-----Original Message-----


>Is the internal PIC oscillator circuit
>designed for series or parallel resonance
>mode crystals -- or both?
>
>I've quickly read the app-note but didn't
>seem to find the answer to my question.
>
>Some of the crystals I've seen listed in the
>Digi-Key catalog are denoted 'series' and
>others list a capacitance value such as '20
>pF'.
>
>I've used PICs and crystals for years, but
>I've never had to address this question until
>now.
>
>Any help would be appreciated -- thank you
>kindly!


They are used in a paralell resonant configuration.

All crystals have both series and paralell resonances.  They are only
guaranteed to perform within spec on one or the other.
If you put a series xtal in a paralell circuit it will probably run, but you
will NEVER get it on the right frequency. The two points
obviously can't be the same frequency.   You need to make sure also that
your caps are properly selected. If the xtal is set
for 20pf and you hang 20pf caps on it, it won't be on the right freq either,
because the chip, and the board tracks have added
a bit more C.  In my experience, 15-18 will work well if short tracks are
used.

If you want to keep your EMI down, return the cap ground to the chip ground
on it's own private track, as short as possible.
Otherwise, the high harmonics will happily use your board as a loop antenna
:)