Raspberry Pi Battery Saving with Wake from Halt

Summary: You can potentially save a lot of energy for battery powered applications by forcing the RPi to halt and then reset whenever a triggering event takes place.

Details

The bootloader supports a wake from halt mode that is triggerd by grounding GPIO3
You can eg short pins 5 and 6 together to connect GPIO3 to ground.
GPIO3 is a special IO pin – it’s normally used for i2c and is pulled high (SCL).
If the RPi is already in HALT state (reached by doing eg sudo shutdown -h now from active state) and theb GPIO3 is grounded, the system will reboot.
So for long-running tasks where you need to eg conserve battery you can:
- Perform your active task as soon as the system reboots
- Force the system to shut down (enter HALT state)
- Externally trigger GPIO3, causing a reboot
- Repeat until your batteries die
The external trigger could be anything from an RTC timer expiring to a PIR sensor firing.
Note that grounding GPIO3 momentarily after the RPi is already active should have no effect (which is different than using the RUN header to reset the device)
Obviously applications are limited by the time it takes to reboot. Simplify your boot sequence and you may speed up your boot. For latency-sensitive applications you could consider using systemd to trigger your application code before networking configuration finishes.