I want an external circuit to know when the Pi has started booting up and finished shutting down. How early/late is it possible to control the pins? And how might it be done?
I'm thinking of writing a simple C program and using systemd to control when it's run.
But how early/late would this actually be able to work? What software alternatives are there for controlling GPIO state as early/late as possible, short of recompiling the kernel or other high-skill methods?
Also I want to avoid hacking the hardware, as I would want to make a hat for the Pi with this circuitry, and I want to be able to install it on a normal Raspbian installation