Texas Instruments | AN-1059 High Speed Transmission with LVDS Link Devices | Application notes | Texas Instruments AN-1059 High Speed Transmission with LVDS Link Devices Application notes

Texas Instruments AN-1059 High Speed Transmission with LVDS Link Devices Application notes
Application Note 1059 High Speed Transmission with LVDS Link Devices
Literature Number: SNLA050
National Semiconductor
Application Note 1059
Susan Poniatowski
June 1998
High speed data transmission requires a designer to be
aware of factors which may impact the correct sampling of
data. In particular, items such as excessive skew and jitter
can lead to data sampling errors, limiting the maximum
bandwidth performance of the LVDS Link. The following is a
discussion of LVDS Link receiver data sampling, skew and
jitter margin, and an explanation of skew and jitter components.
LVDS LINK DATA SAMPLING
The advantage of using the LVDS Link devices (FPD Link
and Channel Link) is the ability to convert many parallel data
lines into a narrower serialized interface. This serialized
LVDS interface is capable of running at seven times the data
rate of the parallel TTL interface. This means that the width
of a data bit on the LVDS interface is one seventh of the
width of the data on the TTL interface. For example, data
transmitted at 65 MHz on the TTL interface has a width of
15.38 ns; the corresponding bit width on the LVDS interface
is 2.19 ns. This requires an accurate strobe to correctly
sample the LVDS data at the receiver. Figure 2 describes
how internal receiver strobes are generated to sample the
LVDS data bits. The LVDS clock and data arrive at the receiver input. The clock moves through a delay element. The
delay lines generate sampling strobes: the first strobe is
based on the rising edge of the receiver input clock (LVDS
clock), and all subsequent strobes are based on the previous
strobe. Each strobe samples one data bit of the next data
cycle. Strobes are resynchronized with the rising clock edge
for each new clock cycle. Any jitter on the incoming clock
source (transmitter input clock) may be directly passed on to
the strobes. Sources of this jitter are discussed later.
The ideal strobe should be targeted at the center of the data
bit to allow for skew and jitter while maintaining a sufficient
margin for sampling data. Figure 1 shows this ideal relationship of the strobe to a single data bit.
AN012885-2
FIGURE 1. Receiver Strobe Position
MARGIN
In an ideal situation, the strobe could occur anywhere in the
data window and sample data correctly. However, there are
several factors which reduce the available sampling window;
this sampling window is “margin”. Margin is required to assure that valid data is sampled correctly by the receiver input
strobe.
The receiver itself has a limitation on sampling of valid data.
There is an inherent internal setup and hold time for the data
with respect to the clock (Rspos). Data must be available for
a period of time before it may be sampled (setup time) and
held for a period of time after sampling (hold time). These
parameters reduce the effective available sampling window.
In addition, if the strobe is not centered within the data bit the
margin is reduced. This receiver input margin is limited by
the smallest data sampling window before or after the
strobe. The transmitter output introduces clock to data skew
which further impacts the available margin (d). The position
of the data bit relative to the clock may not be constant, thus
moving the strobe further from the ideal center of the data
bit. This variation of pulse position is defined as Tppos.
TCCS is the nominal distribution of pulse position skew; this
is within the Tppos distribution. The total margin available after accounting for strobe and pulse position variation is
known as the receiver input skew margin (RSKM).
High Speed Transmission with LVDS Link Devices
High Speed Transmission
with LVDS Link Devices
AN-1059
© 1998 National Semiconductor Corporation
AN012885
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AN012885-1
FIGURE 2. RxCLK IN during Present Cycle Generates Strobes to Sample Data of Next Cycle
AN012885-3
FIGURE 3. Receiver Skew Margin
SYSTEM DESIGN: SKEW AND JITTER
The two primary components of a skew/jitter budget are
cable skew (e) and input clock jitter (f). The system designer
needs to evaluate these components and make trade-offs as
necessary to remain within the budget and maintain the desired margin.
After accounting for the characteristics of the LVDS devices,
(rskm) external cable skew and input clock jitter must be
considered. Figure 3 illustrates the components of the databit sampling window.
Where:
“a”, “b”:rskm is the margin for data sampling at the receiver
inputs. This number is based on the pulse position
(Tppos) and strobe position (Rspos) characteristics
of the device.
“c”:
represents the setup and hold times for the receiver strobe relative to the ideal strobe position
(Rspos min and Rspos max).
“d”:
is the variation of transmitter pulse position from
ideal (Tppos max-ideal and ideal-Tppos min).
“e”:
is the cable skew
“f”:
is the clock jitter
“m”:
The Interconnect
A wide variety of cable options are available for the LVDS interface. The main factors contributing to skew are cable
length, cable type and quality of cable. Skew for a given
cable is typically specified by the manufacturer as skew (in
picoseconds) per unit length. The longer the cable, the
higher the skew. A flat cable in which all signal lines are parallel will typically have less skew per unit length than a
twisted pair cable of comparable quality. The twist in the
cable and the fact that some differential pairs are on the outside of the cable and others inside can result in differences in
signal line lengths. Some twisted pair cables are designed
with special construction to greatly reduce skew between
is the remaining margin for data sampling
and...
m = rskm − (e + f)
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2
cally much less than the contribution of the cable. “Zero
skew” connectors are available on the market today. Such
connectors are configured with a single row of pins/leads.
The PCB traces between the connectors and LVDS device
pins may be routed to ensure equal trace lengths and add
little or no skew (Figure 4).
signal lines. The quality of cable — and the price — can also
make a difference in the skew. Higher quality cables must
meet more stringent requirements to guarantee a lower
skew per unit length. This value is typically 10 ps/ft to 40 ps/ft
depending on the cable quality.
The total skew path also includes the connector skew and
PCB trace skew. The skew at the connector interface is typi-
AN012885-4
FIGURE 4. Interconnect Skew = PCB + Connector + Cable + Connector + PCB
Sample Calculation
Please note that all values are provided for example purposes. These are NOT necessarily typical values.
An example of margin calculation at 40 MHz is shown below.
m = rskm − (e + f) = margin
where:
rskm = 1 ns @ 40 MHz shfclk
e = 30 ps (1m cable)
f = 250 ps
m = 1000 − (30 + 250)
m = 720 ps
Clock Jitter
Jitter on the input clock to the transmitter is also a concern.
Any jitter that is introduced at this point may be passed on to
the LVDS clock and data. Since the rising edge of the LVDS
clock is used to initiate the receiver strobes, the receiver
strobes in turn will jitter as the data jitters. Ideally, this would
appear to maintain the desired relationship between data
and the receiver sampling strobe. However, since sampling
strobes are generated by the clock of the previous cycle, the
receiver sampling strobes will be off-center in proportion to
the cycle-to-cycle input clock jitter. This increases the probability of sampling data incorrectly.
Figure 5 and Figure 6 show the impact of clock jitter on
strobe position relative to the databit. In both figures a cable
skew has been assumed (e). Figure 5 shows the strobe position relative to the databit with a stable clock edge. Additional margin remains for data sampling. Figure 6 shows the
databit and strobe position with jitter on the input clock edge.
The databit has moved relative to the strobe position (set by
the previous clock cycle). No margin for data sampling remains.
It is important to distinguish between short term and long
term clock jitter. Short term jitter, as applies to the LVDS Link
devices, is the variation of a transmitter input clock edge (rising or falling) between adjacent clock cycles and is typically
on the order of 10’s or 100’s of picoseconds. This jitter may
impact the margin. Clock drift describes the long term shift of
a clock edge. This number can be on the order of nanoseconds and may be intentionally introduced as a means of lowering EMI. This long term clock drift does not impact jitter
margin since receiver resynchronizes with every clock cycle.
Total margin remaining is 720 ps.
At 65 MHz the receiver input skew margin (RSKM) will decrease due to the decrease in databit width.
m = rskm − (e + f) = margin
where:
rskm = 500 ps @ 65 MHz shfclk
e = 30 ps (1m cable)
f = 250 ps
m = 500 − (30 + 250)
m = 220 ps
Total margin remaining is 220 ps.
Note: The datasheets for FPD Link and Channel Link products account for
transmitter output skew (“d”) in the Tppos specification.
The FPD Link transmitters have been redesigned to enhance the rejection of cycle-to-cycle jitter. The cycle-to-cycle
jitter at the TxCLK input is not passed directly to the TxCLK
output. TxCLK output cycle-to-cycle jitter is maintained below 250 ps with a TxCLK input cycle-to-cycle jitter of up to
6 ns. This eliminates the variability of clock jitter from the
margin equation (“f” = 250 ps). This feature is implemented
in the 65/66 MHz devices and will be incorporated in future
products.
3
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This has been controlled by redesigning the transmitter to
maintain a cycle-to-cycle jitter below 250 ps. The second
component — cable skew — may be addressed by selecting
a cable based on media, quality and distance requirements.
The designer must decide how to best minimize each
component — cable skew and TTL input clock jitter — while
also minimizing system costs.
CONCLUSION
Designing for high speed data transmission requires attention to factors which will impact the margin for correct data
sampling. Input clock jitter and cable skew are key components to be evaluated when designing to maintain high bandwidth performance. Minimizing this input clock jitter will improve the ability to correctly transmit data at high speeds.
AN012885-6
FIGURE 5. Databit and Strobe Position with Ideal Input Clock
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4
AN012885-7
FIGURE 6. Databit and Strobe Position with Jitter on Input Clock. No Margin Available for Sampling Data.
5
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High Speed Transmission with LVDS Link Devices
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