> Does anybody know the difference between a serial vs a parallel type
crystal?

Yup.  Lots of people I suspect.

> I have tried both types and don't notice any difference,
> but there must be a difference, or they wouldn't be specified
> as parallel or serial crystals.

There is no manufacturing difference between crystals specified for series
or parallel resonant application, and each will oscillate in a circuit
meant for the other.  There is no such thing as parallel or series "cut",
although that is a commonly heard term (including in the Microchip data
sheets).  The difference is in the exact operating conditions required to
achieve the specified frequency.  There is a 90 degree phase shift
difference between the parallel and series operating modes.  Both points
are fairly close to each other in frequency, so you might not notice the
difference without an accurate measurement with a frequency counter or
long term clock, for example.

In theory each crystal has both a parallel and series operating point, but
I've only ever seen one or the other specified.  Series crystals are
usually listed as "series" in catalogs, whereas parallel crystals are
listed with a capacitance value, usually in the 15pF to 30pF range.  This
is the load capacitance required for that crystal to cause a 90 degree
phase shift at the specified operating frequency.


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