> No the peripherals do not keep working while the CPU
> is asleep

I'm not so sure about that. Timer0 rollover interrupt doesn't
work but I think T0CKI still increments (I've not tried it)

> I was thinking of using the PIC18LF1220 running with internal
> RC_OSC at 1 MHz

You might try measuring active current at different frequencies.
A slow clock will keep the PIC awake longer and although a
faster clock may use a little more power, the wake period will
be shorter

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