>
> What happens if I use a 30MHz crystal with a 16c84 (10MHz max?)
>
> of course it will be set to use a HS crystal...
>
> will it work?

Nope.

> what will happen?

It will not run.

> will it work with an external clock?

Nope.

> how accurate will the timing be?

Not accurate at all considering that it doesn't run.

I've found that the best I can do with 16C84's at room temp is about 16 Mhz.
That's with a crystal oscialltor as an external clock and HS clock setting.

The only hope you have of running at that speed is to use a EPROM based PIC.
Then be prepared to take a stack of them and throw out the ones that
don't work.

Sorry that you idea isn't going to fly.

BAJ