Daniel Imfeld <dimfeld@SOFTHOME.NET> wrote:
> Just to clear things up a bit, when the crystal is rated at 20 pF, that
> doesn't mean that it has a 20 pF capacitance.  The rating is the
> recommended value of the capacitors that you will connect to ground from
> the crystal..

Note quite. It's actually the net capacitance that's expected to be in
parallel with the crystal. This includes the series combination of your
external capacitors, plus the distributed capacitance of the PCB traces
and the chip pins.

For example, if you use two 33 pF capacitors, this accounts for 16.5 pF
of actual parallel capacitance across the crystal. This leaves about
3.5 pF for pin/trace capacitance, which is about right.

-- Dave Tweed

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