I basically want to determine if power has been totally disconnected in a low power device I am making that uses an Atmega328 chip.
The theory is that when power gets first applied the ATmega will power up, calibrate and take some readings then power down again. The rest of the circuit will still be powered until the chip wakes up again and takes more readings. What I need to be able to determine is if the battery was disconnected or not. If it was, I need to do a calibration again; if it wasn't, then I just take a reading as normal.
Is there any way of doing this which takes as little power as possible?