> > Is the chip even specced to run at Vdd = 6V ??
>
> Nope. Max is 5.5V. Actually it may be better to parallel the two lithiums
and
> switch to a LF part that runs reliably down to 2V. The lower voltage and
> high available current should combine to extend the available running
time.

Very good idea if the rest of the circuit will go at 2V.

> >
> > You could still run with an RC OSC, but then use the 32.768KHz crystal
to
> > 'calibrate' a timebase for a bit-banged software UART (or, if your baud
rate
> > is low enough, you could even compute an appropriate value to store in
the
> > BRGEN and use the hardware UART).
>
> If that's the case wouldn't a syncronous serial interface be better? Then
> the internal 37 KHz RC oscillator can be used.
>

The OP hasn't told us enough about his 'download' requirements. But if they
can be met using a SYNC interface then this is a great idea.

Of course, you still need accurate timekeeping, which would require the
TIMER1 32.768KHz oscillator (or an external RTC chip (yech!)).

Bob Ammerman
RAm Systems

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