Our solution was to design our carrier board with a super capacitor. When the power is cut, GPIO 9 is pulled high, and the super capacitor supplies the pi with enough power to shutdown.
How long can your super capacitor sustain reliable operation of the CM4? Shutdown can take a considerable amount of time. Running applications have to be notified and allowed to terminate gracefully. Services have to be notified and allowed to stop gracefully. Finally, buffers have to be written out to the storage device.
A fresh Bookworm install on a Raspberry Pi 5 appears to shutdown in approximately a second. An identical system with FreePBX installed can take 10 seconds or more to finish the shutdown sequence.
Unless your super capacitor can keep the CM4 going reliably until the very end, there's no guarantee it will get around to writing out buffers before operation ceases.
Statistics: Posted by RonR — Thu Jun 06, 2024 8:41 am