Pi 2 Design Mercury Streamer, Hardware Reference Manual – P8
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6.3
POWER STATUS
As part of the power control circuit, GPIO24 (*PSW) is used to sense the state of the
Pushbutton Power Switch. A low indicates that the Power Switch is in the on position
(depressed). When high it indicates that the user has placed the switch into the off
position (non-depressed) thus opening the on/off switch. Note that for proper operation
this circuit requires user software to program GPIO24 as an input with the internal
pullup enabled.
6.4
POWER HOLD
GPIO22 (PHLD) allows the CM4 to hold the power on even after the user puts the
Power Switch into the off position. After initialization, software can drive GPIO22 high,
thus holding the power on regardless of the state of the on/off switch. Driving this low,
removes this hold condition. Note that for proper operation this requires user software to
program GPIO24 as an output with the internal pullup disabled.
