How accurate does the 300 seconds have to be?  If you can tolerate some
variation, I would consider using the watchdog timer.  You will have to
calibrate the watchdog on a periodic basis - this is easily done by setting
the watchdog to no pre-scaler, running the PIC, then timing how long it for
the watchdog to trigger.  Use that measured time as the basis for setting
how many watchdog timeouts you need to achieve 300 seconds.

... This will be highly temperature dependent.

Bob Ammerman
RAm Systems

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