To get the 18F series to run at 40MHz, you need a 10MHz crystal and then
to enable the PLL multiplier. It is detailed in the datasheet available
on Microchip's website (http://www.microchip.com).

Hope that helps,

Josh
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"Robert E. Griffith" wrote:
>
> I have a 18F252, programmed for HS, that works fine with a 20mhz xtal, but I
> have tried two different 40mhz xtals without success. One won t oscillator
> (I tried series resistors from 1-330k and caps from 10pF-47pf) and the other
> oscillates at 13.3mhz (1/3 of 40) (also tried all the various res and cap
> values).  What makes an oscillator work at 1/3 the rated frequency?

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