I believe I've seen oscillators with built in selectable dividers, but I cannot
remember where.  You can use a seperate oscillator and divider, and use a pic
output or two to set the divider when necessary.

-Adam

D Lloyd wrote:
>
> Hi,
>
> I've been looking towards using a PIC for a 'dual speed' operation; running the
> PIC from a (small) battery during a power failure (sleep then wake up, even at
> the lower oscillator rate......or maybe run all the time) then resuming full
> power operation when power is restored. As things go, the oscillator speed  for
> achieving the reduced power mode requirements does not provide enough processing
> "oomph" when full power is available. Hence, dual-clock operation would be
> beneficial.
>
> I've had a poke around on the mchip site/checked out a few data sheets and I
> cannot really find too much information or a specific micro that performs this.
> Mitsubishi etc make micros that specifically support this - is it blatantly
> obvious how to do this with the PIC and I have missed it, or is it just not
> possible?
>
> I've seen a posting where an RC oscillator is used where additional resistance
> is switched in and out via an I/O pin (dual speed) but I don't think RC is going
> to give me the clock accuracy that is required.
>
> Any comments would be appreciated.
>
> Regards,
>
> Dan