You'll have to build your own clock oscillator.  Do a search on Google
for crystal oscillator circuits and you should come up with something.
The on-chip crystal oscillator is not intended to directly drive a 40MHz
crystal.

It may be tricky getting a circuit to drive a 3rd overtone crystal,
though.  Good luck!

-Adam

Daniel Sweet wrote:

>Hi,
>
>I am trying to run my PIC(18F252) at 40Mhz, which is locked to an external source using a PLL. I has this working with a 10MHz crystal using HS/PLL mode to give 40Mhz, but have had other issues with the HS/PLL so now want to run with HS mode
>
>The only 40Mhz crystals that I have been able to get hold of in a hurry have been 3rd Overtone mode ones. I only have a vague idea of what this means!
>
>Can anyone tell me how I make it run correctly in the 3rd overtone mode? Or direct me towards some information? I am assuming at the moment that the circuit arrangement needs changing slightly? I've googled and found a little info but not much.
>
>Thanks for your help.
>DAN
>
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