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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 0.8 V when the 12 V 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.
With this solution, you must ensure that by the time the 3.3V is up, all carrier board hardware is fully powered and all clocks are stable.
The conga-TS570 supports the controlling of ATX-style power supplies. If you do not use an ATX power supply, do not connect the conga-TS570
pins SUS_S3/PS_ON, 5V_SB, and PWRBTN#.
SUS_S3#/PS_ON#
The SUS_S3#/PS_ON# (pin A15 on the A-B connector) signal is an active-low output that can be used to turn on the main outputs of an
ATX-style power supply. To accomplish this the signal must be inverted with an inverter/transistor that is supplied by standby voltage and is
located on the carrier board.
PWRBTN#
When using ATX-style power supplies, PWRBTN# (pin B12 on the A-B connector) is used to connect to a momentary-contact, active-low
debounced push-button input while the other terminal on the push-button must be connected to ground. This signal is internally pulled up to
3V_SB using a 100 k resistor. When PWRBTN# is asserted it indicates that an operator wants to turn the power on or off. The response to this
signal from the system may vary as a result of modifications made in BIOS settings or by system software.