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Note:
1. There is a period of time (T6 in Figure 35 and Figure 36 above) during which the Carrier Board circuits
have power but the COM Express Module main internal power rails are not up. This is because
almost all COM Express internal rails are derived from the external VCC_12V and there is a non-zero
start-up time for the Module internal power supplies.
Carrier Board circuits should not drive any COM Express lines during the T6 interval except for those
identified in the COM Express Specification as being powered from a Suspend power rail. Almost all
such signals are active low. Such signals, if used, should be driven low by open drain Carrier Board
circuits to assert them. Pull-ups, if present, should be high value (10K to 100K) and tied to
VCC_5V_SBY.
The line PWR_OK may be used during the T6 interval to hold off a COM Express Module boot.
Sometimes this is done, for example, to allow a Carrier Board device such as an FPGA to be
configured before the Module boots.
The deployment of Carrier Board pull-ups on COM Express signals should be kept to a minimum in
order to avoid back-driving the COM Express signal pins during this interval.
Carrier Board pull-ups on COM Express signal pins are generally not necessary – most signals are
pulled up if necessary on the Module.
