Using a CM3L on a custom project. I currently have a supercap backup setup where I am monitoring a GPIO via a script, and shutting down immediately if the GPIO goes LOW. My current issue is that my supercap has a bit more energy than is required, and it keeps the 3.3V and 1.8V supplies on until the supercap is empty. This usually takes 5-10 seconds or so.
My backup IC has the ability to shutoff output power with a a digital input. Is there a way to have a GPIO on the Pi change state once the system is completely shutdown? I seem to vaguely recall that there may be something like this already implemented, but I cant seem to find any info on it.
So for clarity, I am looking for a pin state change, when the OS is fully shutdown. Basically something telling me that it is ok to completely remove power.