Congatec COM Express conga-TS175 User Manual page 33

The conga-TS175 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 that the input complies with 3.3V CMOS characteristic and also makes it possible to use the module on carrier
board designs that do not drive 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.
The three typical usage scenarios for a carrier board design are:
• Connect PWR_OK to the "power good" signal of an ATX type power supply.
• Connect PWR_OK to the last voltage regulator in the chain on the carrier board.
• Simply pull PWR_OK with a 1k resistor to the carrier board 3.3V power rail.
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