David VanHorn wrote:
> I looked at the app note, and thought it was a little misleading IMHO.

I haven't looked at the app note, but most writeups about crystal caps I've
seen have a big flaw in that they ignore the output impedence of the driver.
>From the comments here, it seems this app note is no different.

For example, a 20pF cap at 8MHz has about 1Kohm impedence.  It seems
entirely reasonable that the PIC oscillator output has an impedence that is
significant when in parallel with 1Kohm.  I expect it's less than 1Kohms at
8MHz, which would actually make it the dominant part of the impedence
driving the crystal.

I don't understand why everyone seems to ignore the driver output impedence.
I don't believe I've ever seen it mentioned by Microchip and I seem to be
the only one saying this every month or two when this topic comes up.  I
think my argument is sound, but if everyone else thinks I'm wrong lets hear
why.


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