Kontron COMe Starterkit Eval T2 - Manual (Page 152)

COM Express Interfaces
The clock trace from the COM Express Module to a PCI bus slot should be 2.5 inches shorter
because PCI cards are specified to have 2.5 inches of clock trace length on the card itself.
PCI clock signals should be routed as a single ended trace with a trace impedance of 55Ω. To
reduce EMI, a single ground referenced internal layer is recommended. The clock traces should
be separated as far as possible from other signal traces.
Refer to Section 6.6.1 'PCI Trace Routing Guidelines' on page 191 below and the 'PCI Local Bus
Specification Revision 2.3' to get more information about this subject.
Note: An approximate value for the signal propagation delay per trace length inch
can be calculated by using the following formula:
t
prop.
=
√ ε
r
'
11.8
ns
inch
ε
r
' can be determined from the dielectric constant of the PCB that is used by
the following approximation. A typical value for the dielectric constant of an
FR4 PCB material is 4.2 < ε
r
< 4.5.
For stripline routing: ε
r
' =ε
r
For microstrip routing: ε
r
' =0.475ε
r
+ 0.67 for 2.0<ε
r
< 6.0
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
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