Kontron COMe Starterkit Eval T2 - Manual (Page 80)

COM Express Interfaces
2.8.3. Avoiding Back-driving Problems
For more information please refer to chapter 2.9.3 'Avoiding Back-driving Problems' on page 86
below.
2.8.4. Routing Considerations
Route USB signals as differential pairs, with a 90-Ω differential impedance and a 45-Ω, single-
ended impedance. Ideally, a USB pair is routed on a single layer adjacent to a ground plane.
USB pairs should not cross plane splits. Keep layer transitions to a minimum. Reference USB
pairs to a power plane if necessary. The power plane should be well-bypassed. Section 6.5.2.
'USB Trace Routing Guidelines' on page 183 summarizes USB routing rules.
2.8.4.1. EMI / ESD Protection
To improve the EMI behavior of the USB interface, a design should include common mode
chokes, which have to be placed as close as possible to the USB connector signal pins.
Common mode chokes can provide required noise attenuation but they also distort the signal
quality of full-speed and high-speed signaling. Therefore, common mode chokes should be
chosen carefully to meet the requirements of the EMI noise filtering while retaining the integrity of
the USB signals on the Carrier Board design.
To protect the USB host interface of the Module from over-voltage caused by electrostatic
discharge (ESD) and electrical fast transients (EFT), low capacitance steering diodes and
transient voltage suppression diodes have to be implemented on the Carrier Board design. In
the USB reference schematics Figure 29 above, this is implemented by using 'SR05 RailClamp
® '
surge rated diode arrays from Semtech (http://semtech.com).
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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