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At 10:21 AM 4/13/00 +0100, Alan B Pearce wrote:
>>2: What is the specified loading value?
>>3: Multiply by two.
>>4: Subtract 4-5 pF (guesstimate of circuit loading)
>>5: Put one cap of this value to the uP's ground pin, on each xtal lead.
>
>You may actually find that using unequal capacitors with the series
equivalent
>capacitance being correct for the crystal loading may work better. This is
then
>similar to a colpitts oscillator where you may have unequal capacitors. If I
>remember correctly the larger value capacitor is put on the driven side of
the
>crystal (i.e. the osc output pin). this will also make attaching a probe
to this
>pin less critical.

This complicates the brew.  The two caps appear to the xtal in series, and
you're
trying to get to XpF using two 2XpF caps in series.  If you make them
unequal, then
you have to make one larger than 2X, and the other smaller.

I'd want to know more about the exact drive circuit before attempting that
trick.

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