Kontron COMe Starterkit Eval T2 - Manual (Page 36)

COM Express Interfaces
Each clock output pair in the example shown is terminated close to the IDT9DB433 buffer pins
with a series resistor (shown as 33 Ω) and a termination to GND (shown as 49.9 Ω), per the
vendor's recommendations. Other vendors may have different recommendations, particularly in
regard to the source termination to GND.
SMBUS software can enable or disable clock-buffer outputs. Configuration resistors or
alternativly the SMBUS also allow software to put the clock buffer into "Bypass Mode", which
experience has shown is needed in some Carrier situations. Please refer to chapter 2.19
'System Management Bus (SMBus)' on page 123 below for more information on SMBUS.
Disable unused outputs to reduce emissions.
The CLKREQ0# and CLKREQ1# should be pulled low to enable the corresponding clock buffer
outputs. For applications in which power management is not a concern, these inputs may be tied
low to permanently enable the outputs.
PICMG ® COM Express ® Carrier Board Design Guide Rev. 2.0 / December 6, 2013
36/218