Hi Graham, imho you are overdoing it a little. All you need is a single
switch if you already have a backup battery. A shottky diode network
(2 of them) should power your circuit both from the battery and from the
regulated power. A pic pin will sense the incoming power going down. You
can use a R divider to sense the voltage before the regulator. The switch
will be a MOSFET (logic level) and will cut off the supply to the
peripherals (leaving the PIC connected to the battery through the backup
system). When the PIC senses power down it will do what it does and then
turn the MOSFET off, then go to sleep. You can arrange for it to come back
when the power comes back, by implementing a wakeup on portb change or
such. You can also arrange for the watchdog to run while sleeping and the
PIC to briefly wake up every 2 seconds or so, check conditions, and sleep
again. This is the way I'd do it anyway.

Peter

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