Advantech SOM-Express Design Guide
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Chapter 5 Carrier Board Design Guidelines
5.10.2 Design
Guidelines
5.10.2.1
PCI Express AC Coupling Capacitor
Each PCI Express lane is AC coupled between its corresponding transmitter (TX)
and receiver (RX). Figure 5-43 shows the connection for SOM-Express PCI Express
signals. It is best to place AC coupling capacitors close to the transmitter (TX) of the
SOM-Express board.
Figure 5-43 PCI Express Interconnect Example
We recommend using size 0603 capacitors. Use the exact same package size for the
capacitor on each signal in a differential pair. Table 5-25 shows the PCI Express
capacitor summary.
Table 5-26 PCI Express Capacitor Summary
Type
Value
Tolerance
Placement
Length Matching
Between Differential
Pair
AC Capacitor
75 nF-
200 nF
20%
Recommended to place
close to the transmit side
As close as possible
between the differential
pairs
5.10.2.2
Bowtie Topology Considerations: Untangling Nets
It is possible that when interconnecting PCI Express devices, certain “bowtie” or
signal-crossing scenarios might occur when the link is routed on the PCB. There are
three main types of bow tie scenarios:
(1) TX+, TX- crisscrossing within a pair.
(2) Crossing of transmitter and receiver pairs within a lane.
(3) Crossing of lanes within a link.
The PCI Express specification provides two different features: Polarity Inverse and
Lane Reversal to help system designers overcome the layout difficulties encountered
