> Yes that was it. A pin had a -1V voltage applied to it.
> Problem solved!
> 
> Thanks, Michael, and Jinx for your time.

You're welcome. The circuit on the page has gone now so I
don't recall whether the wave was labelled as +/- 1 at the
PIC pin. You're lucky you didn't lose the pin or whole PIC
to latch-up

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