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2013
congatec
AG
TU87m10
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The conga-TC87 PWR_OK input circuitry is implemented as shown below:
The voltage divider ensures that the input complies with 3.3V CMOS characteristic and also allows for carrier board designs that are not driving
PWR_OK. 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.
To Module Power Logic
PWR_OK
R5
R1%47k5S02
TB
TBC847
R4
R1%100kS02
+V12.0_S0
R13
R1%1k00S02
R1
R1%47k5S02
R2
R1%20k0S02
