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Texas Instruments An Introduction to M-LVDS and Clock and Data Distribution Applications (Rev. B) Application notes
Application Report
SNLA113B – November 2008 – Revised April 2013
AN-1926 An Introduction to M-LVDS and Clock and Data
Distribution Applications
.....................................................................................................................................................
ABSTRACT
This application note provides an overview of the M-LVDS standard, introduces Texas Instrument’s
current M-LVDS product portfolio, describes common M-LVDS applications, and details important design
guidelines.
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Contents
M-LVDS Standard Overview .............................................................................................. 2
Driver Characteristics ....................................................................................................... 2
Receiver Characteristics ................................................................................................... 4
M-LVDS Portfolio ............................................................................................................ 5
M-LVDS Applications ....................................................................................................... 6
Clock Distribution in AdvancedTCA Systems ........................................................................... 6
Clock Distribution in MicroTCA Systems ................................................................................. 7
M-LVDS as a Short Reach RS-485 Alternative ......................................................................... 9
Signal Distribution with Point-to-Point Links ........................................................................... 10
Wired-OR Implementation ................................................................................................ 11
Design Guidelines ......................................................................................................... 11
Conclusion .................................................................................................................. 12
References ................................................................................................................. 12
List of Figures
1
Multipoint Network .......................................................................................................... 2
2
Driver VOD and VOS Comparison
3
3
Point-to-Point Link
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5
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11
.......................................................................................
..........................................................................................................
M-LVDS Standard Defines Two Receiver Types .......................................................................
DS91D176 Driver Output Amplitude as a Function of Output DC Load .............................................
AdvancedTCA Clock Distribution Interface Example ...................................................................
MicroTCA Non-redundant Clock Distribution Interface Example .....................................................
MicroTCA Redundant Clock Distribution Interface Example ..........................................................
CAT5e Length as a Function of Bit Rate for M-LVDS and RS-485 Point-to-Point Links ..........................
Signal Distribution with Point-to-Point Links ...........................................................................
Wired-OR Circuit with Three M-LVDS Drivers and One Type 2 Receiver .........................................
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List of Tables
1
Comparison of Key Driver Parameters ................................................................................... 4
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Comparison of Key Receiver Parameters ............................................................................... 5
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Summary of the First Generation of TI’s M-LVDS Devices
4
Summary of the Second Generation of TI’s M-LVDS Devices........................................................ 6
5
Summary of the TI’s Fan-out Buffers ................................................................................... 10
...........................................................
5
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1
M-LVDS Standard Overview
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Since it’s been ratified in early 2002, the TIA/EIA-899 (Multipoint Low Voltage Differential Signaling or MLVDS) has become a popular electrical standard for binary data interchange over multipoint clock
distribution and data buses. While keeping many benefits of LVDS circuits (high speed, low power
consumption, and excellent noise immunity), M-LVDS circuits include additional provisions – a stronger
drive, controlled transition times, extended input common mode voltage range, and failsafe – all necessary
for reliable multipoint networks.
.
1
M-LVDS Standard Overview
The M-LVDS standard specifies electrical characteristic of line drivers and receivers intended for general
data transport over a multipoint bus (Figure 1) where up to 32 nodes may be connected. More specifically,
it defines driver output characteristics, and input characteristics of two receiver types. The following two
sections summarize the key M-LVDS driver and receiver characteristics and compare them to the
characteristics of drivers and receivers conforming to the two other popular differential standards: RS-485
(TIA/EIA-485-A) and LVDS (TIA/EIA-644-A).
Node N-1
Node 1
R
D
R
D
Differential
Balanced Bus
RT
RT
R
D
D
Node 2
R
Node N
Figure 1. Multipoint Network
2
Driver Characteristics
Per TIA/EIA-899 standard, an M-LVDS driver generates a differential signal with 480 – 650 mV amplitude
and an offset within the 0.3V to 2.1V range. The signal must have 10% – 90% transition times (rise and
fall) of 1 ns or greater and up to one half of a unit interval (tUI).
When compared to RS-485 drivers, M-LVDS drivers provide significantly reduced signal amplitudes (See
Figure 2) that result in lower power consumption and reduced electromagnetic interference (EMI). The
lower signal amplitudes enable higher signaling rates or signal frequencies. While the M-LVDS standard
specifies maximum signaling rate of 500 Mbps based on the 1 ns minimum transition time, current
commercially available M-LVDS drivers peak at 250 Mbps. On the other side, the fastest RS-485 drivers
typically peak at 10 Mbps with a few unique devices reaching 30 Mbps to 50 Mbps rates. The benefits of
faster speeds, lower power, and reduced EMI come at the expense of reduced noise margins, however,
by following necessary design guidelines given later in this application note, successful M-LVDS networks
can be designed without significant efforts.
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Driver Characteristics
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RS-485
LVDS
1.375V
VOD MIN = 0.25V
1.125V
0.3V
VOD MAX = 0.45V
VOS Range
VOD MIN = 0.48V
VOD MAX = 0.65V
M-LVDS
2.1V
VOS Range
VOD MIN = 1.5V
VOS Range
VOD MAX = 5V
3V
-1V
Figure 2. Driver VOD and VOS Comparison
When compared to LVDS drivers, M-LVDS drivers pose as drivers with a stronger drive (larger IOD). The
stronger drive enables the M-LVDS drivers to drive signals across multipoint networks that are typically
doubly terminated. Doubly terminated networks present a heavier load to the driver, so the stronger drive
is necessary for retaining required signal amplitudes. Both M-LVDS and RS-485 driver output amplitudes
are typically specified with a 50Ω differential load. This is a load that a driver typically sees in a multipoint
network with a double termination as illustrated in Figure 1. LVDS driver output amplitudes are specified
with a 100Ω differential load. This is a load that a driver typically sees in a point-to-point link with a single
termination as shown in Figure 3. M-LVDS drivers also pose as drivers with controlled transition times, a
characteristic that is highly desirable for multipoint networks. On the other side, LVDS drivers with
transition times typically ranging from as low as 100 ps to only several 100 ps are rarely a good fit for any
topology except a point-to-point topology.
D
Differential Transmission Line
RT
R
Figure 3. Point-to-Point Link
Table 1 shows a comparison of key RS-485, M-LVDS, and LVDS driver characteristics:
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Receiver Characteristics
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Table 1. Comparison of Key Driver Parameters
Parameter
3
RS-485
M-LVDS
LVDS
VOD (V)
1.5 to 5.0
0.48 to 0.65
0.25 to 0.45
VOS (V)
-1.0 to 3.0
0.3 to 2.1
1.125 to 1.375
IOD (mA)
28 to 93
9 to 13
2.5 to 4.5
IOS (mA)
<250
<43
<24
tRISE / tFALL Min (ns)
N/A
1
N/A
tRISE / tFALL Typ (ns)
5 to 50
1 to 5
<1
tRISE / tFALL Max (ns)
0.3 tUI
0.5 tUI
0.3 tUI
Typ Data Rate (Mbps)
DC to 10
DC to 500
DC to 3125
Receiver Characteristics
The key receiver specifications are input voltage threshold, input common mode range, and input leakage
current. The input threshold levels differentiate the two types of M-LVDS receivers. Type 1 receivers have
threshold levels centered at 0V differential and provide higher noise margin than Type 2 receivers. Type 1
receivers are used in clock or data transmission applications that either require application specific
external failsafe networks or don’t require failsafe provisions at all. Type 2 receivers have threshold levels
shifted by +100 mV differential. The shift lowers noise margin but provides a known, low output state when
a bus or a transmission line is undriven and having 0V differential bias. In addition to the failsafe, WiredOR function is another possible application of Type 2 receivers. More information about the Wired-OR
implementation using M-LVDS devices is given later in the note.
When compared to RS-485 and LVDS receivers, M-LVDS receivers have the tightest threshold levels.
Figure 4 illustrates threshold levels and maximum recommended differential input amplitude levels of RS485, LVDS, and both M-LVDS receivers.
5V
RS-485
M-LVDS
Type 1
High
M-LVDS
Type 2
2.4 V
High
High
200 mV
LVDS
150 mV
High
2.4V
100 mV
50 mV
0V
VID
0V
0V
-50 mV
-200 mV
Low
Low
-100 mV
Low
-2.4V
-2.4V
Low
Transition
Region
-5V
Figure 4. M-LVDS Standard Defines Two Receiver Types
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M-LVDS Portfolio
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The M-LVDS receiver input common mode range of –1.4V to 3.8V makes M-LVDS a robust interface for
connecting sub-systems that may have a potential difference between their ground references of ±1V.
Given the fact that many M-LVDS drivers have a much tighter VOS specification than what the standard
specifies, the unwanted potential difference between network nodes may be greater than ±1V. RS-485
receivers are suitable for even harsher environments. Their common mode range of −7V to 12V allows for
±7V of unwanted potential difference between the nodes. The LVDS receivers pose as the least robust
receivers given their input common mode range of 0V to 2.4V (0 to VDD is also very common), however,
tight VOS specification of LVDS drivers allows a potential difference between driver and receiver circuit
commons of ±1V.
As all M-LVDS, LVDS and RS-485 devices may be used in multipoint networks, the receivers have to
pose as light loads to the active drivers so that many of them can be connected on a single bus. Based on
this requirement, all three standards specify maximum input leakage current which allows up to 32 loads
(receivers or inactive drivers) on a bus. A load of a single device or one unit load for M-LVDS and LVDS
receivers is equivalent to a 120 kΩ resistor into a 0V to 2.4V voltage source. One unit load for an RS-485
receiver is equivalent to a 12 kΩ resistor into a 0V to 5V source. In addition, the RS-485 devices are
available in 1/2, 1/4 and 1/8 unit loads allowing up to 256 devices on a bus. Table 2 shows key RS-485,
M-LVDS and LVDS receiver characteristics.
Table 2. Comparison of Key Receiver Parameters
4
Parameter
RS-485
M-LVDS
LVDS
VID (V)
0.4 to 5.0
0.1 to 2.4
0.2 to 2.4
VCM (V)
−5.0 to 12.0
−1.4 to 3.8
0 to 2.4
IIN (µA)
<1000
<32
<20
M-LVDS Portfolio
TI’s current M-LVDS product family consists of a wide array of devices ranging from single channel
transceivers and driver/receiver pairs to quad channel transceivers, drivers and 1:4 repeaters/fan-out
buffers.
The first generation of TI’s M-LVDS devices introduced in 2006 includes four single channel devices
providing 100 MHz / 200 Mbps transceivers and driver/receiver pairs with Type 1 and Type 2 receiver
options. Table 3 summarizes the available options. Detailed information on each device can be found at
www.ti.com.
Table 3. Summary of the First Generation of TI’s M-LVDS Devices
Part Number
Description
Package
Features
DS91D176
100 MHz Single Channel MLVDS Transceiver
SOIC-8
`176 Pinout, Type 1 Receiver
DS91C176
100 MHz Single Channel MLVDS Transceiver
SOIC-8
`176 Pinout, Type 2 Receiver
DS91D180
100 MHz Single Channel MLVDS Line Driver / Receiver
Pair
SOIC-8
`180 Pinout, Type 1 Receiver
DS91C180
100 MHz Single Channel MLVDS Line Driver / Receiver
Pair
SOIC-8
`180 Pinout, Type 2 Receiver
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M-LVDS Applications
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The first generation of M-LVDS devices features drivers with transition times (1.8 ns typ) optimized for
multipoint networks, more specifically for ATCA clock distribution networks. In addition, the drivers feature
output amplitude control circuitry that maintains constant VOD over a wide range of loads as illustrated in
Figure 5. This feature helps with the noise margin in heavily loaded backplanes.
Figure 5. DS91D176 Driver Output Amplitude as a Function of Output DC Load
The second generation of M-LVDS devices introduced in 2008 includes four quad channel 125 MHz / 250
Mbps devices. Table 4 summarizes the available options. Similar to the first generation, these devices
also feature controlled transition times (2.0 ns typ) and output amplitude control circuitry for maintaining
constant VOD over a wide range of loads.
Table 4. Summary of the Second Generation of TI’s M-LVDS Devices
Part Number
5
Description
Package
LLP-32
Features
DS91M040
125 MHz Quad M-LVDS
Transceiver
Space saving package, pin
settable receiver type, per
channel output enable
DS91M047
125 MHz Quad M-LVDS Line
Driver
SOIC-16
`047 pinout, per channel output
enable
DS91M124
125 MHz 1:4 M-LVDS
Repeater with LVCMOS Input
SOIC-16
Low skew, per channel output
enable
DS91M125
125 MHz 1:4 M-LVDS
Repeater with LVDS Input
SOIC-16
LVDS input, per channel output
enable
M-LVDS Applications
M-LVDS devices are primarily used in clock distribution networks such as clock distribution interfaces of
AdvancedTCA (ATCA) and MicroTCA (µTCA) based systems. However, the long list of features that MLVDS devices bring, make them ideal for many other applications that can benefit from high-speed, low
power, reduced EMI, controlled transition times and failsafe provisions. This section of the application note
provides a brief overview of common M-LVDS applications.
6
Clock Distribution in AdvancedTCA Systems
AdvancedTCA is PCI Industrial Computer Manufacturers Group’s (PICMG) open standard for modular
communications systems with capacity of up to 2.5 Tbps. The standard specifies a range of system level
design aspects and parameters including mechanical dimensions, power distribution, thermal
considerations and data transport.
As in many communication systems, AdvancedTCA (ATCA) based systems require synchronization of its
internal and external networks. Synchronization Clock Interface is a section of the ATCA Base
Specification (PICMG 3.0) that specifies M-LVDS as the signalling technology of choice. In an ATCA
system, there are three redundant clocks (totaling 6) distributed to up to 16 backplane slots in a multipoint
fashion:
• CLK1A and CLK1B are for redundant 8 kHz standard digital telephony transmission system clocks.
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Clock Distribution in MicroTCA Systems
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•
•
CLK2A and CLK2B are for 19.44 MHz clocks for synchronization of the SONET/SDH networks.
CLK3A and CLK3B are for user-defined signals (clock or data).
Each ATCA clock network consists of up to 16 M-LVDS ports connected to a common clock bus as
illustrated in Figure 6. The clock bus, physically located on the standard backplane, is a 130Ω differential
microstrip terminated on both sides with 80Ω resistors. An M-LVDS port is an I/O of a M-LVDS device
located on a line card. An interconnect that connects an M-LVDS port to a clock bus is referred to as
unterminated stub. A stub in a standard ATCA clock distribution multipoint network consists of a
differential trace connecting the M-LVDS device’s I/O pins to the line card’s standard Zone 2 connector
pins and the connector’s conductors. The standard ATCA Zone 2 connectors are Advanced Differential
Fabric (ADF) connectors such as Tyco’s HM-Zd connectors. Minimizing electrical length of stubs in ATCA
clock distribution networks is critical and is discussed later in the design guidelines. The ATCA standard
specifies 1 inch (25.4 mm) as the maximum stub length.
Line Card in SLOT 1
Line Card in SLOT 16
M-LVDS DRIVERS/RECEIVERS
M-LVDS DRIVERS/RECEIVERS
80:
CLK1A
80:
80:
CLK1B
80:
80:
CLK2A
80:
80:
CLK2B
80:
80:
CLK3A
80:
80:
CLK3B
80:
Differential Bus Impedance, Zo = 130:
AdvancedTCA Backplane
Figure 6. AdvancedTCA Clock Distribution Interface Example
7
Clock Distribution in MicroTCA Systems
MicroTCA standard provides a modular, open platform for Low to Mid-range telecom and datacom
equipment with capacity of up to 144 Gbps. MircoTCA systems are optimized for smaller physical sizes
and more cost sensitive applications.
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Clock Distribution in MicroTCA Systems
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Similar to the ATCA standard, the MicroTCA (uTCA) standard also specifies the use of M-LVDS
technology for clock distribution networks. The MicroTCA specification (PICMG MTCA.0) defines nonredundant and redundant clocking architectures. The non-redundant clocking architecture is for systems
with a single MicroTCA Carrier Hub (MCH). This architecture allows up to three point-to-point links per
Advanced Mezzanine Card (AMC) and up to 36 links per MCH. Figure 7 shows a single point-to-point
clock link between a MCH and AMC in a non-redundant backplane. Note that the clock bus is terminated
on the backplane at the MCH card and on the AMC card. In point-to-point links, M-LVDS device transmit
clock signals with maximum noise margin.
MCH
AMC
M-LVDS Transceiver
M-LVDS Transceiver
100:
100:
Non-Redundant Backplane
Figure 7. MicroTCA Non-redundant Clock Distribution Interface Example
The redundant clock architecture is for dual MCH systems that operate in a redundant manner. In this
clock architecture, each MCH connects to each AMC with a point-to-point link as in Figure 7. However, the
connection between an AMC to each of the MCH cards is implemented with a multipoint network as
illustrated in Figure 8. The effects of unterminated stubs in this multipoint topology variant are minimized
with the use of series resistors. The controlled signal edges of M-LVDS devices further aid in distributing
clocks to all cards within a system.
Primary
MCH
Secondary
MCH
AMC
M-LVDS
M-LVDS
M-LVDS
100:
16.5:
100:
16.5:
16.5:
16.5:
16.5:
16.5:
100:
Redundant Backplane
Figure 8. MicroTCA Redundant Clock Distribution Interface Example
8
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M-LVDS as a Short Reach RS-485 Alternative
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8
M-LVDS as a Short Reach RS-485 Alternative
While RS-485 multipoint differential busses are long reach and typically implemented with cables as
transmission media, M-LVDS devices have found applications in backplane environments. Multipoint links
that utilize cables as interconnects are possible with M-LVDS as well. However, system designers need to
pay extra attention to stub lengths, bus impedance, and potential differences between nodes. Making
stubs as short as possible, spacing the loads evenly, and ensuring less than ±1V of potential difference
between the nodes enables robust designs of M-LVDS multipoint networks outside the backplane arena.
Another common application space that RS-485 and M-LVDS interfaces share is a point-to-point signal
transmission over cables. When it comes to driving signals over long cables, a larger swing of RS-485 and
especially wider input common mode range do help to achieve longer transmission distances; however,
M-LVDS devices have the advantages of higher speed, lower power consumption and lower EMI. These
key M-LVDS characteristics are beneficial in many applications.
Figure 9 shows a typical CAT5e cable length as a function of bit rate for the RS-485 and M-LVDS point-topoint links. The sloped portion of the RS-485 curves is determined based on the maximum attenuation of
9 dB at the frequency of 1/tUI in hertz, where tUI is a unit interval at a given signaling rate. This is an
accepted industry guideline for determining maximum signaling rate for the RS-485 point-to-point links.
The flat portion of the RS-485 curve is based on the ohmic loss of a typical CAT5e cable (9Ω / 100m).
For networks that use low voltage differential drivers (M-LVDS, LVDS), the maximum attenuation of 6 dB
at 1/tUI hertz may be used as a general guideline when determining maximum signaling rate for a given
cable length. The guideline assumes dc-balanced data, point-to-point links, zero crosstalk and pair-to-pair
skew, and no external interference. Note the dashed portion of the M-LVDS curve. In theory, M-LVDS
interfaces can transmit sub-Mbps signals over hundreds of meters of CAT5e cable, however, M-LVDS
receivers can only handle ±1V of ground noise. System designers building M-LVDS interfaces over long
cables need to ensure that the ground noise does not exceed the ±1V limit or they need to fully eliminate
the DC component of signals by implementing either AC-coupled or transformer-coupled interfaces.
CAT5e CABLE LENGTH [m]
10000
1000
RS-485
100
M-LVDS
10
1
0.01
0.1
1
10
100
1000
MAXIMUM SIGNALING RATE [Mbps]
Figure 9. CAT5e Length as a Function of Bit Rate for M-LVDS and RS-485 Point-to-Point Links
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Signal Distribution with Point-to-Point Links
9
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Signal Distribution with Point-to-Point Links
Among many other benefits, signal distribution with multipoint networks reduces connector size and cable
diameter, lowers conductor count and PCB thickness, and ultimately decreases system cost. These
benefits come at the cost of signal integrity, maximum transmission reach and speed. When satisfactory
signal integrity, transmission reach or speed using multipoint networks cannot be achieved, signal
distribution with point-to-point links may be the only option. Signal distribution with point-to-point links is
accomplished using fan-out buffers as illustrated in Figure 10.
Fan-out Buffer
RT
R
RT
R
RT
R
RT
R
Figure 10. Signal Distribution with Point-to-Point Links
The current M-LVDS portfolio offers two 1:4 fan-out buffers/signal repeaters: DS91M124 and DS91M125.
The DS91M124 is the fan-out buffer with an LVCMOS input and four M-LVDS outputs. It provides
LVCMOS-to-M-LVDS level translation and 1:4 signal distribution. It is best suited for taking an LVCMOS
signal from a local signal source and distributing it to local or remote M-LVDS receivers.
The DS91M125 is the fan-out buffer with an LVDS input and four M-LVDS outputs. It can take an LVDS
compatible signal from a local or remote source via a point-to-point or multidrop link and distribute it to
four local or remote M-LVDS receivers or M-LVDS multipoint networks.
In addition to M-LVDS fan-out buffer, there are also several LVDS and Bus LVDS fan-out buffers. Table 5
provides the summary.
Table 5. Summary of the TI’s Fan-out Buffers
Part Number
10
Description
Package
Features
DS91M124
125 MHz 1:4 M-LVDS
Repeater with LVCMOS Input
SOIC-16
Low skew, per channel output
enable
DS91M125
125 MHz 1:4 M-LVDS
Repeater with LVDS Input
SOIC-16
LVDS input, per channel output
enable
DS92CK16
125 MHz Bus LVDS 1:6 Clock
Buffer/Bus Transceiver
TSSOP-24
1:6 LVCMOS Outputs, Bus
LVDS I/O
DS90LV110T
1:10 LVDS Clock / Data
Distributor
TSSOP-28
200 MHz / 400 Mbps
Operation, Low Skew
DS90LV110AT
1:10 LVDS Clock / Data
Distributor with Failsafe
TSSOP-28
Failsafe Operation
DS10BR254
1.5 Gbps 2:4 LVDS Repeater
LLP-40
LOS, 8 kV ESD
DS25BR204
3.125 Gpbs 2:4 LVDS
Repeater, with Transmit Preemphasis and Receive
Equalization
LLP-40
Input Equalization, Output Preemphasis, LOS
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Wired-OR Implementation
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10
Wired-OR Implementation
M-LVDS drivers and Type 2 receivers can be used to implement Wired-OR logic function. Figure 11
illustrates an example of the implementation using three M-LVDS drivers and a single Type 2 receiver
interconnected with a doubly terminated multipoint bus. All driver inputs are set to H while driver output
enable (DE) pins serve as inputs. The receiver output serves as an output of the function. When disabled,
the drivers outputs provide a 0V differential bias to the bus and the Type 2 receiver detects a logic L.
When any of the drivers is enabled, the bus is biased to a H and the receiver detect a logic H.
H
H
DI
IN1
DE
DI
IN3
D
DE
D
RT
RT
IN2
DE
R
D
DI
H
RO
OUT
Figure 11. Wired-OR Circuit with Three M-LVDS Drivers and One Type 2 Receiver
11
Design Guidelines
Multipoint networks provide designers with an economical and simple method to interface multiple devices
using a single interconnect or a bus. While simplicity and low cost make multipoint networks appealing for
many, designing these networks is never a straightforward task. The following list provides design
guidelines for implementing reliable M-LVDS multipoint networks.
• Design or select interconnects that are optimal for multipoint networks. In a multipoint network, each
port presents a load to the bus. The loads are typically capacitive consisting of M-LVDS I/O
capacitance and intrinsic capacitance of the stubs. A capacitive load on a bus with uniform impedance
lowers the impedance of the bus at the port location and creates impedance mismatches. When
multiple loads are connected on a bus with relatively uniform spacing between the loads, the overall
characteristic impedance of the bus becomes lower. Lower characteristic impedance of the bus
requires lower value termination resistors. Lower value termination resistors mean lower DC load for a
signal driver and ultimately lower signal amplitude. Even though the M-LVDS drivers feature control
circuitry that keeps output amplitude constant, the output amplitude is only constant for a load of 40Ω
or higher. This means that the differential characteristic impedance of the loaded bus needs to be 80Ω
or higher. To achieve this, select or design a bus with higher than nominal (100Ω differential)
characteristic impedance so that its impedance does not go below 80Ω when fully loaded.
• Select M-LVDS drivers with the slowest transition time that will satisfy the bandwidth requirements of
the system. Drivers that have transition times of one half of the unit interval (UI) at the bit rate of
interest provide the highest noise margin. For example, the M-LVDS drivers have typical transition
times of 2 ns. This makes them ideal for operation at 250 Mbps/125 MHz (4 ns UI). At 250 Mbps, any
reflections that may occur in the bus have energy that is only at the frequencies that are multiples of
the Nyquist frequency (125 MHz). With this distribution of the signal energy, reflections that have
energy at the frequencies that are higher than the Nyquist frequency are absent. The reflections that
have energy at frequencies higher than the Nyquist frequency are a serious threat to signal distribution
in multipoint networks.
• Minimize the length of stubs as much as possible. M-LVDS devices are typically fine with stubs that
are 1 inch (2.5 cm) or shorter (Connector electrical length should be considered when determining the
total stub length). Anything longer than that may cause a system to fail. Experimental data presented in
the application note AN-1503 has shown that shortening the stubs from 1 to 1/2 inches may increase
noise margin by as much as 50%. In addition, when noise margin is at premium, one should consider
maximizing the stub impedance. This can be accomplished by increasing the dielectric thickness of the
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AN-1926 An Introduction to M-LVDS and Clock and Data Distribution
Applications
Copyright © 2008–2013, Texas Instruments Incorporated
11
Conclusion
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•
12
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material, reducing the stub width, and uncoupling or loosely coupling the individual traces of the stub.
Place M-LVDS drivers next to termination resistors provided other system constrains allow it. The
worst driver location is in the middle of a multipoint network; the receivers adjacent to the driver always
have the worst noise margin. By placing the drivers at one end of the network close to one of the two
termination resistors, network topologies with longer signal paths are created. The longer signal paths
have more loss and as result increase transition times of the signal as it propagates from the driver
toward the furthest receiver. Slower transition times are more “forgiving” when encountering impedance
discontinuities.
Any power supply noise reduces available noise margin. Ensure that M-LVDS devices are properly
decoupled. Each VDD or GND pin of an M-LVDS device should be connected to a printed circuit board
(PCB) through a low inductance path. For best results, one or more vias should be used to connect a
VDD or GND pin to the nearby plane. Ideally, via placement is immediately adjacent to the pin to avoid
adding intrinsic trace inductance. Bypass capacitors should be placed close to VDD pins. Small physical
size capacitors, such as 0402, X7R, surface mount capacitors should be used to minimize package
inductance of capacitors. Each bypass capacitor should be connected to the power and ground planes
through vias tangent to the pads of the capacitor. An X7R surface mount capacitor of size 0402 has
about 0.5 nH of body inductance. At frequencies above 30 MHz or so, X7R capacitors behave as low
impedance inductors. To extend the operating frequency range to a few hundred MHz, an array of
different capacitor values such as 100 pF, 1 nF, 0.03 µF, and 0.1 µF are commonly used in parallel.
The most effective bypass capacitor can be built using sandwiched layers of power and ground at a
separation of 2–3 mils. With a 2 mil FR-4 dielectric, there is approximately 500 pF per square inch of a
PCB. For devices packaged in LLP packages (e.g. DS91M040), the die attach pad (DAP) should be
connected to a ground plane through an array of vias. The via array reduces the effective inductance
to ground and enhances the thermal performance of the LLP package.
Conclusion
M-LVDS is a versatile interface technology optimal for a range of applications including clock distribution
systems in popular AdvancedTCA and MircoTCA backplanes. TI's current M-LVDS portfolio offers devices
that bring all standard and some unique M-LVDS features to engineers looking to solve point-to-point and
multipoint network challenges.
13
References
DS91M040 125 MHz Quad M-LVDS Transceiver (SNLS283)
Application Note AN-1503 – Designing an ATCA Compliant M-LVDS Clock Distribution Network
(SNLA082)
LVDS Owner’s Manual 4th Edition, National Semiconductor Corporation, Summer 2008
TIA/EIA-899, Electrical Characteristics of Multipoint Low Voltage Differential Signaling (M-LVDS) Interface
Circuits, Telecommunications Industry Association. March 2002
TIA/EIA-644-A, Electrical Characteristics of Low Voltage Differential Signaling (LVDS) Interface Circuits,
Telecommunications Industry Association. February 2001
TIA/EIA-485-A, Electrical Characteristics of Generators and Receivers for Use in Balanced Digital
Multipoint Systems, Telecommunications Industry Association. March 1998
PICMG 3.0 Revision 2.0, AdvancedTCA Base Specification, PCI Industrial Computer Manufacturers
Group. March 2005
PICMG MTCA.0 R1.0, Micro Telecommunications Computing Architecture Base Specification, PCI
Industrial Computer Manufacturers Group. July 2006
12
AN-1926 An Introduction to M-LVDS and Clock and Data Distribution
Applications
SNLA113B – November 2008 – Revised April 2013
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