Layout Guideline - Advantech SOM-ETX Series Design Manual

Note:
ESD protection and common mode chokes are only needed if the design does not
pass EMI or ESD testing. Footprints for common mode chokes and/or ESD
suppression components should be included in the event that a problem
occurs(General routing and placement guidelines should be followed).

5.2.3 Layout Guideline

5.2.3.1 Differential pairs
The USB data pairs (USB0 and US0#) should be routed on the baseboard as
differential pairs, with a differential impedance of 90 Ohms. PCB layout software
usually allows determining the correct trace width and spacing to achieve this
impedance, after the PCB stack-up configuration is known.
As per usual differential pair routing practices, the two traces of each USB pair
should be matched in length and kept at uniform spacing. Sharp corners should be
avoided. At the SOM-ETX module and connector ends of the routes, loop areas
should be minimized. USB data pairs should be routed as far from other signals as
possible.
SOM-ETX
5.2.3.2 Over Current Protection
Over current protection on external USB power lines is required to prevent faults in
external USB devices or cables from causing hardware damage and/or crashing the
system. The USBOC# signal is used to signal over current conditions to the system
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Advantech SOM-ETX Design Guide
Figure 5-7 Common Mode Choke
Recommended
USB0
USB0#
USB3
USB3#
Not Recommended
Figure 5-8 USB Layout Guidelines
Chapter 5 Carrier Board Design Guidelines
USB Connector
Route to Minimum