In message  <9702280144.AA12689@zsulink.zsu.edu.cn> PICLIST@MITVMA.MIT.EDU
 writes:
> Hi everyone,
>
> I am designing a project which needs very high (>= 20MHz) running speed. I
 want
>  to use external clock to drive the RC or XT type PICs because of their easy
>  availability. Is there any problem in this method? Thank all those who may
 give
>  me advices.
>

I know people who use the 16C5x range at 50MHz with an external
oscillator.

Their application had a limited temperature range and they were
prepared to select on test and throw away any pics that didn't
work. In practice, all the pics have worked at that frequency.

It is way outside the PIC spec, so you are on your own, but it
does work, reliably too.

Incidently, they pay more for their canned oscillator than they
do for the PICs.

Regards,


Mike Watson