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USB and PS/2 Multimedia
Keyboard Interface
Designer Reference Manual
M68HC08
Microcontrollers
DRM014
Rev. 1
4/2006
freescale.com
USB and PS/2 Multimedia Keyboard Interface
Designer Reference Manual
by: Derek Lau
Freescale Semiconductor, Inc.
Hong Kong
To provide the most up-to-date information, the revision of our documents on the World Wide Web will be
the most current. Your printed copy may be an earlier revision. To verify that you have the latest
information available, refer to http://www.freescale.com
The following revision history table summarizes changes contained in this document. For your
convenience, the page number designators have been linked to the appropriate location.
Revision History
Date
Revision
Level
04/2006
1
Replaced unclear schematic.
26
03/2003
0
Initial release
N/A
Description
Page
Number(s)
USB and PS/2 Multimedia Keyboard Interface, Rev. 1.0
Freescale Semiconductor
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Revision History
USB and PS/2 Multimedia Keyboard Interface, Rev. 1.0
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Freescale Semiconductor
Table of Contents
Chapter 1
USB and PS/2 Multimedia Keyboard Interface
1.1
1.2
1.3
1.4
1.4.1
1.4.2
1.5
1.5.1
1.5.2
1.5.3
1.5.4
1.5.5
1.5.6
1.5.7
1.5.8
1.5.9
1.5.10
1.6
1.7
1.8
1.8.1
1.8.2
1.9
1.10
1.10.1
1.11
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
44-pin QFP MC68HC908JB8 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Hardware Descriptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Key Matrix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
In-Circuit Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Firmware Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
USB and PS/2 Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
PS/2 Main Routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
PS/2 Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Host to Device Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Device to Host Communications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
PS/2 Keyboard Command . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Host to Keyboard Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Keyboard to Host Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
PS/2 Scan Codes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
USB Main Routine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Firmware Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Test Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Customization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Firmware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Extra Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Further Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Related Documents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Appendix A.
Glossary
USB and PS/2 Multimedia Keyboard Interface, Rev. 1
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Table of Contents
USB and PS/2 Multimedia Keyboard Interface
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Chapter 1
USB and PS/2 Multimedia Keyboard Interface
1.1 Introduction
This manual describes a reference design of a Universal Serial Bus and PS/2 multimedia keyboard
interface for Microsoft Windows by using the MC68HC908JB8.
For the full MC68HC908JB8 specification, please refer to the data sheet, Freescale order number:
MC68HC908JB8/D.
1.2 Overview
The Freescale MC68HC908JB8 is a member of the HC08 Family of microcontrollers (MCUs). The
features of the MC68HC908JB8 include a configured Universal Serial Bus (USB) or PS/2 interface, which
makes this MCU suited for personal computer Human Interface Devices (HID), such as mice and
keyboards. The MC68HC908JB8 is available in several packages to fit into various applications. A
multimedia keyboard with USB and PS/2 interface is demonstrated using the MC68HC908JB8 with a
44-pin QFP package. The main features of the keyboard include:
• Fully USB specification 1.1 compliant
• USB or PS/2 interface auto detect
• Windows 98, ME and 2000 compatible
• Power management keys (power, wake and sleep) support
• Multimedia key support
• In-circuit programming for firmware modification
1.3 44-pin QFP MC68HC908JB8 Features
The 44-pin QFP MC68HC908JB8 is targeted for USB and PS/2 interface keyboard applications with
minimum external components needed. Features include:
• USB D+ and D– pins shared with PS/2 data and clock pins
• 27 out of 37 I/O pins with internal pull-up supports up to 8 x 19 key matrix
• 10mA direct drive pins for the Num Lock, Caps Lock and Scroll Lock LEDs
• Internal 1.5K pull-up for USB D– data line
• Internal 5K pull-ups for PS/2 data and clock pins
USB and PS/2 Multimedia Keyboard Interface, Rev. 1
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USB and PS/2 Multimedia Keyboard Interface
1.4 Hardware Descriptions
Scroll LED
Caps LED
Num LED
MC68HC908JB8
8 x 18
Key Matrix
USB Plug
Figure 1-1. . Block Diagram
6
5
USB Plug
USB to PS/2
Converter
3
1 2
4
PS/2 Plug
6-pin PS/2 Plug
1 – Data (USB D– pin)
2 – NC
3 – Ground (USB Ground)
4 – +5V (USB +5V)
5 – Clock (USB D+ pin)
6 – NC
Figure 1-2. USB and PS/2 Connections
Figure 1-1 shows the block diagram of the keyboard. The solution includes the JB8, key button inputs and
LED indicator outputs only. The connections of the corresponding USB and PS/2 signals are shown in
Figure 1-2 The USB to PS/2 converter standard connections are the USB D– and D+ pins connected to
the PS/2 Data and Clock pins respectively.
Figure 1-3. shows the printed circuit board and Figure 1-16 shows the schematic of the keyboard.
• J1 is used for USB connection
• J2 is used for PS/2 mouse connection (for future development)
• J3 and J7 are used for in-circuit programming
• 8 rows x 18 columns key matrix is implemented
– 8 rows implemented in PTA[7:0]
– 18 columns implemented in PTB[7:0], PTC[7:0], PTE0 and PTE2
• Keyboard LEDs
– Scroll Lock at PTD2
– Caps Lock at PTD3
– Num Lock at PTD4
Figure 1-4 shows the key matrix for the 107 standard keyboard with power management keys (Power,
Wake and Sleep). Figure 1-5 shows the key matrix for multimedia keyboard with function key.
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Hardware Descriptions
Figure 1-3. Keyboard PCB
1.4.1 Key Matrix
PTB0 PTB1 PTB2 PTB3 PTB4 PTB5 PTB6 PTB7 PTC0 PTC1 PTC2 PTC3 PTC4 PTC5 PTC6 PTC7 PTE0 PTE2
PTA0
End
9
8
7
4
3
2
1
Page
PWR
SLEEP
Down
Dwn
PTA1
Home
F8
+
=
6
5
F2
F1
~
`
Page
Up
N
B
<
,
M
V
C
F6
H
G
F4
L
K
J
F
D
F7
]}
Y
T
O
I
U
R
PTA2
App
PTA3
>
.
PTA4
PTA5
ENT
PTA6
PTA7
+
INS
Prnt
Scr
DEL
F5
0
F10
Lctrl
_
-
F9
?
/
F12
RALT
Z
*
/
Num
Lck
ESC
.
0
SPC
S
A
3
2
1
RSFT
F3
Caps
Lck
TAB
6
5
4
LSFT
E
W
Q
9
8
7
WAKE
X
ENT
RCTRL
LALT
RGUI
LGUI
Scrl
Lck
Pause
“
‘
F11
:
;
|
\
[{
Bck
Spc
P
Figure 1-4. 107-Key Matrix
USB and PS/2 Multimedia Keyboard Interface, Rev. 1
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9
USB and PS/2 Multimedia Keyboard Interface
PTB0 PTB1 PTB2 PTB3 PTB4 PTB5 PTB6 PTB7 PTC0 PTC1 PTC2 PTC3 PTC4 PTC5 PTC6 PTC7 PTE0 PTE2
PTA0
J
7
1
LCTRL MK7 MK5 MK2
7
JPN3
F4
Z
Q
ESC
Fn
5
_
-
PTA1
K
8
2
MK0 MK8 LGUI
8
INS
F5
X
W
F1
MK3
T
+
=
PTA2
L
9
3
MK6 LSFT
MK9
9
DEL
F6
C
E
F2
SLEEP
Q
Bck
Spc
PTA3
;:
*
+
MK1 RSFT MK18
0
Num
Lck
F7
V
R
F3
MK8
B
[{
PTA4
ENT
4
ENT Home RCTRL MK4 MK23 MK10
U
$64
|
\
F8
A
1
~
`
Caps
Lck
6
]}
PTA5
M
5
0
Page
Up
PWR
Dwn
LALT
MK11
I
JPN1
Prnt
Scr
F10
S
2
TAB
Y
‘“
PTA6
>
.
6
.
Page
SLEEP
Down
RALT
LANG
2
O
$85
Scrl
Lck
F11
D
3
SPC
H
$32
PTA7
Lctrl
/F
End WAKE JPN5 JPN4 JPN2
P
App Pause F12
4
<
,
N
F9
LANG
1
RGUI
Figure 1-5. Multimedia Key Matrix
1.4.2 In-Circuit Programming
J3 and J7 contain all the signals for the MC68HC908JB8 to enter monitor mode for In-Circuit
Programming. The ICP requires J3 and J7 to be connected with a cable to the ICP adaptor board, which
is plugged-into the M68HC08 Serial Programmer (M68SPGRM08) (see Figure 1-6). Running the
MCUScribe software on the PC allows erase, programming and verification of the firmware in the
MC68HC908JB8. The communication baud rate is determined by the jumper setting of J3 of the ICP
adaptor board. Connect pins 1 and 2 of J3 for 9600bps and pins 2 and 3 for 19200bps.
Figure 1-6. In-Circuit Programming Connection
USB and PS/2 Multimedia Keyboard Interface, Rev. 1
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Firmware Description
1.5 Firmware Description
The firmware consists of three main parts:
• USB and PS/2 interface detection
• PS/2 main program and subroutines
• USB main program and subroutines
INITIALIZATION
DELAY 350ms (POWER ON DELAY)
ENABLE USB D– PULLUP
D+ HIGH FOR 1ms ?
YES
PS/2
NO
OVER 10 SECONDS ?
YES
USB
NO
NO
USB RESET?
YES
YES
D+ HIGH FOR 1ms ?
PS/2
NO
SETUP DETECTED ?
YES
USB
NO
NO
OVER 10 SECONDS ?
YES
USB
Figure 1-7. USB and PS/2 Detection
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USB and PS/2 Multimedia Keyboard Interface
1.5.1 USB and PS/2 Detection
PTE3 and PTE4 can be configured as USB D+ and D– pins or as open-drain I/O pins for PS/2 data and
clock lines. Figure 1-7 shows the algorithm for distinguishing between a USB and a PS/2 interface.
After power on, the interface protocol is undetermined and can be either a USB or a PS/2 interface. The
firmware first initializes the registers and the I/O ports. It then performs a 350ms software delay to meet
the PS/2 power on delay requirement. The USB engine and the internal USB pullup resistor are enabled.
PTE3 and PTE4 are configured as USB D+ and D– pins with a 1.5K internal pullup at D– pin.
For a low speed USB interface, D+ pin will not be continuously high for more than 4µs because of the bit
stuffing mechanism. The PS/2 interface is determined by 1ms continuously high at D+ pin while the USB
interface is determined by receiving a SETUP token.
After the interface mode is detected. the firmware enters either the PS/2 or USB main routine.
1.5.2 PS/2 Main Routine
In PS/2 mode, PTE3 and PTE4 are configured as open-drain I/O pins with 5K internal pullup resistors
enabled. Figure 1-8 shows the PS/2 main routine. The main functions of the PS/2 routines are:
• Receive commands from host
• Respond to received commands
• Scan key matrix
• Send make code to host if key pressed
• Send break code to host if key released
1.5.3 PS/2 Protocol
The PS/2 is a bidirectional serial interface using two signals: Clock and Data. The data consists of 11 bits
including 1 start bit, 8 data bits, 1 odd parity bit, and 1 stop bit. PS/2 device generates the clock signal
with a typical cycle of 80µs in both host-to-device or device-to-host communications.
Table 1-1. Clock and Data Line Control
Register Bits
Port Pins
Register
POCR
PTE
DDRE
IOCR
POCR
PTE
DDRE
PTE4
(Data)
PTE3
(Clock)
Bit
PTE4P
PTE4
DDRE4
PTE4IE
PTE3P
PTE3
DDRE3
PTE4
(Data)
PTE3
(Clock)
PTE4 Interrupt
Enabled
—
—
—
1
—
—
—
Input
High
—
Release
Data/Clock High
1
—
0
—
1
—
0
Input
High
Input
High
Drive
Data/Clock Low
—
0
1
—
—
0
1
Output
Low
Output
Low
Table 1-1 shows the setting of the registers for controlling the PTE3 (Clock) and the PTE4 (Data) pins.
Instead of setting DDR3/DDR4 as high to output a high signal, we set DDR3/DDR4 as an input with
internal pullup to perform the same function. In the PS/2 routines, the values of PTE3 and PTE4 are
cleared to zero and the values of PTE3P and PTE4P are set to one. Set DDR3 or DDR4 to one to force
it as output low, or clear DDR3 or DDR4 to zero to make it high impedance and pulled high by the 5K
internal resistor.
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Firmware Description
PS/2 INITIALIZATION
NO
SELF
TEST PASS ?
SEND SELF TEST
FAIL CODE ($FC)
STOP
YES
SEND SELF TEST PASS
CODE ($AA)
VALID COMMAND
FROM HOST ?
NO
YES
ACK AND
HANDLE COMMAND
40ms TIMER TICK ?
YES
SCAN KEY MATRIX
NO
KEYS PRESSED
OR RELEASED
YES
YES
GHOST KEY ?
NO
SEND MAKE CODES
FOR KEYS PRESSED
SEND BREAK CODES
FOR KEYS RELEASED
Figure 1-8. PS/2 Main Routine
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USB and PS/2 Multimedia Keyboard Interface
1.5.4 Host to Device Communications
Data sent from host to device is read while the clock line is high. In an idle state, both the Clock and the
Data lines are pulled high. The host starts sending data by pulling the Clock line low for a minimum of
100 ms. Figure 1-9 shows the signal diagram. Communications steps are shown as below:
1. Host waits until no auxiliary device transmission is in progress.
2. Host pulls the Clock line low.
3. Host pulls the Data line low as the start bit.
4. Host releases the Clock line.
5. Device pulls Clock line low.
6. Host sends out data.
7. Device releases the Clock line high and read Data.
8. Device reads the Clock line and aborts communication if the Clock line is low.
9. Repeat steps 5 to 7 for Data 1 to Data 7 and the parity bit.
10. Device pulls the Clock line low.
11. Host releases the Data line.
12. Device releases the Clock line high.
13. Device reads the Data line high for stop signal and sends error if the Data line is low.
14. Device pull the Data and the Clock lines low.
15. Device release the Clock and the Data lines.
STEP 1
4
7,8
12
CLOCK
2
5
10
15
9
11,13
DATA
3
6
14
START DATA0 DATA1 DATA2 DATA3 DATA4 DATA5 DATA6 DATA7 PARITY STOP ACK
Figure 1-9. Host to Device Communication
KBD_IN is the IRQ1 interrupt routine for receiving data from host. The interrupt is configured to execute
when a falling edge at the PTE4 (Data) pin is detected.
1.5.5 Device to Host Communications
Data sent from device to host is read at the falling edge of the clock. The device checks whether the host
is ready by detecting the clock high before transmitting data. Figure 1-10 shows the signal diagram.
Communications steps are shown as below:
1. Device waits for the Clock line high for a minimum of 50 ms.
2. Abort if the Data line is low.
3. Device sends out data.
4. Device pulls the Clock line low.
5. Device releases the Clock line high.
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Firmware Description
6. Device reads the Clock line and aborts communication if the Clock line is low.
7. Repeat steps 4 to 6 for Data 0 to Data 7, the parity bit and the stop bit.
8. Device releases the Clock line high.
5,6
STEP 1,2
CLOCK
4
8
7
DATA
3
START DATA0 DATA1 DATA2 DATA3 DATA4 DATA5 DATA6 DATA7 PARITY STOP
Figure 1-10. Device to Host Communication
KBD_OUT is the routine for transmitting data to the host. The data to be transmitted is put into V_TxByte
before calling this routine.
1.5.6 PS/2 Keyboard Command
Both the host and the keyboard may send commands to each other. The keyboard transmits an
acknowledge ($FA) after receiving a valid command. When the keyboard receives an invalid command,
it returns a resent command ($FA).
1.5.7 Host to Keyboard Commands
Table 1-2. Host to Keyboard Commands
Code
Description
Implemented
$ED
Set status indicators
YES
$EE
Echo
YES
$F0
Set alternate Scan Code
NO
$F2
Get keyboard ID
YES
$F3
Set typematic repeat rate
YES
$F4
Enable Scan
YES
$F5
Disable Scan
YES
$F6
Set default values
YES
$F7
Set all keys typematic
NO
$F8
Set all keys make/break
NO
$F9
Set all keys make
NO
$FA
Set key type typematic/make/break
NO
$FB
Set key type typematic
NO
$FC
Set key type make/break
NO
$FD
Set key type make
NO
$FE
Resent the last command
YES
$FF
Reset
YES
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USB and PS/2 Multimedia Keyboard Interface
1.5.8 Keyboard to Host Commands
Table 1-3. Keyboard to Host Commands
Code
Description
Implemented
$00
Keyboard detection or overrun error
YES
$AA
Basic assurance test passed
YES
$EE
Echo
YES
$FA
Acknowledge
YES
$FE
Resend
YES
1.5.9 PS/2 Scan Codes
There are three sets of scan codes (code 1, 2, and 3). Most PCs support scan code set 2, hence this is
the only scan code the firmware supports. Make code or break code is sent when any key is pressed or
released. While a key is pressed, its make code is sent out repeatedly and the rate depends on the
typematic repeat value.
In addition to the IBM standard, Microsoft has published standards for the Windows keys, Power
Management keys (sleep, wake, and power), and the Audio Control keys.
Table 1-4. Scan codes supported by Windows 98, ME, and 2000
Description
Left Windows
Make Code
Break Code
$E0, $1F
$E0, $F0, $1F
Right Windows
$E0, $27
$E0, $F0, $27
Applications
$E0, $2F
$E0, $F0, $2F
Power
$E0, $37
$E0, $F0, $37
Sleep
$E0, $3F
$E0, $F0, $3F
Wake
$E0, $5E
$E0, $F0, $5E
Table 1-5. Scan codes supported by Windows ME and 2000
Description
Make Code
Break Code
Scan next track
$E0, $4D
$E0, $F0, $4D
Scan previous track
$E0, $15
$E0, $F0, $15
Stop
$E0, $3B
$E0, $F0, $3B
Play/Pause
$E0, $34
$E0, $F0, $34
Mute
$E0, $23
$E0, $F0, $23
Volume increase
$E0, $32
$E0, $F0, $32
Volume decrease
$E0, $21
$E0, $F0, $21
AL Email Reader
$E0, $48
$E0, $F0, $48
AC search
$E0, $10
$E0, $F0, $10
AC Home
$E0, $3A
$E0, $F0, $3A
AC Forward
$E0, $30
$E0, $F0, $30
AC Stop
$E0, $28
$E0, $F0, $28
AC Refresh
$E0, $20
$E0, $F0, $20
AC Bookmarks
$E0, $18
$E0, $F0, $18
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Firmware Description
Table 1-6. Scan codes supported by Windows ME
Make Code
Break Code
AC Calculator
Description
$E0, $2B
$E0, $F0, $2B
AC Local Browser
$E0, $40
$E0, $F0, $40
AC Consumer Control Configuration
$E0, $50
$E0, $F0, $50
1.5.10 USB Main Routine
Figure 1-11 shows the USB main routine. The routine scans the keyboard every 40 ms. If there are keys
pressed or released, it puts the key codes into a buffer and prepares the input reports for the keys through
endpoint 1 or endpoint 2. If the USB bus idles for more than 6 ms, the routine puts the MC68HC908JB8
into STOP mode until it detects a resume signal from the host or any key pressed for remote wake-up.
Figure 1-12 shows the USB interrupt routines. The USB engine automatically responds to a valid USB
token with either ACK, NAK, or STALL, depending on the registers setting, and ignores it if it is invalid.
The firmware has to set the registers for the USB engine to give a correct response to the token in different
stages. The USB interrupt will be executed whenever there is an EOP, resume signal from host, valid data
received or data transmitted. The USB interrupt routine also makes preparation for the next USB
transaction and handles any valid command or data received.
Figure 1-13 to Figure 1-15 show the routines of handling the Control Transfers. Control transfers have two
or three transaction stages: Setup, Data (optional) and Status as shown below:
• Control Write: SETUP, OUT, OUT, OUT... IN
• Control Read: SETUP, IN, IN, IN... OUT
• No Data Control: SETUP, IN
The firmware first distinguishes the kinds of control transfers and does the corresponding preparation for
the next stage.
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USB and PS/2 Multimedia Keyboard Interface
USB INITIALIZATION
NO
DEVICE
CONFIGURED ?
YES
40ms TIMER TICK ?
YES
SCAN KEY MATRIX
YES
GHOST KEY?
NO
CONVERT SCAN KEY TO
KEYBOARD REPORT
YES
YES
EP1 TX BUFFER
EMPTY ?
NEW ENDPOINT 1
REPORT ?
TX EP1 IN REPORT
NO
YES
YES
EP2 TX BUFFER
EMPTY ?
NEW ENDPOINT 2
REPORT ?
TX EP2 IN REPORT
NO
NO
YES
USB IDLE FOR
6 ms ?
SUSPEND DEVICE
KEY PRESSED OR
RESUME FROM
HOST ?
YES
Figure 1-11. USB Main Routine
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Firmware Description
USB INTERRUPT ROUTINE
NO
EOP ?
YES
RESET SUSPEND COUNTER
YES
SETUP ?
SETUP HANDLER
NO
OUT TOKEN
TO EP0 ?
YES
OUT EP0 HANDLER
NO
EP0 TX COMPLETED ?
YES
IN EP0 HANDLER
NO
EP1 TX COMPLETED ?
YES
DISABLE EP1 TRANSMIT &
CLEAR EP1 TX FLAG
YES
DISABLE EP2 TRANSMIT &
CLEAR EP2 TX FLAG
NO
EP2 TX COMPLETED ?
NO
RESUME FORM
HOST?
YES
CLEAR RESUME FLAG
NO
RETURN FROM INTERRUPT
Figure 1-12. USB Interrupt Routine
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USB and PS/2 Multimedia Keyboard Interface
SETUP HANDLER
1.UNSTALL EP 0 IN & OUT
2.COPY 8 BYTE SETUP
DATA TO RAM BUFFER
3.CLEAR EP0 RX FLAG
4.SET NAK TO IN EP0
STANDARD DEVICE
REQUEST ?
YES
HANDLE STANDARD
DEVICE REQUEST
YES
HANDLE HID
CLASSE REQUEST
NO
HID CLASS
REQUEST ?
NO
RETURN
RETURN STALL
Figure 1-13. Setup Routine
OUT EP0 HANDLER
STATUS STAGE ?
YES
1. SET NAK TO EP0 IN
2. SET STALL TO EP0 OUT
(CONTROL TRANSFER
COMPLETED)
NO
VALID DATA
YES
1. COPY DATA TO BUFFER
2. PROCESS OUT DATA
NO
RETURN
RETURN STALL
Figure 1-14. OUT EP0 HANDLER
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Firmware Description
IN EPO HANDLER
SET NAK TO IN EP0
SET STALL TO EP0 OUT
(CONTROL TRANSFER
COMPLETED)
YES
STATUS STAGE ?
NO [DATA STAGE]
YES
ALL DATA
SENT?
PREPARE FOR
OUT STAGE
NO
PREPARE FOR
NEXT DATA STAGE
RETURN
Figure 1-15. . IN EPO HANDLER
USB Key Codes
The key codes or usage IDs for a basic 104 keyboard are defined in the USB HID Usage Tables. In
addition to the basic key codes, Microsoft has published standards for the Windows keys, Power
Management keys (sleep, wake, and power), and the audio control keys.
ACPI Power Management Control
Table 1-7. Consumer Page HID Controls in Windows ME and 2000
USAGE
USAGE NAME
DATA TYPE
$81
Power
Relative
$82
Sleep
Relative
$83
Wake
Relative
Consumer Page Audio Control
Table 1-8. Consumer Page HID Controls in Windows ME and 2000
USAGE
USAGE NAME
USAGE TYPE
DATA TYPE
$B5
Scan Next Track
One Shot Control
Relative
$B6
Scan Previous Track
One Shot Control
Relative
$B7
Stop
One Shot Control
Relative
$CD
Play/Pause
One Shot Control
Relative
$E0
Volume
Linear Control
Relative
$E2
Mute
On/Off Control
Relative
$E3
Bass
Linear Control
Relative
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USB and PS/2 Multimedia Keyboard Interface
Table 1-8. Consumer Page HID Controls in Windows ME and 2000 (Continued)
USAGE
USAGE NAME
USAGE TYPE
DATA TYPE
Linear Control
Relative
$E
Treble
$E5
Bass Boost
On/Off Control
Relative
$E9
Volume Increment
Re-Trigger Control
Absolute
$EA
Volume Decrement
Re-Trigger Control
Absolute
$152
Bass Increment
Re-Trigger Control
Absolute
$153
Bass Decrement
Re-Trigger Control
Absolute
$154
Treble Increment
Re-Trigger Control
Absolute
$155
Treble Decrement
Re-Trigger Control
Absolute
$18A
AL Email Reader
Selector
Relative
$221
Bass Increment
Selector
Relative
$223
Bass Increment
Selector
Relative
$224
Bass Increment
Selector
Relative
$225
Bass Increment
Selector
Relative
$226
Bass Increment
Selector
Relative
$227
Bass Increment
Selector
Relative
Selector
Relative
Selector
Relative
Selector
Relative
$183
AL Consumer Control
Configuration
$192
AL Calculator(1)
$194
AL Local Browser
(1)
(1)
NOTES:
1. Currently supported in Windows ME only.
USB Keyboard Report
The keyboard implements two HID interfaces on endpoint 1 and 2 in a USB composite-device fashion.
HID interface 0 (endpoint 1) implements a standard HID keyboard with identical report and boot protocols.
HID interface 1 (endpoint 2) implements multimedia and power management keys. This implementation
ensures the keyboard works in BIOS setup and in DOS mode.
Interface 0 will issue 8-byte input reports that are identical to the standard keyboard boot protocol report
(see Table 1-9) as documented in the Device Class Definition for Human Interface Device (HID) version
1.1. This interface also allows the host system to turn on and off the respective LED state indicators, as
specified by the 1-byte output report (see Table 1-10).
Table 1-9. Interface 0 Input Report
Byte
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
0
Right
GUI
Right
ALT
Right
Shift
Right
Control
Left
GUI
Left
ALT
Left
Shift
Left
Control
1
Reserved
2
Keyboard Usage ID (Key Code)
3
Keyboard Usage ID (Key Code)
4
Keyboard Usage ID (Key Code)
5
Keyboard Usage ID (Key Code)
6
Keyboard Usage ID (Key Code)
7
Keyboard Usage ID (Key Code)
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Firmware Description
Table 1-10. Interface 0 Output Report
Byte
Bit 7
Bit 6
Bit 5
0
Bit 4
Kana
Bit 3
Bit 2
Bit 1
Bit 0
Compose
Scroll
Lock
Caps
Lock
Num
Lock
Interface 1 issues power management key or multimedia key input reports, which are distinguished by a
unique Report ID. The power management key uses Report ID number 1 and the multimedia key uses
Report ID number 2 (see Table 1-11 and Table 1-12).
Table 1-11. Interface 1 Power Key Input Report
Byte
Bit 7
Bit 6
Bit 5
0
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
Power
Wake
Sleep
Report ID = 1
1
Table 1-12. Interface 1 Multimedia Key Input Report
Byte
Bit 7
Bit 6
Bit 5
Bit 4
1
M7
M6
M5
M4
2
M15
M14
M13
3
M23
M22
M21
0
Bit 3
Bit 2
Bit 1
Bit 0
M3
M2
M1
M0
M12
M11
M10
M9
M8
M20
M19
M18
M17
M16
Report ID = 2
4
Reserved for M24–M31
Table 1-13 shows some input report examples. Report ID is not used in interface 0. The first byte is the
modifier byte and is set on bit basis. Whenever a modifier key is pressed, the corresponding bit is set to
one. For example, if the Left Control and the character ’A’ keys are pressed, the first byte of the report
equals $01, the second byte is reserved, the third byte equals $04, and the forth to the eight bytes equal
$00.
Power Management keys are reported through interface 1 with report ID 1. For example, if the Wake key
is pressed, the first byte of the report ID equals $01, and the second byte equals $02 since the Wake key
is defined as bit 2 of the second byte
Hot keys are reported through interface 1 with reported ID 2. For example, if hot key 0 and hot key 17 are
pressed, the first byte of report ID equals $02, the second byte equals $01 since hot key 0 is pressed, the
third byte equals $00 since hot keys 8 to 16 are not pressed, and the forth byte equals $02 since the hot
key 17 is pressed.
Table 1-13. Input Report Examples
Keys Pressed
Endpoint
In Report Data
Left Control, ’A’
1
$01,$00,$04,$00,$00,$00,$00,$00
Left Control, Right Alt, ’A’, ’B’
1
$41,$00,$04,$05,$00,$00,$00,$00
Wake
2
$01,$02
Hot Key 0 & Hot Key 17
2
$02,$01,$00,$02
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USB and PS/2 Multimedia Keyboard Interface
1.6 Firmware Files
Firmware is compiled under CASM08Z.EXE ver 3.16 from P & E Microcomputer Systems, Inc.
Table 1-14 summarizes the functions of each firmware files:
Table 1-14. Input Report Examples
Files
Functions
JB8-PSU.ASM
Define constants and variables
USB and PS/2 detection
PS/2 main program
PS/2 key handler
PS2-SCAN.ASM
PS/2 key scan
PS2-KEY.ASM
PS/2 key matrix definition
JB8-USB.ASM
USB main program
USB endpoint 1 and endpoint 2 transmit setting
Suspend and Resume Handler
Timer interrupt
USB-SCAN.ASM
USB key scan
USB key handler
USB-KEY.ASM
USB key matrix
HID-KBD.ASM
USB standard device requests handler
HID class requests handler
JB8-INT.ASM
USB interrupt
USB control transfer handler
JB8-KBD.H
Device, configure, interface, HID, endpoint, string and
report descriptors
JB8-EQS.H
JB8 registers and memory definitions
1.7 Test Description
•
•
•
•
The solution was tested under different Windows operating systems on several brands of PCs.
USB compliance test using Command Verifier beta version.
Compatibility tests under Windows 98SE, 2000 and XP.
Compatibility tests under AMD 750, Intel 810 and 845 chip set Desktops, and IBM Thinkpad 570,
600E, 600X and T23.
1.8 Customization
1.8.1 Hardware
•
Leave unused row and column lines unconnected since they are pulled high by internal resistors.
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Extra Features
1.8.2 Firmware
•
•
•
•
Modify the key matrix tables in "ps2-key.asm" and "usb-key.asm" according to customized key
matrix layout
Change vendor ID, product ID and product revision number in the device descriptor table in
"usb-key.h"
Change vendor name and product name in the string descriptor table in "usb-key.h"
Change the report descriptor in "usb-key.h" if necessary.
1.9 Extra Features
1.10 Further Information
1.10.1 Related Documents
MC68HC908JB8 Technical Data
Device Class Definition for Human Interface Device (HID), Version 1.1
Keyboard Scan Code Specification from Microsoft
Support for Enhanced Keyboard Features under Windows 2000 and Windows ME
USB HID Usage Table
1.11 Schematics
Figure 1-16 shows the schematics for the reference design.
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USB and PS/2 Multimedia Keyboard Interface
Figure 1-16. Keyboard Schematics
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Appendix A.
Glossary
A — See “accumulator (A).”
accumulator (A) — An 8-bit general-purpose register in the CPU08. The CPU08 uses the accumulator
to hold operands and results of arithmetic and logic operations.
acquisition mode — A mode of PLL operation during startup before the PLL locks on a frequency. Also
see "tracking mode."
address bus — The set of wires that the CPU or DMA uses to read and write memory locations.
addressing mode — The way that the CPU determines the operand address for an instruction. The
M68HC08 CPU has 16 addressing modes.
ALU — See “arithmetic logic unit (ALU).”
arithmetic logic unit (ALU) — The portion of the CPU that contains the logic circuitry to perform
arithmetic, logic, and manipulation operations on operands.
asynchronous — Refers to logic circuits and operations that are not synchronized by a common
reference signal.
baud rate — The total number of bits transmitted per unit of time.
BCD — See “binary-coded decimal (BCD).”
binary — Relating to the base 2 number system.
binary number system — The base 2 number system, having two digits, 0 and 1. Binary arithmetic is
convenient in digital circuit design because digital circuits have two permissible voltage levels, low and
high. The binary digits 0 and 1 can be interpreted to correspond to the two digital voltage levels.
binary-coded decimal (BCD) — A notation that uses 4-bit binary numbers to represent the 10 decimal
digits and that retains the same positional structure of a decimal number. For example,
234 (decimal) = 0010 0011 0100 (BCD)
bit — A binary digit. A bit has a value of either logic 0 or logic 1.
branch instruction — An instruction that causes the CPU to continue processing at a memory location
other than the next sequential address.
break module — A module in the M68HC08 Family. The break module allows software to halt program
execution at a programmable point in order to enter a background routine.
breakpoint — A number written into the break address registers of the break module. When a number
appears on the internal address bus that is the same as the number in the break address registers, the
CPU executes the software interrupt instruction (SWI).
break interrupt — A software interrupt caused by the appearance on the internal address bus of the
same value that is written in the break address registers.
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bus — A set of wires that transfers logic signals.
bus clock — The bus clock is derived from the CGMOUT output from the CGM. The bus clock frequency,
fop, is equal to the frequency of the oscillator output, CGMXCLK, divided by four.
byte — A set of eight bits.
C — The carry/borrow bit in the condition code register. The CPU08 sets the carry/borrow bit when an
addition operation produces a carry out of bit 7 of the accumulator or when a subtraction operation
requires a borrow. Some logical operations and data manipulation instructions also clear or set the
carry/borrow bit (as in bit test and branch instructions and shifts and rotates).
CCR — See “condition code register.”
central processor unit (CPU) — The primary functioning unit of any computer system. The CPU controls
the execution of instructions.
CGM — See “clock generator module (CGM).”
clear — To change a bit from logic 1 to logic 0; the opposite of set.
clock — A square wave signal used to synchronize events in a computer.
clock generator module (CGM) — A module in the M68HC08 Family. The CGM generates a base clock
signal from which the system clocks are derived. The CGM may include a crystal oscillator circuit and or
phase-locked loop (PLL) circuit.
comparator — A device that compares the magnitude of two inputs. A digital comparator defines the
equality or relative differences between two binary numbers.
computer operating properly module (COP) — A counter module in the M68HC08 Family that resets
the MCU if allowed to overflow.
condition code register (CCR) — An 8-bit register in the CPU08 that contains the interrupt mask bit and
five bits that indicate the results of the instruction just executed.
control bit — One bit of a register manipulated by software to control the operation of the module.
control unit — One of two major units of the CPU. The control unit contains logic functions that
synchronize the machine and direct various operations. The control unit decodes instructions and
generates the internal control signals that perform the requested operations. The outputs of the control
unit drive the execution unit, which contains the arithmetic logic unit (ALU), CPU registers, and bus
interface.
COP — See "computer operating properly module (COP)."
counter clock — The input clock to the TIM counter. This clock is the output of the TIM prescaler.
CPU — See “central processor unit (CPU).”
CPU08 — The central processor unit of the M68HC08 Family.
CPU clock — The CPU clock is derived from the CGMOUT output from the CGM. The CPU clock
frequency is equal to the frequency of the oscillator output, CGMXCLK, divided by four.
CPU cycles — A CPU cycle is one period of the internal bus clock, normally derived by dividing a crystal
oscillator source by two or more so the high and low times will be equal. The length of time required to
execute an instruction is measured in CPU clock cycles.
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CPU registers — Memory locations that are wired directly into the CPU logic instead of being part of the
addressable memory map. The CPU always has direct access to the information in these registers. The
CPU registers in an M68HC08 are:
A (8-bit accumulator)
H:X (16-bit index register)
SP (16-bit stack pointer)
PC (16-bit program counter)
CCR (condition code register containing the V, H, I, N, Z, and C bits)
CSIC — customer-specified integrated circuit
cycle time — The period of the operating frequency: tCYC = 1/fOP.
decimal number system — Base 10 numbering system that uses the digits zero through nine.
direct memory access module (DMA) — A M68HC08 Family module that can perform data transfers
between any two CPU-addressable locations without CPU intervention. For transmitting or receiving
blocks of data to or from peripherals, DMA transfers are faster and more code-efficient than CPU
interrupts.
DMA — See "direct memory access module (DMA)."
DMA service request — A signal from a peripheral to the DMA module that enables the DMA module to
transfer data.
duty cycle — A ratio of the amount of time the signal is on versus the time it is off. Duty cycle is usually
represented by a percentage.
EEPROM — Electrically erasable, programmable, read-only memory. A nonvolatile type of memory that
can be electrically reprogrammed.
EPROM — Erasable, programmable, read-only memory. A nonvolatile type of memory that can be erased
by exposure to an ultraviolet light source and then reprogrammed.
exception — An event such as an interrupt or a reset that stops the sequential execution of the
instructions in the main program.
external interrupt module (IRQ) — A module in the M68HC08 Family with both dedicated external
interrupt pins and port pins that can be enabled as interrupt pins.
fetch — To copy data from a memory location into the accumulator.
firmware — Instructions and data programmed into nonvolatile memory.
free-running counter — A device that counts from zero to a predetermined number, then rolls over to
zero and begins counting again.
full-duplex transmission — Communication on a channel in which data can be sent and received
simultaneously.
H — The upper byte of the 16-bit index register (H:X) in the CPU08.
H — The half-carry bit in the condition code register of the CPU08. This bit indicates a carry from the
low-order four bits of the accumulator value to the high-order four bits. The half-carry bit is required for
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binary-coded decimal arithmetic operations. The decimal adjust accumulator (DAA) instruction uses the
state of the H and C bits to determine the appropriate correction factor.
hexadecimal — Base 16 numbering system that uses the digits 0 through 9 and the letters A through F.
high byte — The most significant eight bits of a word.
illegal address — An address not within the memory map
illegal opcode — A nonexistent opcode.
I — The interrupt mask bit in the condition code register of the CPU08. When I is set, all interrupts are
disabled.
index register (H:X) — A 16-bit register in the CPU08. The upper byte of H:X is called H. The lower
byte is called X. In the indexed addressing modes, the CPU uses the contents of H:X to determine the
effective address of the operand. H:X can also serve as a temporary data storage location.
input/output (I/O) — Input/output interfaces between a computer system and the external world. A CPU
reads an input to sense the level of an external signal and writes to an output to change the level on an
external signal.
instructions — Operations that a CPU can perform. Instructions are expressed by programmers as
assembly language mnemonics. A CPU interprets an opcode and its associated operand(s) and
instruction.
interrupt — A temporary break in the sequential execution of a program to respond to signals from
peripheral devices by executing a subroutine.
interrupt request — A signal from a peripheral to the CPU intended to cause the CPU to execute a
subroutine.
I/O — See “input/output (I/0).”
IRQ — See "external interrupt module (IRQ)."
jitter — Short-term signal instability.
latch — A circuit that retains the voltage level (logic 1 or logic 0) written to it for as long as power is applied
to the circuit.
latency — The time lag between instruction completion and data movement.
least significant bit (LSB) — The rightmost digit of a binary number.
logic 1 — A voltage level approximately equal to the input power voltage (VDD).
logic 0 — A voltage level approximately equal to the ground voltage (VSS).
low byte — The least significant eight bits of a word.
low voltage inhibit module (LVI) — A module in the M68HC08 Family that monitors power supply
voltage.
LVI — See "low voltage inhibit module (LVI)."
M68HC08 — A Motorola family of 8-bit MCUs.
mark/space — The logic 1/logic 0 convention used in formatting data in serial communication.
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mask — 1. A logic circuit that forces a bit or group of bits to a desired state. 2. A photomask used in
integrated circuit fabrication to transfer an image onto silicon.
mask option — A optional microcontroller feature that the customer chooses to enable or disable.
mask option register (MOR) — An EPROM location containing bits that enable or disable certain MCU
features.
MCU — Microcontroller unit. See “microcontroller.”
memory location — Each M68HC08 memory location holds one byte of data and has a unique address.
To store information in a memory location, the CPU places the address of the location on the address
bus, the data information on the data bus, and asserts the write signal. To read information from a memory
location, the CPU places the address of the location on the address bus and asserts the read signal. In
response to the read signal, the selected memory location places its data onto the data bus.
memory map — A pictorial representation of all memory locations in a computer system.
microcontroller — Microcontroller unit (MCU). A complete computer system, including a CPU, memory,
a clock oscillator, and input/output (I/O) on a single integrated circuit.
modulo counter — A counter that can be programmed to count to any number from zero to its maximum
possible modulus.
monitor ROM — A section of ROM that can execute commands from a host computer for testing
purposes.
MOR — See "mask option register (MOR)."
most significant bit (MSB) — The leftmost digit of a binary number.
multiplexer — A device that can select one of a number of inputs and pass the logic level of that input
on to the output.
N — The negative bit in the condition code register of the CPU08. The CPU sets the negative bit when
an arithmetic operation, logical operation, or data manipulation produces a negative result.
nibble — A set of four bits (half of a byte).
object code — The output from an assembler or compiler that is itself executable machine code, or is
suitable for processing to produce executable machine code.
opcode — A binary code that instructs the CPU to perform an operation.
open-drain — An output that has no pullup transistor. An external pullup device can be connected to the
power supply to provide the logic 1 output voltage.
operand — Data on which an operation is performed. Usually a statement consists of an operator and
an operand. For example, the operator may be an add instruction, and the operand may be the quantity
to be added.
oscillator — A circuit that produces a constant frequency square wave that is used by the computer as
a timing and sequencing reference.
OTPROM — One-time programmable read-only memory. A nonvolatile type of memory that cannot be
reprogrammed.
overflow — A quantity that is too large to be contained in one byte or one word.
page zero — The first 256 bytes of memory (addresses $0000–$00FF).
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parity — An error-checking scheme that counts the number of logic 1s in each byte transmitted. In a
system that uses odd parity, every byte is expected to have an odd number of logic 1s. In an even parity
system, every byte should have an even number of logic 1s. In the transmitter, a parity generator appends
an extra bit to each byte to make the number of logic 1s odd for odd parity or even for even parity. A parity
checker in the receiver counts the number of logic 1s in each byte. The parity checker generates an error
signal if it finds a byte with an incorrect number of logic 1s.
PC — See “program counter (PC).”
peripheral — A circuit not under direct CPU control.
phase-locked loop (PLL) — A oscillator circuit in which the frequency of the oscillator is synchronized
to a reference signal.
PLL — See "phase-locked loop (PLL)."
pointer — Pointer register. An index register is sometimes called a pointer register because its contents
are used in the calculation of the address of an operand, and therefore points to the operand.
polarity — The two opposite logic levels, logic 1 and logic 0, which correspond to two different voltage
levels, VDD and VSS.
polling — Periodically reading a status bit to monitor the condition of a peripheral device.
port — A set of wires for communicating with off-chip devices.
prescaler — A circuit that generates an output signal related to the input signal by a fractional scale factor
such as 1/2, 1/8, 1/10 etc.
program — A set of computer instructions that cause a computer to perform a desired operation or
operations.
program counter (PC) — A 16-bit register in the CPU08. The PC register holds the address of the next
instruction or operand that the CPU will use.
pull — An instruction that copies into the accumulator the contents of a stack RAM location. The stack
RAM address is in the stack pointer.
pullup — A transistor in the output of a logic gate that connects the output to the logic 1 voltage of the
power supply.
pulse-width — The amount of time a signal is on as opposed to being in its off state.
pulse-width modulation (PWM) — Controlled variation (modulation) of the pulse width of a signal with
a constant frequency.
push — An instruction that copies the contents of the accumulator to the stack RAM. The stack RAM
address is in the stack pointer.
PWM period — The time required for one complete cycle of a PWM waveform.
RAM — Random access memory. All RAM locations can be read or written by the CPU. The contents of
a RAM memory location remain valid until the CPU writes a different value or until power is turned off.
RC circuit — A circuit consisting of capacitors and resistors having a defined time constant.
read — To copy the contents of a memory location to the accumulator.
register — A circuit that stores a group of bits.
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reserved memory location — A memory location that is used only in special factory test modes. Writing
to a reserved location has no effect. Reading a reserved location returns an unpredictable value.
reset — To force a device to a known condition.
ROM — Read-only memory. A type of memory that can be read but cannot be changed (written). The
contents of ROM must be specified before manufacturing the MCU.
SCI — See "serial communication interface module (SCI)."
serial — Pertaining to sequential transmission over a single line.
serial communications interface module (SCI) — A module in the M68HC08 Family that supports
asynchronous communication.
serial peripheral interface module (SPI) — A module in the M68HC08 Family that supports
synchronous communication.
set — To change a bit from logic 0 to logic 1; opposite of clear.
shift register — A chain of circuits that can retain the logic levels (logic 1 or logic 0) written to them and
that can shift the logic levels to the right or left through adjacent circuits in the chain.
signed — A binary number notation that accommodates both positive and negative numbers. The most
significant bit is used to indicate whether the number is positive or negative, normally logic 0 for positive
and logic 1 for negative. The other seven bits indicate the magnitude of the number.
software — Instructions and data that control the operation of a microcontroller.
software interrupt (SWI) — An instruction that causes an interrupt and its associated vector fetch.
SPI — See "serial peripheral interface module (SPI)."
stack — A portion of RAM reserved for storage of CPU register contents and subroutine return
addresses.
stack pointer (SP) — A 16-bit register in the CPU08 containing the address of the next available storage
location on the stack.
start bit — A bit that signals the beginning of an asynchronous serial transmission.
status bit — A register bit that indicates the condition of a device.
stop bit — A bit that signals the end of an asynchronous serial transmission.
subroutine — A sequence of instructions to be used more than once in the course of a program. The last
instruction in a subroutine is a return from subroutine (RTS) instruction. At each place in the main program
where the subroutine instructions are needed, a jump or branch to subroutine (JSR or BSR) instruction is
used to call the subroutine. The CPU leaves the flow of the main program to execute the instructions in
the subroutine. When the RTS instruction is executed, the CPU returns to the main program where it left
off.
synchronous — Refers to logic circuits and operations that are synchronized by a common reference
signal.
TIM — See "timer interface module (TIM)."
timer interface module (TIM) — A module used to relate events in a system to a point in time.
timer — A module used to relate events in a system to a point in time.
USB and PS/2 Multimedia Keyboard Interface, Rev. 1
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toggle — To change the state of an output from a logic 0 to a logic 1 or from a logic 1 to a logic 0.
tracking mode — Mode of low-jitter PLL operation during which the PLL is locked on a frequency. Also
see "acquisition mode."
two’s complement — A means of performing binary subtraction using addition techniques. The most
significant bit of a two’s complement number indicates the sign of the number (1 indicates negative). The
two’s complement negative of a number is obtained by inverting each bit in the number and then adding
1 to the result.
unbuffered — Utilizes only one register for data; new data overwrites current data.
unimplemented memory location — A memory location that is not used. Writing to an unimplemented
location has no effect. Reading an unimplemented location returns an unpredictable value. Executing an
opcode at an unimplemented location causes an illegal address reset.
V —The overflow bit in the condition code register of the CPU08. The CPU08 sets the V bit when a two's
complement overflow occurs. The signed branch instructions BGT, BGE, BLE, and BLT use the overflow
bit.
variable — A value that changes during the course of program execution.
VCO — See "voltage-controlled oscillator."
vector — A memory location that contains the address of the beginning of a subroutine written to service
an interrupt or reset.
voltage-controlled oscillator (VCO) — A circuit that produces an oscillating output signal of a frequency
that is controlled by a dc voltage applied to a control input.
waveform — A graphical representation in which the amplitude of a wave is plotted against time.
wired-OR — Connection of circuit outputs so that if any output is high, the connection point is high.
word — A set of two bytes (16 bits).
write — The transfer of a byte of data from the CPU to a memory location.
X — The lower byte of the index register (H:X) in the CPU08.
Z — The zero bit in the condition code register of the CPU08. The CPU08 sets the zero bit when an
arithmetic operation, logical operation, or data manipulation produces a result of $00.
USB and PS/2 Multimedia Keyboard Interface, Rev. 1
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