Spehro Pefhany wrote:
> The capacitors
> are effectively in series

No, this is a common Crystal Myth.

If the crystal is driven with a 0 impedence sine wave on one side, the
specified load capacitance is that value to ground on the other side that
will cause the desired phase shift at the rated frequency.  Therefore, if
the PIC oscillator output pin has a small impedence compared to the load
capacitance, then the capacitor on the oscillator output pin doesn't enter
into consideration.  In that case, the best oscillator input pin capacitor
value would be the rated load capacitance minus the inherent existing
capacitance due to the leads, circuit board, etc.

If the PIC oscillator output had a very high impedence, then both capacitors
need to be considered in series as you said.  In practise, the PIC output is
somewhere in between.  A 18pF capacitance at 20MHz has about 440ohms
impedence.  The PIC output impedence is probably less than that, certainly
not so large compared to 440ohms that it can be ignored.  The effective
drive impedence seen by the crystal is the oscillator output in capacitance
plus stray capacitance all in parallel with the pin output impedence.  All
these parameters can't be know that accurately, but in any case the drive
impedence will likely be no more than half of just the added capacitor in
the 18pF to 22pF range at 20MHz.  At lower crystal frequencies, the PIC
output driver impedence is even less compared to the capacitor.

Also keep in mind the purpose of the oscillator output pin capacitance.
It's not there to provide the crystal load capacitance, although it effects
it in some cases.  It's purpose is to attenuate the harmonics in the crystal
drive waveform.  Excessive harmonics can damage the crystal and can even
cause the oscillator to run at a harmonic instead.  It's important to keep
the loop gain below 1 for harmonics.

There are a lot of subtle interacting effects here with unknown exact
values.  This is why I said earlier that after all the fancy analysis you
still don't really know the right answer, and 22pF on both sides of the
crystal is about right with reasonable assumptions and typical crystals
rated for about 18pF.


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