Congatec COM Express conga-TC175 User Manual page 26

The conga-TC175 PWR_OK input circuitry is implemented as shown below:
+V12.0_S0
R1%10kS02
R1%47k5S02
R1%1k00S02
To Module Power Logic
PWR_OK
R1%20k0S02 R1%47k5S02
TBC847
The voltage divider ensures the input complies with 3.3 V CMOS characteristic. It also makes it possible to use the module on carrier board
designs that do not use the PWR_OK signal. Although the PWR_OK input is not mandatory for the onboard power-up sequencing, it is strongly
recommended that the carrier board hardware drives the signal low until it is safe to let the module boot-up.
When considering the above shown voltage divider circuitry and the transistor stage, the voltage measured at the PWR_OK input pin may be
only around 0.8V when the 12V is applied to the module. Actively driving PWR_OK high is compliant to the COM Express specification but this
can cause back driving. Therefore, congatec recommends driving the PWR_OK low to keep the module in reset and tri-state PWR_OK when
the carrier board hardware is ready to boot.
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