I got my softpower circuit to work finally. After numerous attempts and configurations using only a single N and single P channel I wound using two N-Channel FETs and one P-Channel. That’s more than I wanted, but I have the FETs already in only two chips, so, the heck with it. It works!
I have a working prototype of the schematic above. I don’t have all the caps or the voltage divider in place, but I don’t expect those to be serious problems.
I had a bit of a snafu with the MCU code during testing. I was using a logical AND operation in a conditional expression, instead of bitwise AND, while trying to read the PINB input register. It took me an hour and many experiments to figure out what I had done. The worst part was I knew I must have done something simple wrong but still could not see the obvious error for an hour! Grrr.
Ah, well. Now I have my circuit. It draws no power when its off, can switch off the power if the voltage gets too low and is push for ON, push again for OFF. 11.1V and 18A! I’m excited to get a full fledged PCB designed, etched, populated and smoke tested!