Oh, yeah, one thing to mention - the SHUTDOWN state has a flaw! I'm not sure what to do here, actually. The flaw is that if the PIC negates the "I want you to keep running" signal to the Pi, it then waits for the Pi to negate the "I am running" signal. But, if the Pi never responds by negating that signal, the power controller will just sit in SHUTDOWN state indefinitely.  I'm not sure what to do, though!  I don't want to gank power to the Pi (could corrupt the flash card) yet, really, I'm not sure what else to do... I don't like the fact that the power controller assumes that the Pi will always behave ...