it must be awake and have completed
its reset sequence by the end of reset signaling.
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After completion of the reset sequence, a hub is in the following state:
•
Hub Controller default address is 0.
•
Hub status change bits are set to zero.
•
Hub Repeater is in the WFSOPFU state.
•
Transmitter is in the Inactive state.
•
Downstream facing ports are in the Not Configured state and SE0 driven on all downstream facing
ports.
11.11 Hub Port Power Control
Self-powered hubs may have power switches that control delivery of power downstream facing ports but it
is not required. Bus-powered hubs are required to have power switches. A hub with power switches can
switch power to all ports as a group/gang, to each port individually, or have an arbitrary number of gangs of
one or more ports.
A hub indicates whether or not it supports power switching by the setting of the Logical Power Switching
Mode field in wHubCharacteristics. If a hub supports per-port power switching, then the power to a port is
turned on when a SetPortFeature(PORT_POWER) request is received for the port. Port power is turned off
when the port is in the Powered-off or Not Configured states. If a hub supports ganged power switching,
then the power to all ports in a gang is turned on when any port in a gang receives a
SetPortFeature(PORT_POWER) request. The power to a gang is not turned off unless all ports in a gang
are in the Powered-off or Not Configured states. Note, the power to a port is not turned on by a
SetPortFeature(PORT_POWER) if both C_HUB_LOCAL_POWER and Local Power Status (in
wHubStatus) are set to 1B at the time when the request is executed and the PORT_POWER feature would
be turned on.
Although a self-powered hub is not required to implement power switching, the hub must support the
Powered-off state for all ports. Additionally, the hub must implement the PortPwrCtrlMask (all bits set to
1B) even though the hub has no power switches that can be controlled by the USB System Software.
Note: To ensure compatibility with previous versions of USB Software, hubs must implement the Logical
Power Switching Mode field in wHubCharacteristics. This is because some versions of SW will not use the
SetPortFeature() request if the hub indicates in wHubCharacteristics that the port does not support port
power switching. Otherwise, the Logical Power Switching Mode field in wHubCharacteristics would have
become redundant as of this version of the specification.
The setting of the Logical Power Switching Mode for hubs with no power switches should reflect the
manner in which over-current is reported. For example, if the hub reports over-current conditions on a perport basis, then the Logical Power Switching Mode should be set to indicate that power switching is
controlled on a per-port basis.
For a hub with no power switches, bPwrOn2PwrGood must be set to zero.
11.11.1 Multiple Gangs
A hub may implement any number of power and/or over-current gangs. A hub that implements more than
one over-current and/or power switching gang must set both the Logical Power Switching Mode and the
Over-current Reporting Mode to indicate that power switching and over-current reporting are on a per port
basis (these fields are in wHubCharacteristics). Also, all bits in PortPwrCtrlMask must be set to 1B.
When an over-current condition occurs on an over-current protection device, the over-current is signaled on
all ports that are protected by that device. When the over-current is signaled, all the ports in the group are
placed in the Powered-off state, and the C_PORT_OVER-CURRENT field is set to 1B on all the ports.
When port status is read from any port in the group, the PORT_OVER-CURRENT field will be set to 1B as
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long as the over-current condition exists. The C_PORT_OVER-CURRENT field must be cleared in each
port individually.
When multiple ports share a power switch, setting PORT_POWER on any port in the group will cause the
power to all ports in the group to turn on. It will not, however, cause the other ports in that group to leave
the Powered-off state. When all the ports in a group are in the Powered-off state or the hub is not
configured, the power to the ports is turned off.
If a hub implements both power switching and over-current, it is not necessary for the over-current groups
to be the same as the power switching groups.
If an over-current condition occurs and power switches are present, then all power switches associated with
an over-current protection circuit must be turned off. If multiple over-current protection devices are
associated with a single power switch then that switch will be turned off when any of the over-current
protection circuits indicates an over-current condition.
11.12 Hub Controller
The Hub Controller is logically organized as shown in Figure 11-19.
UPSTREAM CONNECTION
Status Change
Endpoint
ENDPOINT 0:
Configuration
Information
Port N
Port 1
Port 2
Port 3
Figure 11-19. Example Hub Controller Organization
11.12.1 Endpoint Organization
The Hub Class defines one additional endpoint beyond Default Control Pipe, which is required for all hubs:
the Status Change endpoint. The host system receives port and hub status change notifications through the
Status Change endpoint. The Status Change endpoint is an interrupt endpoint. If no hub or port status
change bits are set, then the hub returns an NAK when the Status Change endpoint is polled. When a status
change bit is set, the hub responds with data, as shown in Section 11.12.4, indicating the entity (hub or port)
with a change bit set. The USB System Software can use this data to determine which status registers to
access in order to determine the exact cause of the status change interrupt.
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11.12.2 Hub Information Architecture and Operation
Figure 11-20 shows how status, status change, and control information relate to device states. Hub
descriptors and Hub/Port Status and Control are accessible through the Default Control Pipe. The Hub
descriptors may be read at any time. When a hub detects a change on a port or when the hub changes its
own state, the Status Change endpoint transfers data to the host in the form specified in Section 11.12.4.
Status Information
(static)
s
tu
ta e s
l S ng
Al h a
C
Host Software (e.g., Hub
Driver)
Hub or port status change bits can be set because of hardware or Software events. When set, these bits
remain set until cleared directly by the USB System Software through a ClearPortFeature() request or by a
hub reset. While a change bit is set, the hub continues to report a status change when polled until all change
bits have been cleared by the USB System Software.
Change Information
(due to hardware
events)
Ha r
d wa
re E
vent
s
Hardware Events
Change Device
State
Device Control
Control Information
(change device state)
Software Device
Control
Figure 11-20. Relationship of Status, Status Change, and Control Information to Device States
The USB System Software uses the interrupt pipe associated with the Status Change endpoint to detect
changes in hub and port status.
11.12.3 Port Change Information Processing
Hubs report a port’s status through port commands on a per-port basis. The USB System Software
acknowledges a port change by clearing the change state corresponding to the status change reported by the
hub. The acknowledgment clears the change state for that port so future data transfers to the Status Change
endpoint do not report the previous event. This allows the process to repeat for further changes (see
Figure 11-21).
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Begin
System Software requests Interrupt Pipe notification for Status Change Information
Hub NAKs
status change
IN token
No
Change Data
Available ?
Yes
Interrupt Pipe returns Hub and Port Status Change Bitmap
Interrupt Pipe notification retired
System Software reads Hub or Port status (for affected ports)
Yes
Any Changed
State?
• Accumulate change information
• System Software clears
corresponding change state
No
System Software processes accumulated change information
Re-initialize Interrupt Pipe for Status Change endpoint
Return to
beginning
Figure 11-21. Port Status Handling Method
11.12.4 Hub and Port Status Change Bitmap
The Hub and Port Status Change Bitmap, shown in Figure 11-22, indicates whether the hub or a port has
experienced a status change. This bitmap also indicates which port(s) has had a change in status. The hub
returns this value on the Status Change endpoint. Hubs report this value in byte-increments. That is, if a
hub has six ports, it returns a byte quantity, and reports a zero in the invalid port number field locations.
The USB System Software is aware of the number of ports on a hub (this is reported in the hub descriptor)
and decodes the Hub and Port Status Change Bitmap accordingly. The hub reports any changes in hub
status in bit zero of the Hub and Port Status Change Bitmap.
The Hub and Port Status Change Bitmap size varies from a minimum size of one byte. Hubs report only as
many bits as there are ports on the hub, subject to the byte-granularity requirement (i.e., round up to the
nearest byte).
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N
2
1
0
Port N change detected
Port 2 change detected
Port 1 change detected
Hub change detected
Figure 11-22. Hub and Port Status Change Bitmap
Any time the Status Change endpoint is polled by the host controller and any of the Status Changed bits are
non-zero, the Hub and Port Status Change Bitmap is returned. Figure 11-23 shows an example creation
mechanism for hub and port change bits.
Per-Port Logic
Change
Detect Logic
e
pl
am
Ex
Port N
Logical OR
Change
Information
Hub and Port Status Change Bitmap
N
Figure 11-23. Example Hub and Port Change Bit Sampling
11.12.5 Over-current Reporting and Recovery
USB devices must be designed to meet applicable safety standards. Usually, this will mean that a selfpowered hub implement current limiting on its downstream facing ports. If an over-current condition
occurs, it causes a status and state change in one or more ports. This change is reported to the USB System
Software so that it can take corrective action.
A hub may be designed to report over-current as either a port or a hub event. The hub descriptor field
wHubCharacteristics is used to indicate the reporting capabilities of a particular hub (see Section 11.23.2).
The over-current status bit in the hub or port status field indicates the state of the over-current detection
when the status is returned. The over-current status change bit in the Hub or Port Change field indicates if
the over-current status has changed.
When a hub experiences an over-current condition, it must place all affected ports in the Powered-off state.
If a hub has per-port power switching and per-port current limiting, an over-current on one port may still
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cause the power on another port to fall below specified minimums. In this case, the affected port is placed
in the Powered-off state and C_PORT_OVER_CURRENT is set for the port, but
PORT_OVER_CURRENT is not set. If the hub has over-current detection on a hub basis, then an overcurrent condition on the hub will cause all ports to enter the Powered-off state. However, in this case,
neither C_PORT_OVER_CURRENT nor PORT_OVER_CURRENT is set for the affected ports.
Host recovery actions for an over-current event should include the following:
1.
Host gets change notification from hub with over-current event.
2.
Host extracts appropriate hub or port change information (depending on the information in the
change bitmap).
3.
Host waits for over-current status bit to be cleared to 0.
4.
Host cycles power on to all of the necessary ports (e.g., issues a SetPortFeature(PORT_POWER)
request for each port).
5.
Host re-enumerates all affected ports.
11.12.6 Enumeration Handling
The hub device class commands are used to manipulate its downstream facing port state. When a device is
attached, the device attach event is detected by the hub and reported on the status change interrupt. The host
will accept the status change report and request a SetPortFeature(PORT_RESET) on the port. As part of the
bus reset sequence, a speed detect is performed by the hub’s port hardware.
The Get_Status(PORT) request invoked by the host will return a “not PORT_LOW_SPEED and
PORT_HIGH_SPEED” indication for a downstream facing port operating at high-speed. The
Get_Status(PORT) will report “PORT_LOW_SPEED” for a downstream facing port operating at lowspeed. The Get_Status(PORT) will report “not PORT_LOW_SPEED and not PORT_HIGH_SPEED” for a
downstream facing port operating at full-speed.
When the device is detached from the port, the port reports the status change through the status change
endpoint and the port will be reconnected to the high-speed repeater. Then the process is ready to be
repeated on the next device attach detect.
11.13 Hub Configuration
Hubs are configured through the standard USB device configuration commands. A hub that is not
configured behaves like any other device that is not configured with respect to power requirements and
addressing. If a hub implements power switching, no power is provided to the downstream facing ports
while the hub is not configured. Configuring a hub enables the Status Change endpoint. The USB System
Software may then issue commands to the hub to switch port power on and off at appropriate times.
The USB System Software examines hub descriptor information to determine the hub’s characteristics. By
examining the hub’s characteristics, the USB System Software ensures that illegal power topologies are not
allowed by not powering on the hub’s ports if doing so would violate the USB power topology. The device
status and configuration information can be used to determine whether the hub should be used as a bus or
self-powered device. Table 11-12 summarizes the information and how it can be used to determine the
current power requirements of the hub.
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Table 11-12. Hub Power Operating Mode Summary
Configuration Descriptor
MaxPower
bmAttributes
(Self Powered)
Hub
Device Status
(Self Power)
0
0
N/A
0
1
0
N/A
A device which is only self-powered, but does
not have local power cannot connect to the bus
and communicate.
0
1
1
Self-powered only hub and local power supply is
good. Hub status also indicates local power
good, see Section 11.16.2.5. Hub functionality is
valid anywhere depth restriction is not violated.
>0
0
N/A
Bus-powered only hub. Downstream facing
ports may not be powered unless allowed in
current topology. Hub device status reporting
Self Powered is meaningless in combination of a
zeroed bmAttributes.Self-Powered.
>0
1
0
This hub is capable of both self- and buspowered operating modes. It is currently only
available as a bus-powered hub.
>0
1
1
This hub is capable of both self- and buspowered operating modes. It is currently
available as a self-powered hub.
Explanation
N/A
This is an illegal set of information.
A self-powered hub has a local power supply, but may optionally draw one unit load from its upstream
connection. This allows the interface to function when local power is not available (see Section 7.2.1.2).
When local power is removed (either a hub-wide over-current condition or local supply is off), a hub of this
type remains in the Configured state but transitions all ports (whether removable or non-removable) to the
Powered-off state. While local power is off, all port status and change information read as zero and all
SetPortFeature() requests are ignored (request is treated as a no-operation). The hub will use the Status
Change endpoint to notify the USB System Software of the hub event (see Section 11.24.2.6 for details on
hub status).
The MaxPower field in the configuration descriptor is used to report to the system the maximum power the
hub will draw from VBUS when the configuration is selected. For bus-powered hubs, the reported value
must not include the power for any of external downstream facing ports. The external devices attaching to
the hub will report their individual power requirements.
A compound device may power both the hub electronics and the permanently attached devices from VBUS.
The entire load may be reported in the hubs’ configuration descriptor with the permanently attached devices
each reporting self-powered, with zero MaxPower in their respective configuration descriptors.
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11.14 Transaction Translator
A hub has a special responsibility when it is operating in high-speed and has full-/low-speed devices
connected on downstream facing ports. In this case, the hub must isolate the high-speed signaling
environment from the full-/low-speed signaling environment. This function is performed by the Transaction
Translator (TT) portion of the hub.
This section defines the required behavior of the transaction translator.
11.14.1 Overview
Figure 11-24 shows an overview of the Transaction Translator. The TT is responsible for participating in
high-speed split transactions on the high-speed bus via its upstream facing port and issuing corresponding
full-/low-speed transactions on its downstream facing ports that are operating at full-/low-speed. The TT
acts as a high-speed function on the high-speed bus and performs the role of a host controller for its
downstream facing ports that are operating at full-/low-speed. The TT includes a high-speed handler to deal
with high-speed transactions. The TT also includes a full-/low-speed handler that performs the role of a
host controller on the downstream facing ports that are operating at full-/low-speed.
High Speed Bus
High-Speed Handler
Isoch/Int Isoch/Int
B/C B/C
Start-split Comp.-split In/Out In/Out
...
Full/Low-Speed Handler
Full/Low Speed Bus
Figure 11-24. Transaction Translator Overview
The TT has buffers (shown in gray in the figure) to hold transactions that are in progress and tracks the state
of each buffered transaction as it is processed by the TT. The buffers provide the connection between the
high-speed and full-/low-speed handlers. The state tracking the TT does for each transaction depends on the
specific USB transfer type of the transaction (i.e., bulk, control, interrupt, isochronous). The high-speed
handler accepts high-speed start-split transactions or responds to high-speed complete-split transactions.
The high-speed handler places the start-split transactions in local buffers for the full-/low-speed handler’s
use.
The buffered start-split transactions provide the full-/low-speed handler with the information that allows it
to issue corresponding full-/low-speed transactions to full-/low-speed devices attached on downstream
facing ports. The full-/low-speed handler buffers the results of these full-/low-speed transactions so that
they can be returned with a corresponding complete-split transaction on the high-speed bus.
The general conversion between full-/low-speed transactions and the corresponding high-speed split
transaction protocol is described in Section 8.4.2. More details about the specific transfer types for split
transactions are described later in this chapter.
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The high-speed handler of the TT operates independently of the full-/low-speed handler. Both handlers use
the local transaction buffers to exchange information where required.
Transaction Translator
Bulk &
Control
Interrupt &
Isochronous
Figure 11-25. Periodic and Non-periodic Buffer Sections of TT
The TT has two buffer and state tracking sections (shown in gray in Figure 11-24 and Figure 11-25):
periodic (for isochronous/interrupt full-/low-speed transactions) and non-periodic (for bulk/control full/low-speed transactions). The requirements on the TT for these two buffer and state tracking sections are
different. Each will be described in turn later in this chapter.
11.14.1.1 Data Handling Between High-speed and Full-/low-speed
The host converts transfer requests involving a full-/low-speed device into corresponding high-speed split
transactions to the TT to which the device is attached.
Low-speed Preamble(PRE) packets are never used on the high-speed bus to indicate a low-speed
transaction. Instead, a low-speed transaction is encoded in the split transaction token.
The host can have a single schedule of the transactions that need to be issued to devices. This single
schedule can be used to hold both high-speed transactions and high-speed split transactions used for
communicating with full-/low-speed devices.
11.14.1.2 Host Controller and TT Split Transactions
The host controller uses the split transaction protocol for initiating full-/low-speed transactions via the TT
and then determining the completion status of the full-/low-speed transaction. This approach allows the
host controller to start a full-/low-speed transaction and then continue with other high-speed transactions
while avoiding having to wait for the slower transaction to proceed/complete at its speed. A high-speed
split transaction has two parts: a start-split and a complete-split. Split transactions are only used between
the host controller and a hub. No other high-/full-/low-speed devices ever participate in split transactions.
When the host controller sends a start-split transaction at high-speed, the split transaction is addressed to the
TT for that device. That TT will accept the transaction and buffer it locally. The high-speed handler
responds with an appropriate handshake to inform the host controller that the transaction has been accepted.
Not all split transactions have a handshake phase to the start-split. The start-split transactions are kept
temporarily in a TT transaction buffer.
The full-/low-speed handler processes start-split periodic transactions stored in the periodic transaction
buffer (in order) as the downstream full-/low-speed bus is ready for the “next” transaction. The full-/lowspeed handler accepts any result information from the downstream bus (in response to the full-/low-speed
transaction) and accumulates it in a local buffer for later transmission to the host controller.
At an appropriate future time, the host controller sends a high-speed complete-split transaction to retrieve
the status/data/result for appropriate full-/low-speed transactions. The high-speed handler checks this highspeed complete-split transaction with the response at the head of the appropriate local transaction buffer and
responds accordingly. The specific split transaction sequences are defined for each USB transfer type in
later sections.
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11.14.1.3 Multiple Transaction Translators
A hub has two choices for organizing transaction translators (TTs). A hub can have one TT for all
downstream facing ports that have full-/low-speed devices attached or the hub can have one TT for each
downstream facing port. The hub must report its organization in the hub class descriptor.
11.14.2 Transaction Translator Scheduling
As the high-speed handler accepts start-splits, the full-/low-speed transaction information and data for
OUTs or the transaction information for INs accumulate in buffers awaiting their service on the downstream
bus. The host manages the periodic TT transaction buffers differently than the non-periodic transaction
buffers.
11.14.2.1 TT Isochronous/Interrupt (Periodic) Transaction Buffering
Periodic transactions have strict timing requirements to meet on a full-/low-speed bus (as defined by the
specific endpoint and transfer type). Therefore, transactions must move across the high-speed bus, through
the TT, across the full-/low-speed bus, back through the TT, and onto the high-speed bus in a timely
fashion. An overview of the microframe pipeline of buffering in the TT is shown in Figure 11-26. A
transaction begins as a start-split on the high-speed bus, is accepted by the high-speed handler, and is stored
in the start-split transaction buffer. The full-/low-speed handler uses the next start-split transaction at the
head of the start-split transaction buffer when it is time to issue the next periodic full-/low-speed transaction
on the downstream bus. The results of the transaction are accumulated in the complete-split transaction
buffer. The TT responds to a complete-split from the host and extracts the appropriate response from the
complete-split transaction buffer. This completes the flow for a periodic transaction through the TT. This
is called the periodic transaction pipeline.
High Speed Start-Split
TT
High Speed Complete-Split
Start
Handler
Complete
Handler
Start-split Complete-split
FIFO
FIFO
Full/Low
Handler
Figure 11-26. TT Microframe Pipeline for Periodic Split Transactions
The TT implements a traditional pipeline of transactions with its periodic transaction buffers. There is
separate buffer space for start-splits and complete-splits. The host is responsible for filling the start-split
transaction buffer and draining the complete-split transaction buffer. The host software manages the host
controller to cause high-speed split transactions at the correct times to avoid over/under runs in the TT
periodic transaction buffers. The host controller sends data “just in time” for full-/low-speed OUTs and
retrieves response data from full-/low-speed INs to ensure that the periodic transaction buffer space required
in the TT is the minimum possible. See Section 11.18 for more detailed information.
USB strictly defines the timing requirements of periodic transactions and the isochronous transport
capabilities of the high-speed and full-/low-speed buses. This allows the host to accurately predict when
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data for periodic transactions must be moved on both the full-/low-speed and high-speed buses, whenever a
client requests a data transfer with a full-/low-speed periodic endpoint. Therefore, the host can “pipeline”
data to/from the TT so that it moves in a timely manner with its target endpoint. Once the configuration of
a full-/low-speed device with periodic endpoints is set, the host streams data to/from the TT to keep the
device’s endpoints operating normally.
11.14.2.2 TT Bulk/Control (Non-Periodic) Transaction Buffering
Non-periodic transactions have no timing requirements, but the TT supports the maximum full-/low-speed
throughput allowed. A TT provides a few transaction buffers for bulk/control full-/low-speed transactions.
The host and TT use simple flow control (NAK) mechanisms to manage the bulk/control non-periodic
transaction buffers. The host issues a start-split transaction, and if there is available buffer space, the TT
accepts the transaction. The full-/low-speed handler uses the buffered information to issue the downstream
full-/low-speed transaction and then uses the same buffer to hold any results (e.g., handshake or data or
timeout). The buffer is then emptied with a corresponding high-speed complete-split and the process
continues. Figure 11-27 shows an example overview of a TT that has two bulk/control buffers.
High Speed Start-/Complete-Split
TT
Bulk/Ctrl #1
Bulk/Ctrl #2
Full/Low Speed Transaction
Figure 11-27. TT Nonperiodic Buffering
11.14.2.3 Full-/low-speed Handler Transaction Scheduling
The full-/low-speed handler uses a simple, scheduled priority scheme to service pending transactions on the
downstream bus. Whenever the full-/low-speed handler finishes a transaction on the downstream bus, it
takes the next start-split transaction from the start-split periodic transaction buffer (if any). If there are no
available start-split periodic transactions in the buffer, the full-/low-speed handler may attempt a
bulk/control transaction. If there are start-split transactions pending in the bulk/control buffer(s) and there is
sufficient time left in the full-/low-speed 1 ms frame to complete the transaction, the full-/low-speed handler
issues one of the bulk/control transactions (in round robin order). Figure 11-28 shows pseudo code for the
full-/low-speed handler start-split transaction scheduling algorithm.
The TT also sequences the transaction pipeline based on the high-speed microframe timer to ensure that it
does not start full-/low-speed periodic transactions too early or too late. The “Advance_pipeline” procedure
in the pseudo code is used to keep the TT advancing the microframe “pipeline”. This procedure is described
in more detail later in Figure 11-67.
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While (1) loop
While (not end of microframe) loop
-- process next start-split transaction
If available periodic start-split transaction then
Process next full-/low-speed periodic transaction
Else if (available bulk/control transaction) and
(fits in full-/low-speed 1 ms frame) then
Process one transaction
End if
End loop
Advance_Pipeline();
End loop
-- see description in Figure 11-67(below)
Figure 11-28. Example Full-/low-speed Handler Scheduling for Start-splits
As described earlier in this chapter, the TT derives the downstream bus’s 1 ms SOF timer from the highspeed 125 µs microframe. This means that the host and the TT have the same 1 ms frame time for all TTs.
Given the strict relationship between frames and the zeroth microframe, there is no need to have any
explicit timing information carried in the periodic split transactions sent to the TT. See Section 11.18 for
more information.
11.15 Split Transaction Notation Information
The following sections describe the details of the transaction phases and flow sequences of split transactions
for the different USB transfer types: bulk/control, interrupt, and isochronous. Each description also shows
detailed example host and TT state machines to achieve the required transaction definitions. The diagrams
should not be taken as a required implementation, but to specify the required behavior. Appendix A
includes example high-speed and full-speed transaction sequences with different results to clarify the
relationships between the host controller, the TT, and a full-speed endpoint.
Low-speed is not discussed in detail since beyond the handling of the PRE packet (which is defined in
Chapter 8), there are no packet sequencing differences between low- and full-speed.
For each data transfer direction, reference figures also show the possible flow sequences for the start-split
and the complete-split portion of each split transaction transfer type.
The transitions on the flow sequence figures have labels that correspond to the transitions in the host and TT
state machines. These labels are also included in the examples in Appendix A. The three character labels
are of the form: < S | C >< T | D | H | E ><number>. S indicates that this is a start-split label. C indicates
that this is a complete-split label. T indicates token phase; D indicates data phase; H indicates handshake
phase; E indicates an error case. The number simply distinguishes different labels of the same case/phase in
the same split transaction part.
The flow sequence figures further identify the visibility of transitions according to the legend in
Figure 11-29. The flow sequences also include some indication of states required in the host or TT or
actions taken. The legend shown in Figure 11-29 indicates how these are identified.
Bold indicates host action
Italics indicate <hub status> or <hub action>
Both visible
Hub visible
Host visible
Figure 11-29. Flow Sequence Legend
Figure 11-30 shows the legend for the state machine diagrams. A circle with a three line border indicates a
reference to another (hierarchical) state machine. A circle with a two line border indicates an initial state.
A circle with a single line border is a simple state.
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A diamond (joint) is used to join several transitions to a common point. A joint allows a single input
transition with multiple output transitions or multiple input transitions and a single output transition. All
conditions on the transitions of a path involving a joint must be true for the path to be taken. A path is
simply a sequence of transitions involving one or more joints.
A transition is labeled with a block with a line in the middle separating the (upper) condition and the (lower)
actions. The condition is required to be true to take the transition. The actions are performed if the
transition is taken. The syntax for actions and conditions is VHDL. A circle includes a name in bold and
optionally one or more actions that are performed upon entry to the state.
State
Hierarchy
- Contains other state machines
Initial
State
- Initial state of a state machine
State
- State in a state machine
- Entry and exit of state machine
&
Condition
Actions
- Joint used to connect transitions
- Transition: taken when condition
is true and performs actions
Figure 11-30. Legend for State Machines
The descriptions of the split transactions for the four transfer types refer to the status of the full-/low-speed
transaction on the bus downstream of the TT. This status is used by the high-speed handler to determine its
response to a complete-split transaction. The status is only visible within a TT implementation and is used
in the specification purely for ease of explanation. The defined status values are:
•
Ready – The transaction has completed on the downstream facing full-/low-speed bus with the result
as follows:
•
Ready/NAK – A NAK handshake was received.
•
Ready /trans_err – The full-/low-speed transaction experienced a error in the transaction.
Possible errors are: PID to PID_invert bits check failure, CRC5 check failure, incorrect PID,
timeout, CRC16 check failure, incorrect packet length, bitstuffing error, false EOP.
•
Ready /ACK – An ACK handshake was received.
•
Ready /Stall – A STALL handshake was received.
•
Ready /Data – A data packet was received and the CRC check passed. (bulk/control IN).
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•
Ready /lastdata – A data packet was finished being received. (isochronous/interrupt IN).
•
Ready /moredata – A data packet was being received when the microframe timer occurred
(isochronous/interrupt IN).
•
Old – A complete-split has been received by the high-speed handler for a transaction that previously
had a “ready” status. The possible status results are the same as for the Ready status. This is the
initial state for a buffer before it has been used for a transaction.
•
Pending – The transaction is waiting to be completed on the downstream facing full-/low-speed bus.
The figures use “old/x” and “ready/x” to indicate any of the old or ready status respectively.
The split transaction state machines in the remainder of this chapter are presented in the context of
Figure 11-31. The host controller state machines are located in the host controller. The host controller
causes packets to be issued downstream (labeled as HSD1) and it receives upstream packets (labeled as
HSU2).
The transaction translator state machines are located in the TT. The TT causes packets to be issued
upstream (labeled as HSU1) and it receives downstream packets (labeled as HSD2).
The host controller has commands that tell it what split transaction to issue next for an endpoint. The host
controller tracks transactions for several endpoints. The TT has state in buffers that track transactions for
several endpoints.
Appendix B includes some declarations that were used in constructing the state machines and may be useful
in understanding additional details of the state machines. There are several pseudo-code procedures and
functions for conditions and actions. Simple descriptions of them are also included in Appendix B.
Transaction
Commands
Transaction
Results
HC_cmd
HC_resp
Host
Controller
Host state machines
HSD1
Downstream
High_speed Bus
HSU2
Upstream
High_speed Bus
HSD2
HSU1
TT state machines
BC
Bulk/Ctrl Buffers
SS
CS
Hub
Transaction
Translator
Periodic Pipeline Buffers
Figure 11-31. State Machine Context Overview
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11.16 Common Split Transaction State Machines
There are several state machines common to all the specific split transaction types. These state machines
are used in the host controller and transaction translator to determine the specific split transaction type (e.g.,
interrupt OUT start-split vs. bulk IN complete-split). An overview of the host controller state machine
hierarchy is shown in Figure 11-32. The overview of the transaction translator state machine hierarchy is
shown in Figure 11-33. Each of the labeled boxes in the figures show an individual state machine. Boxes
contained in another box indicate a state machine contained within another state machine. All the state
machines except the lowest level ones are shown in the remaining figures in this section. The lowest level
state machines are shown in later sections describing the specific split transaction type.
HC_Do_start
HC_Do_complete
HC_Do_IsochISS
HC_Do_IsochICS
HC_Do_IntISS
HC_Do_IntICS
HC_Do_BISS
HC_Data_or_timeout
HC_Do_IsochOSS
HC_Do_BICS
HC_Do_IntOSS
HC_Do_IntOCS
HC_Do_BOSS
HC_Do_BOCS
Figure 11-32. Host Controller Split Transaction State Machine Hierarchy Overview
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TT_Process_packet
TT_Do_start
TT_Do_complete
TT_IsochICS
TT_IsochSS
TT_Do_IsochOSS
TT_Do_IsochISS
TT_IntSS
TT_IntCS
TT_Do_IntOSS
TT_Do_IntOCS
TT_Do_IntISS
TT_Do_IntICS
TT_BulkSS
TT_BulkCS
TT_Do_BOSS
TT_Do_BOCS
TT_Do_BISS
TT_Do_BICS
Figure 11-33. Transaction Translator State Machine Hierarchy Overview
11.16.1 Host Controller State Machine
Architecture Declarations
Package List
ieee
std_logic_1164
ieee
numeric_std
usb2statemachines behav_package
ieee
std_logic_arith
HC_Command_ready
Concurrent Statements
HC_wait_for_command
HC_Get_next_command;
Figure 11-34. Host Controller
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11.16.1.1 HC_Process_command State Machine
HC_cmd.cmd = SOF
Issue_packet(HSD1, SOF);
HC_Do_start
Update_Command(HC_done);
HC_cmd.cmd = start_split
HC_cmd.cmd = complete_split
&
HC_Do_complete
HC_cmd.cmd = nonsplit
HC_Do_nonsplit
HC_Process_command
Figure 11-35. HC_Process_Command
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11.16.1.1.1 HC_Do_start State Machine
HC_Do_IsochISS
HC_cmd.ep_type = isochronous
&
HC_cmd.ep_type = interrupt
HC_Do_IntISS
HC_cmd.ep_type = bulk or
HC_cmd.ep_type = control
HC_cmd.direction = in_dir
HC_Do_BISS
HC_Do_IsochOSS
HC_cmd.direction = out_dir
HC_cmd.ep_type = isochronous
&
HC_cmd.ep_type = interrupt
HC_Do_IntOSS
HC_cmd.ep_type = bulk or
HC_cmd.ep_type = control
HC_Do_BOSS
HC_Do_Start
Figure 11-36. HC_Do_Start
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Universal Serial Bus Specification Revision 2.0
11.16.1.1.2 HC_Do_complete State Machine
HC_Do_IsochICS
HC_cmd.ep_type = isochronous
&
HC_cmd.ep_type = interrupt
HC_Do_IntICS
HC_cmd.ep_type = bulk or
HC_cmd.ep_type = control
HC_cmd.direction = in_dir
HC_Do_BICS
HC_cmd.direction = out_dir
HC_cmd.ep_type = isochronous
&
HC_cmd.ep_type = interrupt
HC_Do_IntOCS
HC_cmd.ep_type = bulk or
HC_cmd.ep_type = control
HC_Do_BOCS
HC_Do_complete
Figure 11-37. HC_Do_Complete
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11.16.2 Transaction Translator State Machine
Architecture Declarations
Package List
ieee
std_logic_1164
ieee
numeric_std
usb2statemachines behav_package
Packet_ready(HSD2)
st1/ct1
Save (HSD2, split);
TT_Process_packet
TT_no_packet
Wait_for_Packet(
HSD2, none);
Figure 11-38. Transaction Translator
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Universal Serial Bus Specification Revision 2.0
11.16.2.1 TT_Process_packet State Machine
split.PID /= SSPLIT and split.PID /= CSPLIT
Get_token
Wait_for_packet(
HSD2, ITG);
TT_Do_start
se1/ce1
Packet_ready(HSD2)
Save (HSD2, token);
st2/ct2
HSD2.PID = SSPLIT or
HSD2.PID = CSPLIT
split.PID = SSPLIT
&
Save(HSD2, split);
split.PID = CSPLIT
TT_Do_complete
HSD2.PID = SOF
SS_Buff.saw_split <= false;
not SS_Buff.isochO or
(SS_Buff.isochO and
SS_Buff.saw_split)
&
SS_Buff.isochO and
not SS_Buff.saw_split
DoSawSplit
Down_error;
SS_Buff.isochO <= false;
HSD2.PID /= SSPLIT and
HSD2.PID /= CSPLIT and
HSD2.PID /= SOF
TT_Do_nonsplit
TT_Process_Packet
Figure 11-39. TT_Process_Packet
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11.16.2.1.1 TT_Do_Start State Machine
TT_IsochSS
split.ep_type = isochronous
split.ep_type = interrupt
TT_IntSS
split.ep_type = bulk or
split.ep_type = control
TT_BulkSS
TT_Do_Start
Figure 11-40. TT_Do_Start
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11.16.2.1.2 TT_Do_Complete State Machine
TT_IsochICS
split.ep_type = isochronous
split.ep_type = interrupt
TT_IntCS
split.ep_type = bulk or
split.ep_type = control
TT_BulkCS
TT_Do_complete
Figure 11-41. TT_Do_Complete
11.16.2.1.3 TT_BulkSS State Machine
(token.PID /= tokenOUT and
token.PID /= tokenSETUP and
token.PID /= tokenIN) or
token.timeout
&
token.PID = tokenIN
TT_Do_BISS
token.PID = tokenOUT or
token.PID = tokenSETUP
TT_Do_BOSS
TT_BulkSS
Figure 11-42. TT_BulkSS
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Universal Serial Bus Specification Revision 2.0
11.16.2.1.4 TT_BulkCS State Machine
(token.PID /= tokenOUT and
token.PID /= tokenSETUP and
token.PID /= tokenIN) or
token.timeout
&
token.PID = tokenOUT or
token.PID = tokenSETUP
TT_Do_BOCS
token.PID = tokenIN
TT_Do_BICS
TT_BulkCS
Figure 11-43. TT_BulkCS
11.16.2.1.5 TT_IntSS State Machine
(token.PID /= tokenOUT and
token.PID /= tokenIN) or
token.timeout
&
TT_Do_IntISS
token.PID = tokenIN
token.PID = tokenOUT
TT_Do_IntOSS
TT_IntSS
Figure 11-44. TT_IntSS
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Universal Serial Bus Specification Revision 2.0
11.16.2.1.6 TT_IntCS State Machine
(token.PID /= tokenIN and
token.PID /= tokenOUT) or
token.timeout
TT_Do_IntICS
token.PID = tokenIN
&
token.PID = tokenOUT
TT_Do_IntOCS
TT_IntCS
Figure 11-45. TT_IntCS
11.16.2.1.7 TT_IsochSS State Machine
(token.PID /= tokenIN and
token.PID /= tokenOUT) or
token.timeout
&
token.PID = tokenIN
TT_Do_IsochISS
token.PID = tokenOUT
TT_Do_IsochOSS
TT_IsochSS
Figure 11-46. TT_IsochSS
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11.17 Bulk/Control Transaction Translation Overview
Each TT must have at least two bulk/control transaction buffers. Each buffer holds the information for a
start- or complete-split transaction and represents a single full-/low-speed transaction that is awaiting (or has
completed) transfer on the downstream bus. The buffer is used to hold the transaction information from the
start-split (and data for an OUT) and then the handshake/result of the full-/low-speed transaction (and data
for an IN). This buffer is filled and emptied by split transactions from the high-speed bus via the high-speed
handler. The buffer is also updated by the full-/low-speed handler while the transaction is in progress on the
downstream bus.
The high-speed handler must accept a start-split transaction from the host controller for a bulk/control
endpoint whenever the high-speed handler has appropriate space in a bulk/control buffer.
The host controller attempts a start-split transaction according to its bulk/control high-speed transaction
schedule. As soon as the high-speed handler responds to a complete-split transaction with the results from
the corresponding buffer, the next start-split for some (possibly other) full-/low-speed endpoint can be saved
in the buffer.
There is no method to control the start-split transaction accepted next by the high-speed handler.
Sequencing of start-split transactions is simply determined by available TT buffer space and the current
state of the host controller schedule (e.g., which start-split transaction is next that the host controller tries as
a normal part of processing high-speed transactions).
The host controller does not need to segregate split transaction bulk (or control) transactions from highspeed bulk (control) transactions when building its schedule. The host controller is required to track
whether a transaction is a normal high-speed transaction or a high-speed split transaction.
The following sections describe the details of the transaction phases, flow sequences, and state machines for
split transactions used to support full-/low-speed bulk and control OUT and IN transactions. There are only
minor differences between bulk and control split transactions. In the figures, some areas are shaded to
indicate that they do not apply for control transactions.
11.17.1 Bulk/Control Split Transaction Sequences
The state machine figures show the transitions required for high-speed split transactions for full-/low-speed
bulk/control transfer types for a single endpoint. These figures must not be interpreted as showing any
particular specific timing. They define the required sequencing behavior of different packets of a
bulk/control split transaction. In particular, other high-speed or split transactions for other endpoints occur
before or after these split transaction sequences.
Figure 11-47 shows a sample code algorithm that describes the behavior of the transitions labeled with
Is_new_SS, Is_old_SS and Is_no_space shown in the figures for both bulk/control IN and OUT start-split
transactions buffered in the TT for any endpoint. This algorithm ensures that the TT only buffers a single
bulk/control split transaction for any endpoint. The complete-split protocol definition requires an endpoint
has only a single result buffered in the TT at any time. Note that the “buffer match” test is different for bulk
and control endpoints. A buffer match test for a bulk transaction must include the direction of the
transaction in the test since bulk endpoints are unidirectional. A control transaction must not use direction
as part of the match test.
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procedure Compare_buffs IS
variable match:boolean:=FALSE;
begin
--- Is_new_SS is true when BC_buff.status == NEW_SS
-- Is_old_SS is true when BC_buff.status == OLD_SS
-- Is_no_space is true when BC_buff.status == NO_SPACE
--- Assume nospace and intialize index to 0.
BC_buff.status := NO_SPACE;
BC_buff.index := 0;
FOR i IN 0
IF NOT
-IF
to num_buffs-1 LOOP
match THEN
Re-use buffer with same Device Address/End point.
(token.endpt = cam(i).store.endpt AND
token.dev_addr = cam(i).store.dev_addr AND
((token.direction = cam(i).store.direction AND
split.ep_type /= CONTROL) OR
split.ep_type = CONTROL)) THEN
-- If The buffer is already pending/ready this must be a retry.
IF (cam(i).match.state = READY OR cam(i).match.state = PENDING) THEN
BC_buff.status := OLD_SS;
ELSE
BC_buff.status := NEW_SS;
END IF;
BC_buff.index := i;
match := TRUE;
-- Otherwise use the buffer if it’s old.
ELSIF (cam(i).match.state = OLD) THEN
BC_buff.status := NEW_SS;
BC_buff.index := i;
END IF;
END IF;
END LOOP;
end Compare_buffs;
Figure 11-47. Sample Algorithm for Compare_buffs
Figure 11-48 shows the sequence of packets for a start-split transaction for the full-/low-speed bulk OUT
transfer type. The block labeled SSPLIT represents a split transaction token packet as described in
Chapter 8. It is followed by an OUT token packet (or SETUP token packet for a control setup transaction).
If the high-speed handler times out after the SSPLIT or OUT token packets, and does not receive the
following OUT/SETUP or DATA0/1 packets, it will not respond with a handshake as indicated by the
dotted line transitions labeled “se1” or “se2”. This causes the host to subsequently see a transaction error
(timeout) (labeled “se2” and indicated with a dashed line). If the high-speed handler receives the DATA0/1
packet and it fails the CRC check, it takes the transition “se2” which causes the host to timeout and follow
the “se2” transition.
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Universal Serial Bus Specification Revision 2.0
Start split
st1
SSPLIT
st2
Trans_err
OUT/SETUP
Trans_err
sd1
DATA0/1
se1
Compare_buffs
Is_new_SS
Is_old_SS
Accept_data
sh1
sh2
ACK
Is_no_space
Trans_err
sh3
se2
Inc err
count
NAK
se4
Go to
comp. split
se5
Retry
if err_count < 3 if err_count >= 3
start split retry start split endpoint halt
Host
TT
Figure 11-48. Bulk/Control OUT Start-split Transaction Sequence
The host must keep retrying the start-split for this endpoint until the err_count reaches three for this
endpoint before continuing on to some other start-split for this endpoint. However, the host can issue other
start-splits for other endpoints before it retries the start-split for this endpoint. The err_count is used to
count how many errors have been experienced during attempts to issue a particular transaction for a
particular endpoint.
If there is no space in the transaction buffers to hold the start-split, the high-speed handler responds with a
NAK via transition “sh3”. This will cause the host to retry this start-split at some future time based on its
normal schedule. The host does not increase its err_count for a NAK handshake response. Once the host
has received a NAK response to a start-split, it can skip other start-splits for this TT for bulk/control
endpoints until it finishes a bulk/control complete-split.
If there is buffer space for the start-split, the high-speed handler takes transition “sh1” and responds with an
ACK. This tells the host it must try a complete-split the next time it attempts to process a transaction for
this full-/low-speed endpoint. After receiving an ACK handshake, the host must not issue a further startsplit for this endpoint until the corresponding complete-split has been completed.
If the high-speed handler already has a start-split for this full-/low-speed endpoint pending or ready, it
follows transition “sh2” and also responds with an ACK, but ignores the data. This handles the case where
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an ACK handshake was smashed and missed by the host controller and now the host controller is retrying
the start-split; e.g., a high-speed handler transition of “sh1” but a host transition of “se2”.
In the host controller error cases, the host controller implements the “three strikes and you’re out”
mechanism. That is, it increments an error count (err_count) and, if the count is less than three (transition
“se4”), it will retry the transaction. If the err_count is greater or equal to three (transition “se5”), the host
controller does endpoint halt processing and does not retry the transaction. If for some reason, a host
memory or non-USB bus delay (e.g.,a system memory “hold off”) occurs that causes the transaction to not
be completed normally, the err_count must not be incremented. Whenever a transaction completes
normally, the err_count is reset to zero.
The high-speed handler in the TT has no immediate knowledge of what the host sees, so the “se2”, “se4”,
and “se5” transitions show only host visibility.
This packet flow sequence showing the interactions between the host and hub is also represented by host
and high-speed handler state machine diagrams in the next section. Those state machine diagrams use the
same labels to correlate transitions between the two representations of the split transaction rules.
Figure 11-49 shows the corresponding flow sequence for the complete-split transaction for the full-/lowspeed bulk/control OUT transfer type. The notation “ready/x” or “old/x” indicates that the transaction status
of the split transaction is any of the ready or old states. After a full-/low-speed transaction is run on the
downstream bus, the transaction status is updated to reflect the result of the transaction. The possible result
status is: nak, stall, ack. The “x” means any of the NAK, ACK, STALL full-/low-speed transaction status
results. Each status result reflects the handshake response from the full-/low-speed transaction.
Complete split
ct1
CSPLIT
ct2
Trans_err
OUT/SETUP
ce1
Not applicable
for control-setup
Match_split_state
ready/x or old/x
If status = ready/x => status = old/x Trans_err
No
match
pending
old/stall old/ack
old/nak
ce5 ch2
ch1
ch3
ch4
ce2
Inc err
NYET
STALL
ACK
NAK
count
ce3
Retry
Endpoint
comp. split halt
if err_count < 3
retry immed. ce4
Go to next Retry
cmd
start split comp. split
if err_count >= 3
endpoint halt
Host
TT
Figure 11-49. Bulk/Control OUT Complete-split Transaction Sequence
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Universal Serial Bus Specification Revision 2.0
There is no timeout response status for a transaction because the full-/low-speed handler must perform a
local retry of a full-/low-speed bulk or control transaction that experiences a transaction error. It locally
implements a “three strikes and you’re out” retry mechanism. This means that the full-/low-speed
transaction will resolve to one of a NAK, STALL or ACK handshake results. If the transaction experiences
a transaction error three times, the full-/low-speed handler will reflect this as a stall status result. The full/low-speed handler must not do a local retry of the transaction in response to an ACK, NAK, or STALL
handshake.
Start split
st1
SSPLIT
Trans_err
st2
IN
se1
Compare_buffs
Is_new_SS
Is_old_SS
Accept_data
sh1
sh2
ACK
Is_no_space
Trans_err
sh3
Inc err
count
NAK
se4
se2
Go to
comp. split
Retry
if err_count < 3
start split retry start split
Host
TT
se3
if err_count >= 3
endpoint halt
Figure 11-50. Bulk/Control IN Start-split Transaction Sequence
If the high-speed handler receives the complete-split token packet (and the token packet) while the full/low-speed transaction has not been completed (e.g., the transaction status is “pending”), the high-speed
handler responds with a NYET handshake. This causes the host to retry the complete-split for this endpoint
some time in the future.
If the high-speed handler receives a complete-split token packet (and the token packet) and finds no local
buffer with a corresponding transaction, the TT responds with a STALL to indicate a protocol violation.
Once the full-/low-speed handler has finished a full-/low-speed transaction, it changes the transaction status
from pending to ready and saves the transaction result. This allows the high-speed handler to respond to the
complete-split transaction with something besides NYET. Once the high-speed handler has seen a
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complete-split, it changes the transaction status from ready/x to old/x. This allows the high-speed handler to
reuse its local buffer for some other bulk/control transaction after this complete-split is finished.
If the host times out the transaction or does not receive a valid handshake, it immediately retries the
complete-split before going on to any other bulk/control transactions for this TT. The normal “three strikes”
mechanism applies here also for the host; i.e., the err_count is incremented. If for some reason, a host
memory or non-USB bus delay (e.g., a system memory “hold off”) occurs that causes the transaction to not
be completed normally, the err_count must not be incremented.
Complete split
ct1
CSPLIT
ct2
IN
Match_split_state
No match
ce1
Trans_err ready/x or old/x or pending
If status = ready/x => status = old/x
old/ack
old/data
old/nak
old/stall
cd1
ch2
ch3 ce5
DATA0/1
NAK
pending
ch1
STALL
NYET
Trans_err
Retry
Endpoint
start split halt
ce6
ce2 Trans_err
Inc err
count
ce3
if err_count >= 3
endpoint halt
ce4
Retry
comp. split
not trans_err not trans_err and
and
Datax = toggle
Datax /=
HC_Accept_data
toggle
ch4
if err_count < 3 Retry
retry immed.
start split
comp. split
ch5
Host
TT
Go to next
cmd
Figure 11-51. Bulk/Control IN Complete-split Transaction Sequence
If the host receives a STALL handshake, it performs endpoint halt processing and will not issue any more
split transactions for this full-/low-speed endpoint until the halt condition is removed.
If the host receives an ACK, it records the results of the full-/low-speed transaction and advances to the next
split transaction for this endpoint. The next transaction will be issued at some time in the future according
to normal scheduling rules.
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If the host receives a NAK, it will retry the start-split transaction for this endpoint at some time in the future
according to normal scheduling rules. The host must not increment the err_count in this case.
The host must keep retrying the current start-split until the err_count reaches three for this endpoint before
proceeding to the next split transaction for this endpoint. However, the host can issue other start-splits for
other endpoints before it retries the start-split for this endpoint.
After the host receives a NAK, ACK, or STALL handshake in response to a complete-split transaction, it
may subsequently issue a start-split transaction for the same endpoint. The host may choose to instead issue
a start-split transaction for a different endpoint that is not awaiting a complete-split response.
The shaded case shown in the figure indicates that a control setup transaction should never encounter a
NAK response since that is not allowed for full-/low-speed transactions.
Figure 11-50 and Figure 11-51 show the corresponding flow sequences for bulk/control IN split
transactions.
11.17.2 Bulk/Control Split Transaction State Machines
The host and TT state machines for bulk/control IN and OUT split transactions are shown in the following
figures. The transitions for these state machines are labeled the same as in the flow sequence figures.
st1
HC_cmd.ep_type = control and
HC_cmd.setup
Issue_packet(HSD1, SSPLIT);
HSU2.PID = ACK
HC_cmd.ep_type = bulk or
(HC_cmd.ep_type = control and
not HC_cmd.setup)
RespondHC(Do_complete);
DoSetup
Issue_packet(
HSD1, SSPLIT);
sh1/sh2
st2
&
DoOut
Issue_packet(
HSD1, tokenSETUP);
sh3
HSU2.PID = NAK
RespondHC(Do_start);
se1/se2
st2
ErrorCount < 3
Issue_packet(
HSD1, tokenOUT);
RespondHC(Do_start);
Dodata
sd1
(HSU2.PID /= ACK and
HSU2.PID /= NAK) or
HSU2.timeout
Issue_packet(HSD1, DATAx);
se4
ErrorCount >= 3
RespondHC(Do_halt);
packet_ready(HSU2)
se5
BSSO_error
BSSO_Wait_hndshk
IncError;
Wait_for_packet(
HSU2, ITG);
HC_Do_BOSS
Figure 11-52. Bulk/Control OUT Start-split Transaction Host State Machine
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Universal Serial Bus Specification Revision 2.0
HSU2.PID = NYET
ct1
RespondHC(Do_complete);
HC_cmd.ep_type = control and
HC_cmd.setup
Issue_packet(HSD1, CSPLIT);
HSU2.PID = STALL
ch2/ce5
HC_cmd.ep_type = bulk or
(HC_cmd.ep_type= control and
NOT HC_cmd.setup)
RespondHC(Do_halt);
ch1
DoSetupCS
Issue_packet(HSD1, CSPLIT);
&
HSU2.PID = ACK
ch3
RespondHC(Do_next_cmd);
ct2
ch4
Issue_packet(HSD1, tokenSETUP);
ce2
HSU2.PID = NAK
RespondHC(Do_start);
DoOUTCS
ErrorCount < 3
RespondHC(Do_complete_immediate);
ct2
Issue_packet(HSD1, tokenOUT);
Packet_ready(HSU2)
(HSU2.PID /= NYET and
HSU2.PID /= STALL and
HSU2.PID /= ACK and
HSU2.PID /= NAK) or
HSU2.timeout
ce3
ErrorCount >= 3
RespondHC(Do_halt);
ce4
BCSO_Wait_for_resp
Wait_for_packet(
HSU2, ITG);
BCSO_error
Not allowed for control
setup transaction
IncError;
HC_DO_BOCS
Figure 11-53. Bulk/Control OUT Complete-split Transaction Host State Machine
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Universal Serial Bus Specification Revision 2.0
se1
&
HSD2.PID /= DATAx or
HSD2.timeout or
HSD2.CRC16 = bad
HSD2.PID = DATAx
Packet_ready (HSD2)
Is_new_SS(BC_buff)
sh1
TT_BSSO_Check_Buffs
sd1
Accept_data;
Issue_packet(HSU1, ACK);
Compare_BC_buff;
sh2
Is_old_SS(BC_buff)
TT_SS_wait_pkt3
Issue_packet(HSU1, ACK);
sh3
Wait_for_packet(HSD2, ITG);
Is_no_space(BC_buff)
Issue_packet(HSU1, NAK);
TT_Do_BOSS
Figure 11-54. Bulk/Control OUT Start-split Transaction TT State Machine
&
TT_BOCS_Match
BC_Buff.match.state = no_match
Issue_packet(HSU1, STALL);
Match_split_state;
ce5
BC_Buff.match.down_result = r_stall
ch2
BC_Buff.match.down_result = r_ack
&
ch3
Issue_packet(HSU1, ACK);
ch4
BC_Buff.match.state /= ready
BC_Buff.match.down_result = r_nak
Issue_packet(HSU1, NAK);
BC_Buff.match.state = ready
BC_Buff.match.state = old
BC_Buff.match.state := old;
DidOld
ch1
BC_buff.match.state = pending
Issue_packet(HSU1, NYET);
TT_Do_BOCS
Figure 11-55. Bulk/Control OUT Complete-split Transaction TT State Machine
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Universal Serial Bus Specification Revision 2.0
st1
Issue_packet(HSD1, SSPLIT);
HSU2.PID = NAK
RespondHC(Do_start);
DoINISS
sh3
st2
&
Issue_packet(HSD1, tokenIN);
sh1/sh2
HSU2.PID = ACK
RespondHC(Do_complete);
Packet_ready(HSU2)
ErrorCount < 3
se4
RespondHC(Do_start);
(HSU2.PID /= ACK and
HSU2.PID /= NAK) or
HSU2.timeout
ErrorCount >= 3
RespondHC(Do_halt);
se2
BSSI_Wait_hndshk
Wait_for_packet(
HSU2, ITG);
se3
BSSI_error
IncError;
HC_Do_BISS
Figure 11-56. Bulk/Control IN Start-split Transaction Host State Machine
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Universal Serial Bus Specification Revision 2.0
ch4
&
HSU2.x /= HC_cmd.toggle
ch5
HSU2.CRC16 = ok
RespondHC(Do_start);
HSU2.x = HC_cmd.toggle
HC_Accept_data;
&
Dotoggled
HSU2.CRC16 = bad
RespondHC(Do_next_cmd);
ce3
ct1
HC_BSSI_error
ErrorCount >= 3
IncError;
Issue_packet(HSD1, CSPLIT);
ce4
RespondHC(Do_halt);
ErrorCount < 3
DoINBSS
RespondHC(Do_complete_immediate);
HSU2.PID = DATAx
ct2
Issue_packet(HSD1, tokenIN);
(HSU2.PID /= DATAx and
HSU2.PID /= NAK and
HSU2.PID /= NYET and
HSU2.PID /= STALL) or
HSU2.timeout
HSU2.PID = STALL
RespondHC(Do_halt);
ce6
BICS_wait_response
Wait_for_packet(
HSU2, ITG);
cd1
HSU2.PID = NAK
ch3/ce5
RespondHC(Do_start);
ch2
Packet_ready(HSU2)
&
ch1
HSU2.PID = NYET
RespondHC(Do_complete);
HC_Do_BICS
Figure 11-57. Bulk/Control IN Complete-split Transaction Host State Machine
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Universal Serial Bus Specification Revision 2.0
Is_no_space(BC_buff)
Issue_packet(HSU1, NAK);
sh3
Is_new_SS(BC_buff)
sh1
TT_BISS_check
Compare_BC_buff;
Accept_data;
Issue_packet(HSU1, ACK);
sh2
Is_old_SS(BC_buff)
Issue_packet(HSU1, ACK);
TT_Do_BISS
Figure 11-58. Bulk/Control IN Start-split Transaction TT State Machine
BC_buff.match.state = no_match
Issue_packet(HSU1, STALL);
&
BC_buff.match.down_result = r_stall or
BC_buff.match.down_result = r_ack
Match_split_state;
BC_buff.match.down_result = r_data
Issue_packet(HSU1, DATAx);
TT_BICS_match
ch3
BC_buff.match.down_result = r_nak
cd1
Issue_packet(HSU1, NAK);
ch2
ce5
BC_buff.match.state /= ready
&
BC_buff.match.state = old
BC_buff.match.state = ready
BC_buff.match.state := old;
Donyet ch1
BC_buff.match.state = pending
Issue_packet(HSU1, NYET);
TT_Do_BICS
Figure 11-59. Bulk/Control IN Complete-split Transaction TT State Machine
11.17.3 Bulk/Control Sequencing
Once the high-speed handler has received a start-split for an endpoint and saved it in a local buffer, it
responds with an ACK split transaction handshake. This tells the host controller to do a complete-split
transaction next time this endpoint is polled.
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As soon as possible (subject to scheduling rules described previously), the full-/low-speed handler issues the
full-/low-speed transaction and saves the handshake status (for OUT) or data/handshake status (for IN) in
the same buffer.
Some time later (according to the host controller schedule), this endpoint will be polled for the completesplit transaction. The high-speed handler responds to the complete-split to return the full-/low-speed
endpoint status for this transaction (as recorded in the buffer). If the host controller polls for the completesplit transaction for this endpoint before the full-/low-speed handler has finished processing this transaction
on the downstream bus, the high-speed handler responds with a NYET handshake. This tells the host
controller that the transaction is not yet complete. In this case, the host controller will retry the completesplit again at some later time.
When the full-/low-speed handler finally finishes the full-/low-speed transaction, it saves the data/status in
the buffer to be ready for the next host controller complete-split transaction for this endpoint. When the
host sends the complete-split, the high-speed handler responds with the indicated data/status as recorded in
the buffer. The buffer transaction status is updated from ready to old so the high-speed handler is ready for
either a retry or a new start-split transaction for this (or some other) full-/low-speed endpoint.
If there is an error on the complete-split transaction, the host controller will retry the complete-split
transaction for this bulk/control endpoint “immediately” before proceeding to some other bulk/control split
transaction. The host controller may issue other periodic split transactions or other non-split transactions
before doing this complete-split transaction retry.
If there is a bulk/control transaction in progress on the downstream facing bus when the EOF time occurs,
the TT must adhere to the definition in Section 11.3 for its behavior on the downstream facing bus. This
will cause an increase in the error count for this transaction. The normal retry rules will determine if the
transaction will be retried or not on the downstream facing bus.
11.17.4 Bulk/Control Buffering Requirements
The TT must provide at least two transactions of non-periodic buffering to allow the TT to deliver
maximum full-/low-speed throughput on a downstream bus when the high-speed bus is idle.
As the high-speed bus becomes busier, the throughput possible on downstream full-/low-speed buses will
decrease.
A TT may provide more than two transactions of non-periodic buffering and this can improve throughput
for downstream buses for specific combinations of device configurations.
11.17.5 Other Bulk/Control Details
When a bulk/control split transaction fails, it can leave the associated TT transaction buffer in a busy
(ready/x) state. This buffer state will not allow the buffer to be reused for other bulk/control split
transactions. Therefore, as part of endpoint halt processing for full-/low-speed endpoints connected via a
TT, the host software must use the Clear_TT_Buffer request to the TT to ensure that the buffer is not in the
busy state.
Appendix A shows examples of packet sequences for full-/low-speed bulk/control transactions and their
relationship with start-splits and complete-splits in various normal and error conditions.
11.18 Periodic Split Transaction Pipelining and Buffer Management
There are requirements on the behavior of the host and the TT to ensure that the microframe pipeline
correctly sequences full-/low-speed isochronous/interrupt transactions on downstream facing full-/lowspeed buses. The host must determine the microframes in which a start-split and complete-split transaction
must be issued on high-speed to correctly sequence a corresponding full-/low-speed transaction on the
downstream facing bus. This is called “scheduling” the split transactions.
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In the following descriptions, the 8 microframes within each full-speed (1 ms.) frame are referred to as
microframe Y0, Y1, Y2, …, Y7. This notation means that the first microframe of each full-speed frame is
labeled Y0. The second microframe is labeled Y1, etc. The last microframe of each full-speed frame is
labeled Y7. The labels repeat for each full-speed frame.
This section describes details of the microframe pipeline that affect both full-speed isochronous and full/low-speed interrupt transactions. Then the split transaction rules for interrupt and isochronous are
described.
Bulk/control transactions are not scheduled with this mechanism. They are handled as described in the
previous section.
11.18.1 Best Case Full-Speed Budget
A microframe of time allows at most 187.5 raw bytes of signaling on a full-speed bus. In order to estimate
when full-/low-speed transactions appear on a downstream bus, the host must calculate a best case fullspeed budget. This budget tracks in which microframes a full-/low-speed transaction appears. The best case
full-speed budget assumes that 188 full-speed bytes occur in each microframe. Figure 11-60 shows how a
1 ms frame subdivided into microframes of budget time. This estimate assumes that no bit stuffing occurs
to lengthen the time required to move transactions over the bus.
The maximum number of bytes in a 1 ms frame is calculated as:
1157 maximum_periodic_bytes_per_frame = 12 Mb/s * 1 ms / 8 bits_per_byte *
6 data_bits / 7 bit-stuffed_data_bits * 90% maximum_periodic_data_per_frame
Microframes
Y0
Max wire time
Best case wire budget,
1157 bytes w/ no
bitstuffing
Y1
Y2
Y3
Y4
187.5
187.5
187.5
187.5
187.5
188
188
188
188
188
Y5
Y6
187.5
32
188
29
Y7
Figure 11-60. Best Case Budgeted Full-speed Wire Time With No Bit Stuffing
11.18.2 TT Microframe Pipeline
The TT implements a microframe pipeline of split transactions in support of a full-/low-speed bus. Startsplit transactions are scheduled a microframe before the earliest time that their corresponding full-/lowspeed transaction is expected to start. Complete-split transactions are scheduled in microframes that the
full-/low-speed transaction can finish.
When a full-/low-speed device is attached to the bus and configured, the host assigns some time on the
full-/low-speed bus at some budgeted time, based on the endpoint requirements of the configured device.
The effects of bit stuffing can delay when the full-/low-speed transaction actually runs. The results of other
previous full-/low-speed transactions can cause the transaction to run earlier or later on the full-/low-speed
bus.
The host always uses the maximum data payload size for a full-/low-speed endpoint in doing its budgeting.
It does not attempt to schedule the actual data payloads that may be used in specific transactions to full/low-speed endpoints. The host must include the maximum duration interpacket gap, bus turnaround times,
and “TT think time”. The TT requires some time to proceed to the next full-/low-speed transaction. This
time is called the “TT think time” and is specified in the hub descriptor field wHubCharacteristics bit 5 and
6.
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#1: A full/low-speed transaction
budgeted to run here on the classic bus,...
(Y-1)7
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
Best case budget
HS
Start-split
HS Complete-splits
#2: …has a HS start-split scheduled
in this microframe and ...
#3: …has 3 HS complete-split transactions
scheduled in the possible microframes
for this full/low-speed transaction
Figure 11-61. Scheduling of TT Microframe Pipeline
Figure 11-61 shows an example of a new endpoint that is assigned some portion of a full-/low-speed frame
and where its start- and complete-splits are generally scheduled. The act of assigning some portion of the
full-/low-speed frame to a particular transaction is called determining the budget for the transaction. More
precise rules for scheduling and budgeting are presented later. The start-split for this example transaction is
scheduled in microframe Y-17, the transaction is budgeted to run in microframe Y0, and complete-splits are
scheduled for microframes Y1, Y2, and Y3. Section 11.18.4 describes the scheduling rules more completely.
The host must determine precisely when start- and complete- splits are scheduled to avoid overruns or
underruns in the periodic transaction buffers provided by the TT.
11.18.3 Generation of Full-speed Frames
The TT must generate SOFs on the full-speed bus to establish the 1 ms frame clock within the defined jitter
tolerances for full-speed devices. The TT has its own frame clock that is synchronized to the microframe
SOFs on the high-speed bus. The SOF that reflects a change in the frame number it carries is identified as
the zeroth microframe SOF. The zeroth high-speed microframe SOF corresponds to the full-speed SOF on
the TT’s downstream facing bus. The TT must adhere to all timing/jitter requirements of a host controller
related to frames as defined in other parts of this specification.
The TT must stop issuing full-speed SOFs after it detects 250 µs of high-speed idle. This is required to
ensure that the full-/low-speed downstream facing bus enters suspend no more than 250 µs after the highspeed bus enters suspend.
The TT must generate a full-speed SOF on the downstream facing bus based on its frame timer. The
generation of the full-speed SOF must occur within +/-3 full-speed bit time from the occurrence of the
zeroth high-speed SOF. See Section 11.22.1 for more information about TT SOF generation.
11.18.4 Host Split Transaction Scheduling Requirements
Scheduling of split transactions is done by the host (typically in software) based on a best-case estimate of
how the full-/low-speed transactions can be run on the downstream facing bus. This best-case estimate is
called the best case budget. The host is free to issue the split transactions anytime within the scheduled
microframe, but each split transaction must be issued sometime within the scheduled microframe. This
description of the scheduling requirements applies to the split transactions for a single full-/low-speed
transaction at a time.
1.
374
The host must never schedule a start-split in microframe Y6. Some error conditions may result in the
host controller erroneously issuing a start-split in this microframe. The TT response to this start-split is
undefined.
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2.
The host must compute the start-split schedule by determining the best case budget for the transaction
and:
a.
For isochronous OUT full-speed transactions, for each microframe in which the transaction is
budgeted, the host must schedule a 188 (or the remaining data size) data byte start-split transaction.
The start-split transaction must be scheduled in the microframe before the data is budgeted to begin
on the full-speed bus. The start-split transactions must use the beginning/middle/end/all split
transaction token encodings corresponding to the piece of the full-speed data that is being sent on
the high-speed bus. For example, if only a single start-split is required, an “all” encoding is used.
If multiple start-splits are required, a “beginning” encoding is used for the first start-split and an
“end” encoding is used for the final start-split. If there are more than two start-splits required, the
additional start-splits that are not the first or last use a “middle” encoding. A zero length full-speed
data payload must only be scheduled with an “all” start-split. A start-split transaction for a
beginning, middle, or end start-split must always have a non-zero length data payload.
Figure 11-62 shows an example of an isochronous OUT that would appear to have budgeted a zero
length data payload in a start-split (end). This example instead must be scheduled with a startsplit(all) transaction.
Isoch OUT transaction with 187 data
bytes has 196 byte budget.
Transaction budgeted for Y1 and Y2.
(Y-1)7
Y0
Best case budget
Y1
T
Y2
Y3
Y4
Y5
Y6
Y7
Data
HS SS-all
Start-split
Schedule SS-all with 187 data bytes, not SS-begin(187 data) and SS-end (0 data).
An Isoch OUT only ever has zero length data in SS-all.
Figure 11-62. Isochronous OUT Example That Avoids a Start-split-end With Zero Data
b.
3.
For isochronous IN and interrupt IN/OUT full-/low-speed transactions, a single start-split must be
scheduled in the microframe before the transaction is budgeted to start on the full-/low-speed bus.
The host never schedules more than one complete-split in any microframe for the same full-/low-speed
transaction.
a.
For isochronous OUT full-speed transactions, the host must never schedule a complete-split. The
TT response to a complete-split for an isochronous OUT is undefined.
b.
For interrupt IN/OUT full-/low-speed transactions, the host must schedule a complete-split
transaction in each of the two microframes following the first microframe in which the full-/lowspeed transaction is budgeted. An additional complete-split must also be scheduled in the third
following microframe unless the full-/low-speed transaction was budgeted to start in microframe
Y6. Figure 11-63 shows an example with only two complete-splits.
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#1: A full/low-speed transaction
budgeted to run here on the classic bus,...
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
(Y+1)0
Previously budgeted transactions
Best case budget
HS
Start-split
HS Complete-splits
#2: …has a HS start-split scheduled
in this microframe and ...
#3: …has 2 HS complete-split transactions
scheduled in the possible microframes
for this full/low-speed transaction
Figure 11-63. End of Frame TT Pipeline Scheduling Example
c.
For isochronous IN full-speed transactions, for each microframe in which the full-speed transaction
is budgeted, a complete-split must be scheduled for each following microframe. Also, determine
the last microframe in which a complete-split is scheduled, call it L. If L is less than Y6, schedule
additional complete-splits in microframe L+1 and L+2.
If L is equal to Y6, schedule one complete-split in microframe Y7. Also, schedule one completesplit in microframe Y0 of the next frame, unless the full-speed transaction was budgeted to start in
microframe Y0.
If L is equal to Y7, schedule one complete-split in microframe Y0 of the next frame, unless the fullspeed transaction was budgeted to start in microframe Y0. Figure 11-64 and Figure 11-65 show
examples of the cases for L= Y6 and L=Y7.
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Microframe with
last complete-split
from budget (L)
#1: A full/low-speed transaction
budgeted to run here on the classic bus,...
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
(Y+1)0
Previously budgeted transactions
Best case budget
HS Complete-splits
HS
Start-split
“Extra” complete-splits
#2: …has a HS start-split scheduled
in this microframe and ...
#3: …has 4 HS complete-split transactions
scheduled in the possible microframes
for this full/low-speed transaction
Figure 11-64. Isochronous IN Complete-split Schedule Example at L=Y6
Microframe with
last complete-split
from budget (L)
#1: A full/low-speed transaction
budgeted to run here on the classic bus,...
Y0
Y1
Y2
Y3
Y4
Y5
Y6
Y7
(Y+1)0
Previously budgeted transactions
Best case budget
HS
Start-split
HS Complete-splits
“Extra”
complete-split
#2: …has a HS start-split scheduled
in this microframe and ...
#3: …has 4 HS complete-split transactions
scheduled in the possible microframes
for this full/low-speed transaction
Figure 11-65. Isochronous IN Complete-split Schedule Example at L=Y7
4.
The host must never issue more than 16 start-splits in any high-speed microframe for any TT.
5.
The host must only issue a split transaction in the microframe in which it was scheduled.
6.
As precisely identified in the flow sequence and state machine figures, the host controller must
immediately retry a complete-split after a high-speed transaction error (“trans_err”).
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The “pattern” of split transactions scheduled for a full-/low-speed transaction can be computed once when
each endpoint is configured. Then the pattern does not change unless some change occurs to the collection
of currently configured full-/low-speed endpoints attached via a TT.
Finally, for all periodic endpoints that have split transactions scheduled within a particular microframe, the
host must issue complete-split transactions in the same relative order as the corresponding start-split
transactions were issued.
11.18.5 TT Response Generation
The approach used for full-speed isochronous INs and interrupt INs/OUTs ensures that there is always an
opportunity for the TT to return data/results whenever it has something to return from the full-/low-speed
transaction. Then whenever the full-/low-speed handler starts the full-/low-speed transaction, it simply
accumulates the results in each microframe and then returns it in response to a complete-split from the host.
The TT acts similar to an isochronous device in that it uses the microframe boundary to "carve up" the full/low-speed data to be returned to the host. The TT does not do any computation on how much data to return
at what time. In response to the "next" high-speed complete-split, the TT simply returns the endpoint data it
has received from the full-/low-speed bus in a microframe.
Whenever the TT has data to return in response to a complete-split for an interrupt full-/low-speed or
isochronous full-speed transaction, it uses either a DATA0/1 or MDATA for the data packet PID.
If the full-/low-speed handler completes the full-/low-speed isochronous/interrupt IN transaction during a
microframe with a valid CRC16, it uses the DATA0/1 PID for the data packet of the complete-split
transaction. This indicates that this is the last data of the full-/low-speed transaction. A DATA0 PID is
always used for isochronous transactions. For interrupt transactions, a DATA0/1 PID is used corresponding
to the full-/low-speed data packet PID received.
If the full-/low-speed handler completes the full-/low-speed isochronous/interrupt IN transaction during a
microframe with a bad CRC16, it uses the ERR response to the complete-split transaction and does not
return the data received from the full-/low-speed device.
If the TT is still receiving data on the downstream facing bus at the microframe boundary, the TT will
respond with either an MDATA PID or a NYET for the corresponding complete-split. If the TT has
received more than two bytes of the data field of the full-/low-speed data packet, it will respond with an
MDATA PID. Further, the data packet that will be returned in the complete-split must contain the data
received from the full-/low-speed device minus the last two bytes. The last two bytes must not be included
since they could be the CRC16 field, but the TT will not know this until the next microframe. The CRC16
field received from the full-/low-speed device is never returned in a complete-split data packet for
isochronous/interrupt transactions. If less than three data bytes of the full-/low-speed data packet have been
received at the end of a microframe, the TT must respond with a NYET to the corresponding high-speed
complete-split. Both of these responses indicate to the host that more data is being received and another
complete-split transaction is required.
When the host controller receives a DATA0/1 PID for interrupt or isochronous IN complete-splits (and
ACK, NAK, STALL, ERR for interrupt IN/OUT complete-splits), it stops issuing any remaining completesplits that might be scheduled for that endpoint for this full-/low-speed transaction.
If the TT has not started the full-/low-speed transaction when it receives a complete-split, the TT will not
find an entry in the complete-split pipeline stage. When this happens, the protocol state machines show that
the TT responds with a NYET (e.g., the “no match” case). This NYET response tells the host that there are
no results available currently, but the host should continue with other scheduled split transactions for this
endpoint in subsequent microframes.
In general, there will be two (or more) complete-split transactions scheduled for a periodic endpoint.
However, for interrupt endpoints, the maximum size of the full-/low-speed transaction guarantees that it can
never require more than two complete-split transactions. Two complete-split transactions are only required
when the transaction spans a microframe boundary. In cases where the full-/low-speed transaction actually
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starts and completes in the same microframe, only a single complete-split will return data; any other earlier
complete-splits will have a NYET response.
For isochronous IN transactions, more complete-split transactions may be scheduled based on the length of
the full-speed transaction. A full-speed isochronous IN transaction can be up to 1023 data bytes, which can
require portions of up to 8 microframes of time on the downstream facing bus (with the worst alignment in
the frame and worst case bit stuffing). Such a maximum sized full-speed transaction can require
8 complete-split transactions. If the device generates less data, the host will stop issuing complete-splits
after the one that returns the final data from the device for a frame.
11.18.6 TT Periodic Transaction Handling Requirements
The TT has two methods it must use to react to timing related events that affect the microframe pipeline:
current transaction abort and freeing pending start-splits. These methods must be used to manage the
microframe pipeline.
The TT must also react (as described in Section 11.22.1) when its microframe or frame timer loses
synchronization with the high-speed bus.
The TT must not issue too many full-/low-speed transactions in any microframe.
Each of these requirements are described below.
11.18.6.1 Abort of Current Transaction
When a current transaction is in progress on the downstream facing bus and it is no longer appropriate for
the TT to continue the transaction, the transaction is “aborted.”
The TT full-/low-speed handler must abort the current full-/low-speed transaction:
1.
For all periodic transaction types, if the full-speed frame EOF time occurs
2.
If the transaction is an interrupt transaction and the start-split for the transaction was received in some
microframe (call it X) and the TT microframe timer indicates the X+4 microframe
Note that no additional abort handling is required for isochronous transactions besides the generic IN/OUT
handling described below. Abort has different processing requirements with regards to the downstream
facing bus for IN and OUT transactions. For any type of transaction, the TT must not generate a completesplit response for an aborted transaction; e.g., no entry is made in the complete-split pipeline stage for an
aborted transaction.
1.
At the time the TT decides to abort an IN transaction, the TT must not issue the handshake packet for
the transaction if the handshake has not already been started on the downstream facing bus. The TT
may choose to not issue the IN token packet, if possible. If the transaction is in the data phase (e.g., in
the middle of the target device generated DATA packet), the TT simply awaits the completion of that
packet and ignores any data received and must not respond with a full-/low-speed handshake. The TT
must not make an entry in the complete-split pipeline stage. This processing will cause a NYET
response to the corresponding complete-split on the high-speed bus.
2.
At the time the TT decides to abort an OUT transaction, the TT may choose to not issue the TOKEN or
DATA packets, if possible. If the TT is in the middle of the DATA packet, it must stop issuing data
bytes as soon as possible and force a bit-stuffing error on the downstream facing bus. In any case, the
TT must not make an entry in the complete-split pipeline stage. This processing will cause a NYET
response to the corresponding complete-split on the high-speed bus.
11.18.6.2 Free of Pending Start-splits
A start-split can be buffered in the start-split pipeline stage that is no longer appropriate to cause a full-/lowspeed transaction on the downstream facing bus. Such a start-split transaction must be “freed” from the
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start-split pipeline stage. This means the start-split is simply ignored by the TT and the TT must respond to
a corresponding complete-split with a NYET. For example, no entry is made in the complete-split pipeline
stage for the freed start-split.
A start-split in the start-split pipeline must be freed:
1.
If the full-speed frame EOF time occurs, except for start-splits received in (Y-1)7
2.
If the start-split transaction was received in some microframe (call it X) and the TT microframe
timer indicates the X+4 microframe
If the TT receives a periodic start-split transaction in microframe Y6, its behavior is undefined. This is a
host scheduling error.
11.18.6.3 Maximum Full-/low-speed Transactions per Microframe
The TT must not start a full-/low-speed transaction unless it has space available in the complete-split
pipeline stage to hold the results of the transaction. If there is not enough space, the TT must wait to issue
the transaction until there is enough space. The maximum number of normally operating full-speed
transactions that can ever be completed in a microframe is 16.
11.18.7 TT Transaction Tracking
Figure 11-66 shows the TT microframe pipeline of transactions. The 8 high-speed microframes that
compose a full-/low-speed frame are labeled with Y0 through Y7 assuming the microframe timer has
occurred at the point in time shown by the arrow (e.g., time “NOW”).
As shown in the figure, a start-split high-speed transaction that the high-speed handler receives in
microframe Y0 (e.g., a start-split “B”) can run on the full-/low-speed bus during microframe times Y1 or Y2
or Y3. This variation in starting on the full-/low-speed bus is due to bit stuffing and bulk/control
reclamation that can occur on the full-/low-speed bus. Once the full-/low-speed transaction finishes, its
complete-split transactions (if they are required) will run on the high-speed bus during microframes Y2, Y3,
or Y4.
Y0
Start-splits
FS/LS transaction
Complete-splits
B
A
F’, G’
Y1
C
A, B
A
Y2
D
A, B, C
A, B
Y3
Y4
Y5
E
B, C, D
A, B, C
F
C, D, E
B, C, D
G
D, E, F
C, D, E
NOW-4
NOW-3
NOW-2
Y6
Y7
None,
E, F, G
D, E, F
NOW-1
A’’
F, G
E, F, G
NOW
Figure 11-66. Microframe Pipeline
When the microframe timer indicates a new microframe, the high-speed handler must mark any start-splits
in the start-split pipeline stage it received in the previous microframe as “pending” so that they can be
processed on the full-/low-speed bus as appropriate. This prevents the full-/low-speed transactions from
running on the downstream bus too early.
At the beginning of each microframe (call it “NOW”), the high-speed handler must free (as defined in
Section 11.18.6.2) any start-split transactions from the start-split pipeline stage that are still pending from
microframe NOW-4 (or earlier) and ignore them. If the transaction is in progress on the downstream facing
bus, the transaction must be aborted (with full-/low-speed methods as defined in Chapter 8). This is
described in more detail in the previous sections. This ensures that even if the full-/low-speed bus has
encountered a babble condition on the bus (or other delay condition), the TT keeps its periodic transaction
pipeline running on time (e.g., transactions do not run too late). This also ensures that when the last
scheduled complete-split transaction is received by the TT, the full-/low-speed transaction has been
completed (either successfully or by being aborted).
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Finally, at the beginning of each microframe, the high-speed handler must change any complete-split
transaction responses in the complete-split pipeline stage from microframe NOW-2 to the free state so that
their space can be reused for responses in this microframe.
This algorithm is shown in pseudo code in Figure 11-67. This pseudo-code corresponds to the
Advance_pipeline procedure identified previously.
-- Clean up start-split state in case full-/low-speed bus fell behind
while start-splits in pending state received by TT before microframe-4 loop
Free start-split entry
End loop
-- Clean up complete-split pipeline in case no complete-splits were received
While complete-split transaction states from (microframe-2) loop
Free complete-split response transaction entry
End loop
-- Enable full-/low-speed transactions received in previous microframe
While start-split transactions from (previous_microframe) loop
Set start-split entry to pending status
End loop
Figure 11-67. Advance_Pipeline Pseudocode
11.18.8 TT Complete-split Transaction State Searching
A host must issue complete-split transactions in a microframe for a set of full-/low-speed endpoints in the
same relative order as the start-splits were issued in a microframe for this TT. However, errors on start- or
complete-splits can cause the high-speed handler to receive a complete-split transaction that does not
“match” the expected next transaction according to the TT’s transaction pipeline.
The TT has a pipeline of complete-split transaction state that it is expecting to use to respond to completesplit transactions. Normally the host will issue the complete-split that the high-speed handler is expecting
next and the complete-split will correspond to the entry at the front of the complete-split pipeline.
However, when errors occur, the complete-split transaction that the high-speed handler receives might not
match the entry at the front of the complete-split pipeline. This can happen for example, when a start-split
is damaged on the high-speed bus and the high-speed handler does not receive it successfully. Or the highspeed handler might have a match, but the matching entry is located after the state for other expected
complete-splits that the high-speed handler did not receive (due to complete-split errors on the high-speed
bus).
The high-speed handler must respond to a complete-split transaction with the results of a full-/low-speed
transaction that it has completed. This means that the high-speed handler must search to find the correct
state tracking entry among several possible complete-split response entries. This searching takes time. The
high-speed handler only needs to search the complete-split responses accumulated during the previous
microframe. There only needs to be at most 1 microframe of complete-split response entries; the
microframe of responses that have already been accumulated and are awaiting to be returned via high-speed
complete-splits.
The split transaction protocol is defined to allow the high-speed handler to timeout the first high-speed
complete-split transaction while it is searching for the correct response. This allows the high-speed handler
time to complete its search and respond correctly to the next (retried) complete-split.
The following interrupt and isochronous flow sequence figures show these cases with the transitions labeled
“Search not complete in time” and “No split response found”.
The high-speed handler matches the complete-split transaction with the correct entry in the complete-split
pipeline stage and advances the pipeline appropriately. There are five cases the TT must handle correctly:
1. If the high-speed complete-split token and first entry of the complete-split pipeline match, the high-speed
handler responds with the indicated data/status. This case occurs the first time the TT receives a
complete-split.
381
Universal Serial Bus Specification Revision 2.0
2. Same as above, but this is a retry of a complete-split that the TT has already received due to the host
controller not receiving the (previous) response information.
3. If the complete-split transaction matches some other entry in the complete-split pipeline besides the first,
the high-speed handler advances the complete-split pipeline (e.g., frees response information for previous
complete-split entries) and responds with the information for the matching entry. This case can happen
due to normal or missed previous complete-split transactions. An example abnormal case could be that
the host controller was unsuccessful in issuing a complete-split transaction to the high-speed handler and
has done endpoint halt processing for that endpoint. This means the next complete-split will not match
the first entry of the complete-split pipeline stage.
4. The high-speed handler can also receive a complete-split before it has started a full-/low-speed
transaction. If there is not an entry in the complete-split pipeline, the high-speed handler responds with a
NYET handshake to inform the host that it has no status information. When the host issues the last
scheduled complete-split for this endpoint for this frame, it must interpret the NYET as an error
condition. This stimulates the normal “three strikes” error handling. If there have been more than three
errors, the host halts this endpoint. If there have been less than three errors, the host continues processing
the scheduled transactions of this endpoint (e.g., a start-split will be issued as the next transaction for this
endpoint at the next scheduled time for this endpoint). Note that a NYET response is possible in this case
due to a transaction error on the start-split or a host (or TT) scheduling error.
5. The high-speed handler can timeout its first high-speed complete-split transaction while it is searching the
complete-split pipeline stage for a matching entry. However, the high-speed handler must respond
correctly to the subsequent complete-split transaction. If the high-speed handler did not respond correctly
for an interrupt IN after it had acknowledged the full-/low-speed transaction, the endpoint software and
the device would lose data synchronization and more catastrophic errors could occur.
The host controller must issue the complete-split transactions in the same relative order as the original
corresponding start-split transactions.
11.19 Approximate TT Buffer Space Required
A transaction translator requires buffer and state tracking space for its periodic and non-periodic portions.
The TT microframe pipeline requires less than:
•
752 data bytes for the start-split stage
•
2x 188 data bytes for the complete-split stage
•
16x 4x transaction status (<4 bytes for each transaction) for start-split stage
•
16x 2x transaction status (<4 bytes for each transaction) for complete-split stage
There are, at most, 4 microframes of buffering required for the start-split stage of the pipeline and, at most,
2 microframes of buffering for the complete-split stage of the pipeline. There are, at most, 16 full-speed
(minimum sized) transactions possible in any microframe.
The non-periodic portion of the TT requires at least:
•
2x (64 data + 4 transaction status) bytes
Different implementations may require more or less buffering and state tracking space.
11.20 Interrupt Transaction Translation Overview
The flow sequence and state machine figures show the transitions required for high-speed split transactions
for full-/low-speed interrupt transfer types for a single endpoint. These figures must not be interpreted as
showing any particular specific timing. In particular, high-speed or full-/low-speed transactions for other
endpoints may occur before or after these split transactions. Specific details are described as appropriate.
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Universal Serial Bus Specification Revision 2.0
In contrast to bulk/control processing, the full-/low-speed handler must not do local retry processing on the
full-/low-speed bus in response to a transaction error for full-/low-speed interrupt transactions.
11.20.1 Interrupt Split Transaction Sequences
The interrupt IN and OUT flow sequence figures use the same notation and have descriptions similar to the
bulk/control figures.
In contrast to bulk/control processing, the full-speed handler must not do local retry processing on the fullspeed bus in response to a transaction errors (including timeout) of an interrupt transaction.
Start split
st1
SSPLIT
st2
OUT
sd1
DATA0/1
not trans_err,
Data_into_SS_pipe
Trans_err
se1
Trans_err
se2
sh1
Go to
comp. split
Host
TT
Figure 11-68. Interrupt OUT Start-split Transaction Sequence
383
Universal Serial Bus Specification Revision 2.0
Complete split
ct1
CSPLIT
Trans_err
ct2
OUT
Fast_match
Search not complete in time
No split response found
old/stall
ch1
STALL
old/ack
old/nak
ch2
ch3
ACK
ch4
NAK
Trans_err
old/trans_err
ch5
ERR
NYET
Last
ce2
Endpoint
halt
Go to next
cmd
Retry
start split
Host
ce1
ce7
Inc err
count
Inc err
Not last count
ch6
Next
comp. split
TT
ce3
ce4
if err_count < 3
retry start split
ce6
ce5
if err_count < 3
retry immed.
comp. split
if err_count >= 3
endpoint halt
Figure 11-69. Interrupt OUT Complete-split Transaction Sequence
384
Universal Serial Bus Specification Revision 2.0
Start split
st1
SSPLIT
Trans_err
st2
IN
se1
Data_into_SS_pipe
Go to
comp. split
Host
TT
Figure 11-70. Interrupt IN Start-split Transaction Sequence
Complete split
ct1
Trans_err
CSPLIT
ct2
IN
Fast_match
Host
Search not
complete in time
old/moredata
Trans_err cd2
MDATA
No split response found
old/lastdata
cd1
old/nak
ch1
DATA0/1
NAK
Trans_ not trans_err
ch5
err
ce1
HC_Accept_data
Trans_
Next
comp.
err
ce4
Split
ce5
Inc err count
ce7
if err_count >= 3
endpoint halt
if err_count < 3
retry immed.
comp. split
old/stall
ch2
STALL
Endpoint
Retry
start split halt
not
trans_err,
Datax =
toggle
ch7
ce8
TT
old/trans_err
ch3
ERR
ch4
NYET
Last
ce3
Inc err
not
count
trans_err,
ce2
Datax /=
toggle
ce6
ch8
if err_count < 3
Not last
ch6
Next
comp. split
retry start split
Retry
Go to next cmd start split
ce9
HC_Accept_data HC_reject_data
if err_count >= 3
endpoint halt
Figure 11-71. Interrupt IN Complete-split Transaction Sequence
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Universal Serial Bus Specification Revision 2.0
11.20.2 Interrupt Split Transaction State Machines
st1
Issue_packet(HSD1, SSPLIT);
DoOUTSS
st2
Issue_packet(HSD1, tokenOUT);
DodataSS
sd1
Issue_packet(HSD1, DATAx);
Doupdate
sh1
RespondHC(Do_complete);
HC_Do_IntOSS
Figure 11-72. Interrupt OUT Start-split Transaction Host State Machine
386
Universal Serial Bus Specification Revision 2.0
HSU2.PID = NAK
ch3
RespondHC(Do_start);
ct1
HSU2.PID = STALL
ch1
RespondHC(Do_halt);
&
ch2
Issue_packet(HSD1, CSPLIT);
ch5
HSU2.PID = ACK
RespondHC(Do_next_cmd);
HSU2.PID = NYET
Dooutintcs
ch6
&
ce2
not HC_cmd.last
RespondHC(Do_next_complete);
ce7
ct2
ce1
HC_cmd.last
Issue_packet(HSD1, tokenOUT);
ErrorCount < 3
HSU2.PID = ERR
RespondHC(Do_start);
Packet_ready(HSU2)
ce3
ICSO_error_2
(HSU2.PID /= STALL and
HSU2.PID /= NAK and
HSU2.PID /= ACK and
HSU2.PID /= ERR and
HSU2.PID /= NYET) or
HSU2.timeout
RespondHC(Do_halt);
IncError;
ce4
ErrorCount >= 3
&
ErrorCount >= 3
ce5
ICSO_wait
Wait_for_packet(HSU2, ITG);
ErrorCount < 3
RespondHC(Do_comp_immed_now);
ICSO_error
IncError;
ce6
HC_Do_IntOCS
Figure 11-73. Interrupt OUT Complete-split Transaction Host State Machine
387
Universal Serial Bus Specification Revision 2.0
HSD2.PID /= DATAx or
HSD2.timeout
se1
&
HSD2.PID = DATAx
Packet_ready(HSD2)
HSD2.CRC16 = ok
sh1
&
TT_IntOSS_wait
Wait_for_packet(
HSD2, ITG);
Data_into_SS_pipe;
se2
HSD2.CRC16 = bad
TT_Do_IntOSS
Figure 11-74. Interrupt OUT Start-split Transaction TT State Machine
388
Universal Serial Bus Specification Revision 2.0
CS_Buff.match.down_result = r_trans_err
Issue_packet(HSU1, ERR);
ch4
ch3
CS_Buff.match.down_result = r_nak
&
Issue_packet(HSU1, NAK);
ch2
CS_Buff.match.down_result = r_ack
ch1
CS_Buff.match.state = old
Issue_packet(HSU1, ACK);
CS_Buff.match.down_result = r_stall
Issue_packet(HSU1, STALL);
ch5
CS_Buff.match.state = no_match
TT_IntOCS_match
Issue_packet(HSU1, NYET);
ce1
Fast_match;
CS_Buff.match.state = match_busy
TT_Do_IntOCS
Figure 11-75. Interrupt OUT Complete-split Transaction TT State Machine
st1
Issue_packet(HSD1, SSPLIT);
DoinINSS
st2
Issue_packet(HSD1, tokenIN);
DoinupdateSS
RespondHC(Do_complete);
HC_Do_IntISS
Figure 11-76. Interrupt IN Start-split Transaction Host State Machine
389
Universal Serial Bus Specification Revision 2.0
HC_IntICS_error
ce2
HSU2.PID = ERR
ce6
IncError;
ce9
ce3
HC_cmd.last
ErrorCount < 3
RespondHC(Do_start);
ErrorCount >= 3
&
RespondHC(Do_halt);
ch6
not HC_cmd.last
RespondHC(Do_next_complete);
ct1
Issue_packet(HSD1, CSPLIT);
HSU2.PID = NYET
HSU2.PID = NAK
RespondHC(Do_start);
ch2
DoinINcs
HSU2.PID = STALL
ct2
RespondHC(Do_halt);
Issue_packet(HSD1, tokenIN);
HC_Data_or_error
ch4
ch1
HC_IntICS_wait
Wait_for_packet(
HSU2, ITG);
Packet_ready(HSU2)
&
(HSU2.PID /= NAK and
HSU2.PID /= STALL and
HSU2.PID /= NYET and
HSU2.PID /= ERR) or
HSU2.timeout
HC_Do_IntICS
Figure 11-77. Interrupt IN Complete-split Transaction Host State Machine
390
Universal Serial Bus Specification Revision 2.0
HSU2.x = HC_cmd.toggle
RespondHC(Do_next_cmd);
RespondHC(Do_start);
HSU2.x /= HC_cmd.toggle
Dostartss
HC_Reject_data;
ch7
ch8
Acceptdata
ErrorCount >= 3
RespondHC(Do_next_complete);
HSU2.PID = DATAx and
HSU2.CRC16 = ok
RespondHC(Do_halt);
ErrorCount < 3
Docmpl
RespondHC(Do_comp_immed_now);
HC_Accept_data;
ce8
HSU2.PID = MDATA and
HSU2.CRC16 = ok
ce7
HC_Accept_data;
ch5
(HSU2.PID = MDATA or
HSU2.PID = DATAx) and
HSU2.CRC16 = bad
ce4/ce5
ce1
HC_IntICS_err3
IncError;
(HSU2.PID /= MDATA and
HSU2.PID /= DATAx) or
HSU2.timeout
HC_Data_or_error
Figure 11-78. HC_Data_or_Error State Machine
Data_into_SS_pipe;
TT_Do_IntISS
Figure 11-79. Interrupt IN Start-split Transaction TT State Machine
391
Universal Serial Bus Specification Revision 2.0
CS_Buff.match.down_result = r_moredata
Issue_packet(HSU1, MDATA);
CS_Buff.match.down_result = r_lastdata
cd2
Issue_packet(HSU1, DATAx);
cd1
&
ch3
ch1
CS_Buff.match.down_result = r_trans_err
Issue_packet(HSU1,ERR);
CS_Buff.match.down_result = r_nak
Issue_packet(HSU1, NAK);
CS_Buff.match.state = old
ch2
CS_Buff.match.down_result = r_stall
Issue_packet(HSU1, STALL);
CS_Buff.match.state = no_match
ch4
Issue_packet(HSU1, NYET);
TT_IntICS_match
Fast_match;
ce1
CS_Buff.match.state = match_busy
TT_Do_IntICS
Figure 11-80. Interrupt IN Complete-split Transaction TT State Machine
11.20.3 Interrupt OUT Sequencing
Interrupt OUT split transactions are scheduled by the host controller as normal high-speed transactions with
the start- and complete-splits scheduled as described previously.
When there are several full-/low-speed transactions allocated for a given microframe, they are saved by the
high-speed handler in the TT in the start-split pipeline stage. The start-splits are saved in the order they are
received until the end of the microframe. At the end of the microframe, these transactions are available to
be issued by the full-/low-speed handler on the full-/low-speed bus in the order they were received.
In a following microframe (as described previously), the full-/low-speed handler issues the transactions that
had been saved in the start-split pipeline stage on the downstream facing full-/low-speed bus. Some
transactions could be leftover from a previous microframe since the high-speed schedule was built assuming
best case bit stuffing and the full-/low-speed transactions could be taking longer on the full-/low-speed bus.
As the full-/low-speed handler issues transactions on the downstream facing full-/low-speed bus, it saves the
results in the periodic complete-split pipeline stage and then advances to the next transaction in the startsplit pipeline.
In a following microframe (as described previously), the host controller issues a high-speed complete-split
transaction and the TT responds appropriately.
392
Universal Serial Bus Specification Revision 2.0
High
Speed SS
Bus
C
R
C
1
6
64 bytes w/
HS CRC16
125us microframe
Full/LowSpeed
Bus
Int. OUT
data packet
C
R
C
1
6
64 bytes
Figure 11-81. Example of CRC16 Handling for Interrupt OUT
The start-split transaction for an interrupt OUT transaction includes a normal CRC16 field for the highspeed data packet of the data phase of the start-split transaction. However, the data payload of the data
packet contains only the data payload of the corresponding full-/low-speed data packet; i.e., there is only a
single CRC16 in the data packet of the start-split transaction. The TT high-speed handler must check the
CRC on the start-split and ignore the start-split if there is a failure in the CRC check of the data packet. If
the start-split has a CRC check failure, the full-speed transaction must not be started on the downstream bus.
Figure 11-81 shows an example of the CRC16 handling for an interrupt OUT transaction and its start-split.
11.20.4 Interrupt IN Sequencing
When the high-speed handler receives an interrupt start-split transaction, it saves the packet in the start-split
pipeline stage. In this fashion, it accumulates some number of start-split transactions for a following
microframe.
At the beginning of the next microframe (as described previously), these transactions are available to be
issued by the full-/low-speed handler on the downstream full-/low-speed bus in the order they were saved in
the start-split pipeline stage. The full-/low-speed handler issues each transaction on the downstream facing
bus. The full-/low-speed handler responds to the full-/low-speed transaction with an appropriate handshake
as described in Chapter 8. The full-/low-speed handler saves the results of the transaction (data, NAK,
STALL, trans_err) in the complete-split pipeline stage.
During following microframes, the host controller issues high-speed complete-split transactions to retrieve
the data/handshake from the high-speed handler. When the high-speed handler receives s complete-split
transaction, the TT returns whatever data it has received during a microframe. If the full-/low-speed
transaction was started and completed in a single microframe, the TT returns all the data for the transaction
in the complete-split response occurring in the following microframe. If the full-/low-speed CRC check
passes, the appropriate DATA0/1 PID for the data packet is used. If the full-/low-speed CRC check fails, an
ERR handshake is used and there is no data packet as part of the complete-split transaction.
If the full-/low-speed transaction spanned a microframe, the TT requires two complete-splits (in two
subsequent microframes) to return all the data for the full-/low-speed transaction. The data packet PID for
the first complete-split must be an MDATA to tell the host controller that another complete-split is required
for this endpoint. This MDATA response is made without performing a CRC check (since the CRC16 field
has not yet been received on the full-/low-speed bus). The complete-split in the next microframe must use a
DATA0/1 PID if the CRC check passes. If the CRC check fails, an ERR handshake response is made
instead and there is no data packet as part of the complete-split transaction. Since full-speed interrupt
transactions are limited to 64 data bytes or less (and low-speed interrupt transactions are limited to 8 data
393
Universal Serial Bus Specification Revision 2.0
bytes or less), no full-/low-speed interrupt transaction can span more than a single microframe boundary;
i.e., no more than two microframes are ever required to complete the transaction.
The complete-split transaction for an interrupt IN transaction must not include the CRC16 field received
from the full-/low-speed data packet (i.e., only a high-speed CRC16 field is used in split transactions). The
TT must use a high-speed CRC16 on each complete-split data packet. If the full-speed handler detects a
failed CRC check, it must use an ERR handshake response in the complete-split transaction to reflect that
error to the high-speed host controller. The host controller must check the CRC16 on each returned
complete-split data packet. A CRC failure (or ERR handshake) on any (partial) complete-split is reflected
as a CRC failure on the total full-/low-speed transaction. This means that for a case where a full-/low-speed
interrupt spans a microframe boundary, the host controller can accept the first complete-split without
errors, then the second complete-split can indicate that the data from the first complete-split must be
rejected as if it were never received by the host controller. Figure 11-82 shows an example of an interrupt
IN and its CRC16 handling with corresponding complete-split responses.
High
Speed
Bus
CS
NYET
64 bytes w/
HS CRC16
125us microframe
Full/LowSpeed
Bus
C
R
C
1
6
Int. IN
data packet
2 bytes
C
R
C
1
6
62 bytes
Figure 11-82. Example of CRC16 Handling for Interrupt IN
11.21 Isochronous Transaction Translation Overview
Isochronous split transactions are handled by the host by scheduling start- and complete-split transactions as
described previously. Isochronous IN split transactions have more than two schedule entries:
•
One entry for the start-split transaction in the microframe before the earliest the full-speed transaction
can occur
•
Other entries for the complete-splits in microframes after the data can occur on the full-speed bus
(similar to interrupt IN scheduling)
Furthermore, isochronous transactions are split into microframe sized pieces; e.g., a 300 byte full-speed
transaction is budgeted multiple high-speed split transactions to move data to/from the TT. This allows any
alignment of the data for each microframe.
Full-speed isochronous OUT transactions issued by a TT do not have corresponding complete-split
transactions. They must only have start-split transaction(s).
The host controller must preserve the same order for the complete-split transactions (as for the start-split
transactions) for IN handling.
394
Universal Serial Bus Specification Revision 2.0
Isochronous INs have start- and complete- split transactions. The “first” high-speed split transaction for a
full-speed endpoint is always a start-split transaction and the second (and others as required) is always a
complete-split no matter what the high-speed handler of the TT responds.
The full-/low-speed handler recombines OUT data in its local buffers to recreate the single full-speed data
transaction and handle the microframe error cases. The full-/low-speed handler splits IN response data on
microframe boundaries.
Microframe buffers always advance no matter what the interactions with the host controller or the full-speed
handler.
11.21.1 Isochronous Split Transaction Sequences
The flow sequence and state machine figures show the transitions required for high-speed split transactions
for a full-speed isochronous transfer type for a single endpoint. These figures must not be interpreted as
showing any particular specific timing. In particular, high-speed or full-speed transactions for other
endpoints may occur before or after these split transactions. Specific details are described as appropriate.
In contrast to bulk/control processing, the full-speed handler must not do local retry processing on the fullspeed bus in response to transaction errors (including timeout) of an isochronous transaction.
Start split
If all of
payload
If beginning
of payload
st1
st2
SSPLIT-all
If middle
of payload
If last
of payload
st3
st4
SSPLIT -begin SSPLIT -mid
SSPLIT -end
st5
OUT
sd1
DATA0
not trans_err,
Data_into_SS_pipe
sh1
Host
TT
Trans_err
se1
Trans_err
Down_error
se2
Go to next
cmd
Figure 11-83. Isochronous OUT Start-split Transaction Sequence
395
Universal Serial Bus Specification Revision 2.0
Start split
st1
SSPLIT
Trans_err
st2
IN
se1
Data_into_SS_pipe
Host
TT
Go to
complete split
Figure 11-84. Isochronous IN Start-split Transaction Sequence
In Figure 11-85, the high-speed handler returns an ERR handshake for a “transaction error” of the full-speed
transaction.
The high-speed handler returns an NYET handshake when it cannot find a matching entry in the completesplit pipeline stage. This handles the case where the host controller issued the first high-speed completesplit transaction, but the full-/low-speed handler has not started the transaction yet or has not yet received
data back from the full-speed device. This can be due to a delay from starting previous full-speed
transactions.
The transition labeled "TAdvance" indicates that the host advances to the next transaction for this full-speed
endpoint.
The transition labeled "DAdvance" indicates that the host advances to the next data area of the current
transaction for the current full-speed endpoint.
396
Universal Serial Bus Specification Revision 2.0
Complete split
ct1
CSPLIT
Trans_err
ct2
IN
Fast_,match
ce1
Search not complete
in time
Trans_err
ce7
Inc err
count
ce3
old/Trans_err
ce2
ERR
ce4
if err_count < 3
retry immed.
comp. split
Host
TT
No split response found
old/lastdata
cd1
DATA0
old/moredata
cd2
MDATA
ch4
NYET
Not trans_err
not
Trans_err DAdvance
ch2
trans_err
Last
last
to ce7 ce8
ce6
if err_count >= 3
Record error
ch1
Not last
ch4
Not last
ch3
TAdvance
Go to next
cmd
Go to next
comp. split
Figure 11-85. Isochronous IN Complete-split Transaction Sequence
397
Universal Serial Bus Specification Revision 2.0
11.21.2 Isochronous Split Transaction State Machines
HC_cmd.datapart = alldata
Issue_packet(HSD1, SSPLIT); -- all
HC_cmd.datapart = enddata
st1
st4
Issue_packet(HSD1, SSPLIT); -- end
DoOUTisSS
st2
st3
HC_cmd.datapart = begindata
Issue_packet(HSD1, SSPLIT); -- begin
st5
HC_cmd.datapart = middata
Issue_packet(HSD1, SSPLIT); -- middata
Issue_packet(HSD1, tokenOUT);
DoDATAisSS
sd1
Issue_packet(HSD1, DATAx);
DonxtcmdSS
sh1
RespondHC(Do_next_cmd);
HC_Do_IsochOSS
Figure 11-86. Isochronous OUT Start-split Transaction Host State Machine
398
Universal Serial Bus Specification Revision 2.0
HSD2.PID = DATAx and HSD2.CRC16 = ok and
split.datapart = enddata and SS_Buff.isochO and
(SS_Buff.lastdata = middata or
SS_Buff.lastdata = begindata)
Doend
SS_Buff.isochO <= false;
SS_Buff.saw_split <=false;
Data_into_SS_pipe;
HSD2.PID = DATAx and HSD2.CRC16 = ok and
split.datapart = middata and SS_Buff.isochO and
(SS_Buff.lastdata = begindata or
SS_Buff.lastdata = middata)
Didmid
SS_Buff.lastdata <= middata;
SS_Buff.saw_split <= true;
Data_into_SS_pipe;
st3
HSD2.PID = DATAx and
HSD2.CRC16 = ok and
split.datapart = begindata and
(not SS_Buff.isochO)
Didbegin
st4
SS_Buff.isochO <= true;
SS_Buff.lastdata <= begindata;
SS_Buff.saw_split <= true;
Data_into_SS_pipe;
st2
st1
HSD2.PID = DATAx and HSD2.CRC16 = ok and
split.datapart = alldata and (not SS_Buff.isochO)
&
se1/se2
Packet_ready(HSD2)
HSD2.PID /= DATAx or
HSD2.timeout or
HSD2.CRC16 = bad or
Bad_IsochOut(SS_Buff, split)
Didall
Data_into_SS_pipe;
not SS_Buff.isochO
TT_IsochOSS_wait
Wait_for_packet(
HSD2, ITG);
SS_Buff.isochO
Didbad
SS_Buff.isochO <= false;
Down_error;
TT_Do_IsochOSS
Figure 11-87. Isochronous OUT Start-split Transaction TT State Machine
There is a condition in Figure 11-87 on transition se1/se2 labeled “Bad_IsochOut”. This condition is true
when none of the conditions on transitions st1 through st4 are true. The action labeled “Down_error”
records an error to be indicated on the downstream facing full-speed bus for the transaction corresponding
to this start-split.
399
Universal Serial Bus Specification Revision 2.0
st1
Issue_packet(HSD1, SSPLIT);
DoisINSS
st2
Issue_packet(HSD1, tokenIN);
DonxtisSS
RespondHC(Do_complete);
HC_Do_IsochISS
Figure 11-88. Isochronous IN Start-split Transaction Host State Machine
400
Universal Serial Bus Specification Revision 2.0
HSU2.PID = DATAx and
HSU2.CRC16 = ok
DoisnxtCS
HC_Accept_data;
HSU2.PID = MDATA and
HSU2.CRC16 = ok
cd1
RespondHC(Do_next_cmd);
not HC_cmd.last
ch3
&
RespondHC(Do_next_complete);
HC_Accept_data;
ce6/ce8
ch2
RespondHC(Do_next_cmd);
ch4
&
ce2
HSU2.PID = NYET
HC_cmd.last
ce5
ce7
HSU2.PID = ERR
HC_IsochICS_error
Record_error;
(HSU2.PID = MDATA or
HSU2.PID = DATAx) and
HSU2.CRC16 = bad
Packet_ready(HSU2)
ErrorCount < 3
RespondHC(Do_comp_immed_now);
HC_IsochICS_wait
ErrorCount >= 3
Wait_for_packet(
HSU2, ITG);
Issue_packet(HSD1, tokenIN);
ct2
ce3
ce4
(HSU2.PID /= NYET and
HSU2.PID /= DATAx and
HSU2.PID /= MDATA and
HSU2.PID /= ERR) or
HSU2.timeout
HC_IsochICS_err2
IncError;
DoisINCS
ct1
Issue_packet(HSD1, CSPLIT);
HC_Do_IsochICS
Figure 11-89. Isochronous IN Complete-split Transaction Host State Machine
In Figure 11-89, the transition “ce8” occurs when the high-speed handler responds with an MDATA to
indicate there is more data for the full-speed transaction, but the host controller knows that this is the last
scheduled complete-split for this endpoint for this frame. If a DATA0 response from the high-speed
handler is not received before the last scheduled complete-split, the host controller records an error and
proceeds to the next transaction for this endpoint (in the next frame).
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Universal Serial Bus Specification Revision 2.0
Data_into_SS_pipe;
TT_Do_IsochISS
Figure 11-90. Isochronous IN Start-split Transaction TT State Machine
token.PID /= tokenIN or
token.timeout
ce1
cd2
&
CS_Buff.match.down_result = r_moredata
cd1
CS_Buff.match.state = old
ce2
Issue_packet(HSU1, MDATA);
CS_Buff.match.down_result = r_lastdata
Issue_packet(HSU1, DATAx); -- Data0
TT_IsochICS_1
CS_Buff.match.down_result = r_trans_err
Fast_match;
Issue_packet(HSU1, ERR);
ch4
token.PID = tokenIN
ce7
&
CS_Buff.match.state = no_match
Issue_packet(HSU1, NYET);
CS_Buff.match.state = match_busy
TT_IsochICS
Figure 11-91. Isochronous IN Complete-split Transaction TT State Machine
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Universal Serial Bus Specification Revision 2.0
11.21.3 Isochronous OUT Sequencing
The host controller and TT must ensure that errors that can occur in split transactions of an isochronous fullspeed transaction translate into a detectable error. For isochronous OUT split transactions, once the highspeed handler has received an “SSPLIT-begin” start-split transaction token packet, the high-speed handler
must track start-split transactions that are received for this endpoint. The high-speed handler must track that
a start-split transaction is received each and every microframe until an “SSPLIT-end” split transaction token
packet is received for this endpoint. If a microframe passes without the high-speed handler receiving a
start-split for this full-speed endpoint, it must ensure that the full-speed handler forces a bitstuff error on the
full-speed transaction. Any subsequent “SPLIT-middle” or “SPLIT-end” start-splits for the same endpoint
must be ignored until the next non “SPLIT-middle” and non “SPLIT-end” is received (for any endpoint
supported by this TT).
The start-split transaction for an isochronous OUT transaction must not include the CRC16 field for the fullspeed data packet. For a full-speed transaction, the host would compute the CRC16 of the data packet for
the full data packet (e.g., a 1023 byte data packet uses a single CRC16 field that is computed once by the
host controller). For a split transaction, any isochronous OUT full-speed transaction is subdivided into
multiple start-splits, each with a data payload of 188 bytes or less. For each of these start-splits, the host
computes a high-speed CRC16 field for each start-split data packet. The TT high-speed handler must check
each high-speed CRC16 value on each start-split. The TT full-speed handler must locally generate the
CRC16 value for the complete full-speed data packet. Figure 11-92 shows an example of a full-speed
isochronous OUT packet and the high-speed start-splits with their CRC16 fields.
If there is a CRC check failure on the high-speed start-split, the high-speed handler must indicate to the fullspeed handler that there was an error in the start-split for the full-speed transaction. If the transaction has
been indicated as having a CRC failure (or if there is a missed start-split), the full-speed handler uses the
defined mechanism for forcing a downstream corrupted packet. If the first start-split has a CRC check
failure, the full-speed transaction must not be started on the downstream bus.
Additional high-speed start-split transactions for the same endpoint must be ignored after a CRC check fails,
until the high-speed handler receives either an “SSPLIT-end” start-split transaction token packet for that
endpoint or a start-split for a different endpoint.
High
Speed SS
Bus
C
R
C
1
6
188 byte w/
HS CRC16
C
R
C
1
6
SS
2 bytes w/
HS CRC16
125us microframe
Full
Speed
Bus
C
R
C
1
6
Isoch. OUT
data packet
188 bytes
2 bytes
Figure 11-92. Example of CRC16 Isochronous OUT Data Packet Handling
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Universal Serial Bus Specification Revision 2.0
11.21.4 Isochronous IN Sequencing
The complete-split transaction for an isochronous IN transaction must not include the CRC16 field for the
full-speed data packet (e.g., only a high-speed CRC16 field is used in split transactions). The TT must not
pass the full-speed value received from the device and instead only use high-speed CRC16 values for
complete-split transactions. If the full-speed handler detects a failed CRC check at the end of the data
packet (e.g., after potentially several complete-split transactions on high-speed), the handler must use an
ERR handshake response to reflect that error to the high-speed host controller. The host controller must
check the CRC16 on each returned high-speed complete-split. A CRC failure (or ERR handshake) on any
(partial) complete-split is reflected by the host controller as a CRC failure on the total full-speed transaction.
Figure 11-93 shows an example of the relationships of the full-speed data packet and the high-speed
complete-splits and their CRC16 fields.
High
Speed
Bus
CS
125us microframe
Full
Speed
Bus
C
R
C
1
6
2+186 bytes w/
HS CRC16
1 byte w/
HS CRC16
Isoch.IN
data packet
3 bytes
CS
C
R
C
1
6
CS
C
R
C
1
6
1 byte w/
HS CRC16
C
R
C
1
6
188 bytes
Figure 11-93. Example of CRC16 Isochronous IN Data Packet Handling
11.22 TT Error Handling
The TT has the same requirements for handling errors as a host controller or hub. In particular:
•
If the TT is receiving a packet at EOF2 of the downstream facing bus, it must disable the downstream
facing port that is currently transmitting.
•
If the TT is transmitting a packet near EOF1 of the downstream facing bus, it must force an abnormal
termination sequence as defined in Section 11.3.3 and stop transmitting.
•
If the TT is going to transmit a non-periodic full-/low-speed transaction, it must determine that there is
sufficient time remaining before EOF1 to complete the transaction. This determination is based on
normal sequencing of the packets in the transaction. Since the TT has no information about data
payload size for INs, it must use the maximum allowed size allowed for the transfer type in its
determination. Periodic transactions do not need to be included in this test since the microframe
pipeline is maintained separately.
11.22.1 Loss of TT Synchronization With HS SOFs
The hub has a timer it uses for (micro)frame maintenance. It has a 1 ms frame timer when operating at full/low-speed for enforcing EOF with downstream connected devices. It has a 125 µs microframe timer when
operating at high-speed for enforcing EOF with high-speed devices. It also uses the 125 µs microframe
timer to create a 1 ms frame timer for enforcing EOF with downstream full-/low-speed devices when
operating at high-speed. The hub (micro)frame timer must always stay synchronized with host generated
SOFs to keep the bus operating correctly
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Universal Serial Bus Specification Revision 2.0
In normal hub repeater (full- or high-speed) operation (e.g., not involving a TT), the (micro)frame timer
loses synchronization whenever it has missed SOFs for three consecutive microframes. While timer
synchronization is lost, the hub does not establish upstream connectivity. Downstream connectivity is
established normally, even when timer synchronization is lost. When the timer is synchronized, the hub
allows upstream connectivity to be established when required. The hub is responsible for ensuring that
there is no signaling being repeated/transmitted upstream from a device after the EOF2 point in any
(micro)frame. The hub must not establish upstream connectivity if it has lost (micro)frame timer
synchronization since it no longer knows accurately where the EOF2 point is.
11.22.2 TT Frame and Microframe Timer Synchronization Requirements
When the hub is operating at high-speed and has full-/low-speed devices connected on its downstream
facing ports (e.g., a TT is active), the hub has additional responsibilities beyond enforcement of the (highspeed) EOF2 point on its upstream facing port in every microframe. The TT must also generate full-speed
SOFs downstream and ensure that the TT operates correctly (in bridging high-speed and full-/low-speed
operation).
A high-speed operating hub synchronizes its microframe timer to 125 µs SOFs. However, in order to
generate full-speed downstream SOFs, it must also have a 1 ms frame timer. It generates this 1 ms frame
timer by recognizing zeroth microframe SOFs, e.g., a high-speed SOF when the frame number value
changes compared to SOF of the immediately previous microframe.
In order to create the 1 ms frame timer, the hub must successfully receive a zeroth microframe SOF after its
microframe timer is synchronized. In order to recognize a zeroth microframe SOF, the hub must
successfully receive SOFs for two consecutive microframes where the frame number increments by 1 (mod
2^11). When the hub has done this, it knows that the second SOF is a zeroth microframe SOF and thereby
establishes a 1 ms frame timer starting time. Note that a hub can synchronize both timers with as few as
two SOFs if the SOFs are for microframe 7 and microframe 0, i.e., if the second SOF is a zeroth
microframe SOF.
Once the hub has synchronized its 1 ms frame timer, it can keep that timer synchronized as long as it keeps
its 125 µs microframe timer synchronized (since it knows that every 8 microframes from the zeroth
microframe SOF is a 1 ms frame). In particular, the hub can keep its frame timer synchronized even if it
misses zeroth microframe SOFs (as long as the microframe timer stays synchronized).
So in summary, the hub can synchronize its 125 µs microframe timer after receiving SOFs of two
consecutive microframes. It synchronizes its 1 ms frame timer when it receives a zeroth microframe SOF
(and the microframe timer is synchronized). The 125 µs microframe timer loses synchronization after three
SOFs for consecutive microframes have been missed. This also causes the 1 ms frame timer to lose
synchronization at the same time.
The TT must only generate full-speed SOFs downstream when its 1 ms frame timer is synchronized.
Correct internal operation of the TT is dependent on both timers. The TT must accurately know when
microframes occur to enforce its microframe pipeline abort/free rules. It knows this based on a
synchronized microframe timer (for generally incrementing the microframe number) and a synchronized
frame timer (to know when the zeroth microframe occurs).
Since loss of microframe timer synchronization immediately causes loss of frame timer synchronization, the
TT stops normal operation once the microframe timer loses synchronization. In an error free environment,
microframe timer synchronization can be restored after receiving the two SOFs for the next two consecutive
microframes (e.g., synchronization is restored at least 250 µs after synchronization loss). As long as SOFs
are not missed, frame timer synchronization will be restored in less than 1 ms after microframe
synchronization. Note that frame timer synchronization can be restored in a high-speed operating case in
much less time (0.250-1.250 ms) than the 2-3 ms required in full-speed operation. Once the frame timer is
synchronized, SOFs can be issued on downstream facing full-speed ports for the beginning of the next
frame.
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Universal Serial Bus Specification Revision 2.0
Once the hub detects loss of microframe timer synchronization, its TT(s):
1.
Must respond to periodic complete-splits with any responses buffered in the periodic pipeline (only
good for at most 1 microframe of complete-splits).
2.
Must abort any buffered periodic start-split transactions in the periodic pipeline.
3.
Must ignore any high-speed periodic start-splits.
4.
Must stop issuing full-speed SOFs on downstream facing full-speed ports (and low-speed keep-alives
on low-speed ports).
5.
Must not start issuing subsequent periodic full-/low-speed transactions on downstream facing full-/lowspeed ports.
6.
Must respond to high-speed start-split bulk/control transactions.
7.
Buffered bulk/control results must respond to high-speed complete-split transactions.
8.
Pending bulk/control transactions must not be issued to full-/low-speed downstream facing ports. The
TT buffers used to hold bulk/control transactions must be preserved until the microframe timer is resynchronized. (Or until a Clear_TT_Buffer request is received for the transaction).
Note that in any case a TT must not issue transactions of any speed on downstream facing ports when its
upstream facing port is suspended.
A TT only restores normal operation on downstream facing full-/low-speed ports after both microframe and
frame timers are synchronized. Figure Figure 11-94 summarizes the relationship between high-speed SOFs
and the TT frame and microframe timer synchronization requirements on start-splits.
For suspend sequencing of a hub, a hub will first lose microframe/frame timer synchronization at the same
time. This will cause its TT(s) to stop issuing SOFs (which should be the only transactions keeping the
downstream facing full-/low-speed ports out of suspend). Then the hub (along with any downstream
devices) will enter suspend.
Upon a resume, the hub will first restore its microframe timer synchronization (after high-speed transactions
continue). Then in less than 1 ms (assuming no errors), the frame timer will be synchronized and the TT
can start normal operation (including SOFs/keep-alives on downstream facing full-/low-speed ports).
Microframes
Y0
SOF
Y1
No
SOF
Y2
No
SOF
Y3
No
SOF
Lose Microframe & Frame
Timer Synchronization,
Ignore start-splits
Y4
SOF
Y5
SOF
Y6
SOF
Microframe Timer
Re-synchronized;
Frame timer unsynchronized,
Ignore start-splits
Y7
SOF
SOF
Microframe & Frame
Timer Synchronized,
Accept start-splits
Figure 11-94. Example Frame/Microframe Synchronization Events
406
(Y+1)0
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Universal Serial Bus Specification Revision 2.0
11.23 Descriptors
Hub descriptors are derived from the general USB device framework. Hub descriptors define a hub device
and the ports on that hub. The host accesses hub descriptors through the hub’s default pipe.
The USB specification (refer to Chapter 9) defines the following descriptors:
•
Device
•
Configuration
•
Interface
•
Endpoint
•
String (optional)
The hub class defines additional descriptors (refer to Section 11.23.2). In addition, vendor-specific
descriptors are allowed in the USB device framework. Hubs support standard USB device commands as
defined in Chapter 9.
11.23.1 Standard Descriptors for Hub Class
The hub class pre-defines certain fields in standard USB descriptors. Other fields are either
implementation-dependent or not applicable to this class.
A hub returns different descriptors based on whether it is operating at high-speed or full-/low-speed. A hub
can report three different sets of the descriptors: one descriptor set for full-/low-speed operation and two
sets for high-speed operation.
A hub operating at full-/low-speed has a device descriptor with a bDeviceProtocol field set to zero(0) and an
interface descriptor with a bInterfaceProtocol field set to zero(0). The rest of the descriptors are the same
for all speeds.
A hub operating at high-speed can have one of two TT organizations: single TT or multiple TT. All hubs
must support the single TT organization. A multiple TT hub has an additional interface descriptor (with a
corresponding endpoint descriptor). The first set of descriptors shown below must be provided by all hubs.
A hub that has a single TT must set the bDeviceProtocol field of the device descriptor to one(1) and the
interface descriptor bInterfaceProtocol field set to 0.
A multiple TT hub must set the bDeviceProtocol field of the device descriptor to two (2). The first interface
descriptor has the bInterfaceProtocol field set to one(1). Such a hub also has a second interface descriptor
where the bInterfaceProtocol is set to two(2). When the hub is configured with an interface protocol of
one(1), it will operate as a single TT organized hub. When the hub is configured with an interface protocol
of two(2), it will operate as a multiple TT organized hub. The TT organization must not be changed while
the hub has full-/low-speed transactions in progress.
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Universal Serial Bus Specification Revision 2.0
Note: For the descriptors and fields shown below, the bits in a field are organized in a little-endian fashion;
that is, bit location 0 is the least significant bit and bit location 7 is the most significant bit of a byte value.
Full-/Low-speed Operating Hub
Device Descriptor (full-speed information):
bLength
12H
bDescriptorType
1
bcdUSB
0200H
bDeviceClass
HUB_CLASSCODE (09H)
bDeviceSubClass
0
bDeviceProtocol
0
bMaxPacketSize0
64
bNumConfigurations
1
Device_Qualifier Descriptor (high-speed information):
bLength
0AH
bDescriptorType
6
bcdUSB
200H
bDeviceClass
HUB_CLASSCODE (09H)
bDeviceSubClass
0
bDeviceProtocol
1 (for single TT) or 2 (for
multiple TT)
bMaxPacketSize0
64
bNumConfigurations
1
Configuration Descriptor (full-speed information):
408
bLength
09H
bDescriptorType
2
wTotalLength
N
bNumInterfaces
1
bConfigurationValue
iConfiguration
bmAttributes
bMaxPower
X
Y
Z
The maximum amount of bus
power the hub will consume in
full-/low-speed configuration
Universal Serial Bus Specification Revision 2.0
Interface Descriptor:
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
0
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
0
iInterface
i
Endpoint Descriptor (for Status Change Endpoint):
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B)
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
Other_Speed_Configuration Descriptor (High-speed information):
bLength
09H
bDescriptorType
7
wTotalLength
N
bNumInterfaces
1 (for single TT) or 2 (for
multiple TT)
bConfigurationValue
iConfiguration
bmAttributes
bMaxPower
X
Y
Z
The maximum amount of bus
power the hub will consume in
high-speed configuration
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Universal Serial Bus Specification Revision 2.0
Interface Descriptor:
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
0
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
0 (for single TT)
1 (for multiple TT)
iInterface
i
Endpoint Descriptor (for Status Change Endpoint):
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B )
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
Interface Descriptor (present if multiple TT hub):
410
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
0
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
2
iInterface
i
Universal Serial Bus Specification Revision 2.0
Endpoint Descriptor (present if multiple TT hub):
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B )
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
High-speed Operating Hub with Single TT
Device Descriptor (High-speed information):
bLength
12H
bDescriptorType
1
bcdUSB
200H
bDeviceClass
HUB_CLASSCODE (09H)
bDeviceSubClass
0
bDeviceProtocol
1
bMaxPacketSize0
64
bNumConfigurations
1
Device_Qualifier Descriptor (full-speed information):
bLength
0AH
bDescriptorType
6
bcdUSB
200H
bDeviceClass
HUB_CLASSCODE (09H)
bDeviceSubClass
0
bDeviceProtocol
0
bMaxPacketSize0
64
bNumConfigurations
1
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Universal Serial Bus Specification Revision 2.0
Configuration Descriptor (high-speed information):
bLength
09H
bDescriptorType
2
wTotalLength
N
bNumInterfaces
1
bConfigurationValue
iConfiguration
bmAttributes
bMaxPower
X
Y
Z
The maximum amount of bus
power the hub will consume in
this configuration
Interface Descriptor:
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
0
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
0 (single TT)
iInterface
i
Endpoint Descriptor (for Status Change Endpoint):
412
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B)
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
Universal Serial Bus Specification Revision 2.0
Other_Speed_Configuration Descriptor (full-speed information):
bLength
09H
bDescriptorType
7
wTotalLength
N
bNumInterfaces
1
bConfigurationValue
iConfiguration
bmAttributes
bMaxPower
X
Y
Z
The maximum amount of bus
power the hub will consume in
high-speed configuration
Interface Descriptor:
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
0
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
0
iInterface
i
Endpoint Descriptor (for Status Change Endpoint):
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B )
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
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Universal Serial Bus Specification Revision 2.0
High-speed Operating Hub with Multiple TTs
Device Descriptor (High-speed information):
bLength
12H
bDescriptorType
1
bcdUSB
200H
bDeviceClass
HUB_CLASSCODE (09H)
bDeviceSubClass
0
bDeviceProtocol
2 (multiple TTs)
bMaxPacketSize0
64
bNumConfigurations
1
Device_Qualifier Descriptor (full-speed information):
bLength
0AH
bDescriptorType
6
bcdUSB
200H
bDeviceClass
HUB_CLASSCODE (09H)
bDeviceSubClass
0
bDeviceProtocol
0
bMaxPacketSize0
64
bNumConfigurations
1
Configuration Descriptor (high-speed information):
414
bLength
09H
bDescriptorType
2
wTotalLength
N
bNumInterfaces
1
bConfigurationValue
iConfiguration
bmAttributes
bMaxPower
X
Y
Z
The maximum amount of bus
power the hub will consume in
this configuration
Universal Serial Bus Specification Revision 2.0
Interface Descriptor:
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
0
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
1 (single TT)
iInterface
i
Endpoint Descriptor (for Status Change Endpoint):
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B)
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
Interface Descriptor:
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
1
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
2 (multiple TTs)
iInterface
i
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Universal Serial Bus Specification Revision 2.0
Endpoint Descriptor:
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B )
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
Other_Speed_Configuration Descriptor (full-speed information):
bLength
09H
bDescriptorType
7
wTotalLength
N
bNumInterfaces
1
bConfigurationValue
iConfiguration
bmAttributes
bMaxPower
X
Y
Z
The maximum amount of bus
power the hub will consume in
high-speed configuration
Interface Descriptor:
416
bLength
09H
bDescriptorType
4
bInterfaceNumber
0
bAlternateSetting
0
bNumEndpoints
1
bInterfaceClass
HUB_CLASSCODE (09H)
bInterfaceSubClass
0
bInterfaceProtocol
0
iInterface
i
Universal Serial Bus Specification Revision 2.0
Endpoint Descriptor (for Status Change Endpoint):
bLength
07H
bDescriptorType
5
bEndpointAddress
Implementation-dependent;
Bit 7: Direction = In(1)
bmAttributes
Transfer Type = Interrupt
(00000011B)
wMaxPacketSize
Implementation-dependent
bInterval
FFH (Maximum allowable
interval)
The hub class driver retrieves a device configuration from the USB System Software using the
GetDescriptor() device request. The only endpoint descriptor that is returned by the GetDescriptor() request
is the Status Change endpoint descriptor.
11.23.2 Class-specific Descriptors
11.23.2.1 Hub Descriptor
Table 11-13 outlines the various fields contained by the hub descriptor.
Table 11-13. Hub Descriptor
Offset
Field
Size
Description
0
bDescLength
1
Number of bytes in this descriptor, including this byte
1
bDescriptorType
1
Descriptor Type, value: 29H for hub descriptor
2
bNbrPorts
1
Number of downstream facing ports that this hub
supports
3
wHubCharacteristics
2
D1...D0: Logical Power Switching Mode
00: Ganged power switching (all ports’ power at
once)
01: Individual port power switching
1X: Reserved. Used only on 1.0 compliant hubs
that implement no power switching
D2:
0:
1:
Identifies a Compound Device
Hub is not part of a compound device.
Hub is part of a compound device.
D4...D3: Over-current Protection Mode
00: Global Over-current Protection. The hub
reports over-current as a summation of all
ports’ current draw, without a breakdown of
individual port over-current status.
01: Individual Port Over-current Protection. The
hub reports over-current on a per-port basis.
Each port has an over-current status.
1X: No Over-current Protection. This option is
allowed only for bus-powered hubs that do not
implement over-current protection.
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Universal Serial Bus Specification Revision 2.0
Offset
Field
Size
Description
D6...D5: TT Think TIme
00: TT requires at most 8 FS bit times of inter
transaction gap on a full-/low-speed
downstream bus.
01: TT requires at most 16 FS bit times.
10: TT requires at most 24 FS bit times.
11: TT requires at most 32 FS bit times.
D7: Port Indicators Supported
0:
Port Indicators are not supported on its
downstream facing ports and the
PORT_INDICATOR request has no effect.
1:
Port Indicators are supported on its
downstream facing ports and the
PORT_INDICATOR request controls the
indicators. See Section 11.5.3.
D15...D8:
Reserved
5
bPwrOn2PwrGood
1
Time (in 2 ms intervals) from the time the power-on
sequence begins on a port until power is good on that
port. The USB System Software uses this value to
determine how long to wait before accessing a
powered-on port.
6
bHubContrCurrent
1
Maximum current requirements of the Hub Controller
electronics in mA.
7
DeviceRemovable
Variable,
depending
on
number of
ports on
hub
Indicates if a port has a removable device attached.
This field is reported on byte-granularity. Within a
byte, if no port exists for a given location, the field
representing the port characteristics returns 0.
Bit value definition:
0B - Device is removable.
1B - Device is non-removable
This is a bitmap corresponding to the individual ports
on the hub:
Bit 0: Reserved for future use.
Bit 1: Port 1
Bit 2: Port 2
....
Bit n: Port n (implementation-dependent, up to a
maximum of 255 ports).
Variable
418
PortPwrCtrlMask
Variable,
depending
on
number of
ports on
hub
This field exists for reasons of compatibility with
software written for 1.0 compliant devices. All bits in
this field should be set to 1B. This field has one bit for
each port on the hub with additional pad bits, if
necessary, to make the number of bits in the field an
integer multiple of 8.
Universal Serial Bus Specification Revision 2.0
11.24 Requests
11.24.1 Standard Requests
Hubs have tighter constraints on request processing timing than specified in Section 9.2.6 for standard
devices because they are crucial to the "time to availability" of all devices attached to USB. The worst case
request timing requirements are listed below (apply to both Standard and Hub Class requests):
1.
Completion time for requests with no data stage:
50 ms
2.
Completion times for standard requests with data stage(s)
Time from setup packet to first data stage:
Time between each subsequent data stage:
Time between last data stage and status stage:
50 ms
50 ms
50 ms
As hubs play such a crucial role in bus enumeration, it is recommended that hubs average response times be
less than 5 ms for all requests.
Table 11-14 outlines the various standard device requests.
Table 11-14. Hub Responses to Standard Device Requests
bRequest
Hub Response
CLEAR_FEATURE
Standard
GET_CONFIGURATION
Standard
GET_DESCRIPTOR
Standard
GET_INTERFACE
Undefined. Hubs are allowed to support only one
interface.
GET_STATUS
Standard
SET_ADDRESS
Standard
SET_CONFIGURATION
Standard
SET_DESCRIPTOR
Optional
SET_FEATURE
Standard
SET_INTERFACE
Undefined. Hubs are allowed to support only one
interface.
SYNCH_FRAME
Undefined. Hubs are not allowed to have isochronous
endpoints.
Optional requests that are not implemented shall return a STALL in the Data stage or Status stage of the
request.
419
Universal Serial Bus Specification Revision 2.0
11.24.2 Class-specific Requests
The hub class defines requests to which hubs respond, as outlined in Table 11-15. Table 11-16 defines the
hub class request codes. All requests in the table below except SetHubDescriptor() are mandatory.
Table 11-15. Hub Class Requests
Request
bmRequestType
bRequest
wValue
wIndex
wLength
Data
ClearHubFeature
00100000B
CLEAR_ FEATURE
Feature
Selector
Zero
Zero
None
ClearPortFeature
00100011B
CLEAR_ FEATURE
Feature
Selector
Selector,
Port
Zero
None
ClearTTBuffer
00100011B
CLEAR_TT_BUFFER
Dev_Addr,
EP_Num
TT_port
Zero
None
GetHubDescriptor
10100000B
GET_DESCRIPTOR
Descriptor
Type and
Descriptor
Index
Zero or
Language
ID
Descriptor
Length
Descriptor
GetHubStatus
10100000B
GET_ STATUS
Zero
Zero
Four
Hub
Status and
Change
Status
GetPortStatus
10100011B
GET_ STATUS
Zero
Port
Four
Port
Status and
Change
Status
ResetTT
00100011B
RESET_TT
Zero
Port
Zero
None
SetHubDescriptor
00100000B
SET_DESCRIPTOR
Descriptor
Type and
Descriptor
Index
Zero or
Language
ID
Descriptor
Length
Descriptor
SetHubFeature
00100000B
SET_ FEATURE
Feature
Selector
Zero
Zero
None
SetPortFeature
00100011B
SET_ FEATURE
Feature
Selector
Selector,
Port
Zero
None
GetTTState
10100011B
GET_TT_STATE
TT_Flags
Port
TT State
Length
TT State
StopTT
00100011B
STOP_TT
Zero
Port
Zero
None
420
Universal Serial Bus Specification Revision 2.0
Table 11-16. Hub Class Request Codes
bRequest
Value
GET_ STATUS
0
CLEAR_ FEATURE
1
RESERVED (used in previous
2
specifications for
GET_STATE)
SET_ FEATURE
3
Reserved for future use
4-5
GET_DESCRIPTOR
6
SET_DESCRIPTOR
7
CLEAR_TT_BUFFER
8
RESET_TT
9
GET_TT_STATE
10
STOP_TT
11
Table 11-17 gives the valid feature selectors for the hub class. See Section 11.24.2.6 and Section 11.24.2.7 for a
description of the features.
Table 11-17. Hub Class Feature Selectors
Recipient
Value
C_HUB_LOCAL_POWER
Hub
0
C_HUB_OVER_CURRENT
Hub
1
PORT_CONNECTION
Port
0
PORT_ENABLE
Port
1
PORT_SUSPEND
Port
2
PORT_OVER_CURRENT
Port
3
PORT_RESET
Port
4
421
Universal Serial Bus Specification Revision 2.0
Table 11-17. Hub Class Feature Selectors (continued)
Recipient
Value
PORT_POWER
Port
8
PORT_LOW_SPEED
Port
9
C_PORT_CONNECTION
Port
16
C_PORT_ENABLE
Port
17
C_PORT_SUSPEND
Port
18
C_PORT_OVER_CURRENT
Port
19
C_PORT_RESET
Port
20
PORT_TEST
Port
21
PORT_INDICATOR
Port
22
11.24.2.1 Clear Hub Feature
This request resets a value reported in the hub status.
bmRequestType
bRequest
wValue
wIndex
wLength
Data
00100000B
CLEAR_ FEATURE
Feature
Selector
Zero
Zero
None
Clearing a feature disables that feature; refer to Table 11-17 for the feature selector definitions that apply to
the hub as a recipient. If the feature selector is associated with a status change, clearing that status change
acknowledges the change. This request format is used to clear either the C_HUB_LOCAL_POWER or
C_HUB_OVER_CURRENT features.
It is a Request Error if wValue is not a feature selector listed in Table 11-17 or if wIndex or wLength are not
as specified above.
If the hub