
Advantech SOM-Express Design Guide
82 Chapter 5 Carrier Board Design Guidelines
5.10.3.2 Board Stack-up Considerations
Table 5-27 shows the PCI Express Trace Width and Spacing for Micro-strip and Strip-
line base on the six layer board stack-up. Please refer to chapter 4 to get more
information. Keep the required impedance based on the different board stack-up.
Table 5-27 PCI Express Trace Width and Spacing for Micro-strip and Strip-line
Trace
Width
Differential
Pair Trace
Spacing
Adjacent
Pair /
Trace
Spacing
Differential
Pair Length
Matching
Breakout
Guideline
Nominal
Trace
Impedance
(Zo)
Micro-
strip
5 mils 6 mils 20 mils 5 mils 5 mil trace width, 5mil
separation to both the
differential pair signals
and adjacent traces
for up to 250 mils
100
Ω±20%
(Differential)
Strip-
line
4 mils 8 mils 20 mils 5 mils Only 4 mil trace width
on 8 mils spacing is
allowed
100
Ω±20%
(Differential)
5.10.3.3 PCI Express Topology #1 – Device Down Routing
Guidelines
The device down topology allows a maximum of 15 inches from SOM-Express pin to
the pin of the down device. This max length takes into account all routing, including
the breakout region, which should not exceed 0.25 inches per device. The TX and
RX pairs can be routed “interleaved”, such that the pairs alternate between TX and
RX on the carrier board, or “non-interleaved”, where TX and RX pairs are routed next
to each other. Only interleaved routing can used for microstrip routing topologies. For
stripline routing, It is preferable to route the TX and RX differential pairs in an
interleaved fashion to reduce crosstalk. Figure 5-48 shows the example.
TX
[ 0 ]
RX
[ 0 ]
TX
[ 1 ]
RX
[ 1 ]
.
.
TX
[ 0 ]
TX
[ 1 ]
.
.
RX
[
0
]
RX
[ 1 ]
..
Can be used on strip
-
line ,
but i t is
not preferable
Must be used on the micro-strip, prefereably used on
the strip-line
Figure 5-48 Example of “interleaved” and “non-interleaved”
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