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Texas Instruments SN65LVCP114 lines for Skew Compensation Application notes
Application Note
January 2012
SN65LVCP114 Guidelines for Skew Compensation
Communications Interface
ABSTRACT
Channel to channel skew in high speed data paths must be controlled to ensure
robust system performance. Skew can arise from the integrated circuit, package or
the circuit board. This application note provides information on channel to channel
skew compensation of the SN65LVCP114.
Contents
Introduction .................................................................................................................................... 2
PCB Material Selection.................................................................................................................. 2
Transmission Line ......................................................................................................................... 3
Propagation Delay ......................................................................................................................... 4
SN65LVCP114 Inter-Pair Skew ..................................................................................................... 4
Summary......................................................................................................................................... 7
Figures
Figure 1. Microstrip Structure....................................................................................................... 3
Figure 2. Stripline Structure.......................................................................................................... 3
Figure 3. Microstrip Propagation Delay Equation....................................................................... 4
Figure 4. Stripline Propagation Delay Equation.......................................................................... 4
Tables
Table 1. Dielectric Constant and Loss Tangent Values for Different Materials ....................... 2
Table 2. SN65LVCP114 Expected Channel Skew per Port ......................................................... 5
Table 3. SN65LVCP114 Skew Compensation Example (εr = 3.66) ............................................ 6
Table 4. SN65LVCP114 Skew Compensation Example (εr = 3.66) ............................................ 6
1
Introduction
Today’s leading edge serial interconnect standards have some very demanding
requirements on a copper link’s ability to deliver usable output waveforms while
achieving jitter, ISI, attenuation, reflection, crosstalk, and skew requirements. In the
past, at lower data rates, circuit designers had the luxury of implementing channel links
with large skew budgets; but with today’s higher data transmission speeds much less
time is available between clocking events and designers must take care to align data
edges.
PCB Material Selection
Signal propagation delay depends, in part, on dielectric constant (εr) of the PCB
material. A dielectric material can be assigned a dielectric constant that is related to the
force of attraction between two opposite charges separated by a distance in a uniform
medium. Each PCB substrate can have a different relative dielectric constant. The
dielectric constant compares the effect of capacitance of a capacitor using that material
as a dielectric, compared to a similar capacitor which has a vacuum as its dielectric. The
dielectric constant affects the impedance of a transmission line. Also a signal will
propagate faster in PCB designed with materials that have a lower dielectric constant.
FR-4 is the most widely used dielectric material in PCBs, and has a dielectric constant
between 4.1 and 4.5. Table 1 shows the dielectric constant of a sample of materials.
Material
DE156
GETEK
FR408HR
IS680 3.33
IS680 2.8
Dielectric Constant (εr)
4.0
3.6
3.68
3.33
2.8
Loss Tangent
0.02
0.009
0.0092
0.003
0.0025
Table 1. Dielectric Constant and Loss Tangent Values for Different Materials 1
1
2
www.isola-group.com
SN65LVCP114 Guidelines for Skew Compensation
.
Transmission Line
Many different structures of trace routing are possible on a PCB; two common structures
are show on Figure 1 and 2. A microstrip has one reference, often a ground plane, and
these elements are separated by a dielectric. A stripline has two references, often
multiple ground planes, and are surrounded with the dielectric.
Microstrip Transmission Line Layout
W
W
T
S
εr
H
Figure 1. Microstrip Structure
Stripline Transmission Line Layout
Figure 2. Stripline Structure
SN65LVCP114 Guidelines for Skew Compensation
3
Propagation Delay
Propagation delay is the time required for a signal to travel from one point to another.
Transmission line propagation delay is a function of the dielectric constant of the
material.
Microstrip Propagation Delay Equation (ps/in):
Figure 3. Microstrip Propagation Delay Equation
Stripline Propagation Delay Equation (ps/in):
Figure 4. Stripline Propagation Delay Equation
SN65LVCP114 Inter-Pair Skew
Inter-pair skew is the difference between the fastest and slowest pairs in the link. Its
control becomes critical when a link uses a separate clock line (e.g. non-PLL based
devices) which must be synchronized accurately in relation to the data lines to correctly
sample the data. Serial links have clocks embedded with the data stream and thus do
not have tight restrictions on Inter-pair skew.
In order to optimize the overall package size, the SN65LVCP114 package substrate
signal trace lengths are not matched, refer to Table 1. To ensure timing alignment for all
channels per port, both the substrate trace length plus the PCB trace length for each
signal must be matched to meet the trace length skew tolerance for all signals within the
clock domain.
4
SN65LVCP114 Guidelines for Skew Compensation
.
Channel
Skew (ps)
AIN0
AIN1
AIN2
AIN3
49.6
34.8
4.7
0.0
BIN0
BIN1
BIN2
BIN3
0.0
16.3
29.2
40.7
CIN0
CIN1
CIN2
CIN3
0.0
2.8
6.4
3.7
AOUT0
AOUT1
AOUT2
AOUT3
23.9
10.5
17.4
0.0
BOUT0
BOUT1
BOUT2
BOUT3
0.0
4.8
18.5
15.4
COUT0
COUT1
COUT2
COUT3
3.4
11.8
9.2
0.0
Table 2. SN65LVCP114 Expected Channel Skew per Port
A procedure for optimizing inter-pair skew for SN65LVCP114 is provided below.
1. Route signals following proper high speed layout techniques.
2. Match all high speed trace length channels per port.
3. Use microstrip or stripline equations, Figures 3 or 4 respectively, to calculate the
trace length deltas based on Table 2.
a. Table 3, below, shows an example using the microstrip equations and an
effective dielectric constant of 3.66 to calculate the trace lenghts.
4. Calculate the compensated trace length by adding the matched trace length from
step 2 with the trace length deltas calculated in step 3.
a. Table 4 shows an example for output port C using microstrip, effective
dielectric constant of 3.66, and an original trace length match (step 2) of
4,000mils.
SN65LVCP114 Guidelines for Skew Compensation
5
Channel
Microstrip Recommended Trace Delay Addition
Skew (ps)
Inches
Millimeters
Mils
Microns
AIN0
AIN1
AIN2
AIN3
49.6
34.8
4.7
0.0
0.376
0.264
0.036
0.000
9.550
6.701
0.905
0.000
376.001
263.807
35.629
0.000
9550.435
6700.708
904.981
0.000
BIN0
BIN1
BIN2
BIN3
0.0
16.3
29.2
40.7
0.000
0.124
0.221
0.309
0.000
3.139
5.622
7.837
0.000
123.565
221.356
308.533
0.000
3138.550
5622.433
7836.748
CIN0
CIN1
CIN2
CIN3
0.0
2.8
6.4
3.7
0.000
0.021
0.049
0.028
0.000
0.539
1.232
0.712
0.000
21.226
48.516
28.048
0.000
539.137
1232.314
712.432
AOUT0
AOUT1
AOUT2
AOUT3
23.9
10.5
17.4
0.0
0.181
0.080
0.132
0.000
4.602
2.022
3.350
0.000
181.178
79.597
131.904
0.000
4601.923
2021.765
3350.354
0.000
BOUT0
BOUT1
BOUT2
BOUT3
0.0
4.8
18.5
15.4
0.000
0.036
0.140
0.117
0.000
0.924
3.562
2.965
0.000
36.387
140.242
116.742
0.000
924.236
3562.158
2965.256
COUT0
COUT1
COUT2
COUT3
3.4
11.8
9.2
0.0
0.026
0.089
0.070
0.000
0.655
2.272
1.771
0.000
25.774
89.452
69.742
0.000
654.667
2272.079
1771.452
0.000
Table 3. SN65LVCP114 Skew Compensation Example (εr = 3.66)
Channel
Skew (ps)
Skew (Mils)
Trace Length
(Mils)
COUT0
COUT1
COUT2
COUT3
3.4
11.8
9.2
0.0
25.774
89.452
69.742
0.000
4000
4000
4000
4000
Compensated
Trace Length
(Mils)
4025.77
4089.45
4069.74
4000.00
Table 4. SN65LVCP114 Skew Compensation Example (εr = 3.66)
6
SN65LVCP114 Guidelines for Skew Compensation
.
Summary
This application note provides guidance for inter-pair skew compensation of the
SN65LVCP114 channels when used with a non-PLL based device.
For any concerns, a question can be submitted in the TI E2E Community forum
(e2e.ti.com) under the High Speed Interface section in the Interface forum.
SN65LVCP114 Guidelines for Skew Compensation
7
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