Are you sure that any of your "over 20 MHz crystals"
are fundamental crystrals at this "over 20 MHz"
frequency?

Odds are, that all those "higher-frequency crystals"
are actually "overtone" crystals and as such their
"overtone frequency" is stamped on the outside.

I would then place money on a bet that the simple
little oscillator used on the PIC chips *only*
oscillates at the lower "fundamental" frequency
of those crystals ...

(All this comes from the perspective of a radio engineer
who has done work with crystals in communications gear
in the (now-distant) past ...)

RF Jim


----- Original Message -----
From: "Michael Simpson" <msimpson@HIS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Saturday, August 03, 2002 1:44 PM
Subject: Re: [PIC]:Has any one ever got a PIC 18F244 chip to work at 40mhz


> Done that, did that.   I have about 100 crystal,cap and resonators.  I
tried
> using the HS/PLL option.  Cant get it to go above 20mhz.
>
> ----- Original Message -----
> From: "M. Adam Davis" <adampic@UBASICS.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Saturday, August 03, 2002 9:05 AM
> Subject: Re: [PIC]:Has any one ever got a PIC 18F244 chip to work at 40mhz
>
>
> > You'll note, if you inspect the datasheet, that Microchip hasn't listed
> > any values for oscillators (crystal or resonator) above 20MHz, the
> > closest they get is listing a 25MHz crystal in the capacitor value table
> > with valuse of "TBD" as in the capacitor column
> >
> > I seem to remember that if you wanted to run the chip above 20MHz you
> > had to use a lower oscillator value and use the PLL to multiply it by 4.
> >
> > However, you can probably get it going faster if you try a few different
> > capacitors, and perhaps use a resistor in series.  Check the archives
> > for crystal problems.
> >
> > -Adam
> >
> > Michael Simpson wrote:
> >
> > >I have tried everything including the HS/PLL option using just about
> every Resonator and Crystal I could get my hands on.
> > >
> > >Im using MPLAB and a PICSTART Plus.
> > >
> > >The best I seem to do is 20Mhz
> > >

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