>>As I probably don't need super accuracy I am going to try a couple of 22pf
>>caps and see if they work. Basically if I can get serial coms working at
>>9600baud on my 20mhz 16F628 then I'll be happy.

>They almost certainly will. You cannot "pull" a quartz crystal far enough
to
>cause a problem with serial coms. Timekeeping, measuring instruments (my

....
>Just the same CL/2 - 5pF isn't very difficult to calculate, assuming you
know
>CL (it is always specified on the crystal data sheet and should be in any

Almost every crystal that you are likely to use will be specified to use a
load capacitance value in the region of 20-30pF. It is pretty safe to assume
30pF as the likely specified capacitance, most manufacturers seem to use
this value unless manufacturing to a specific specification that requires a
different capacitance. The exception is likely to be at lower frequencies,
below about 500kHz, where this value of capacitance will have minimal effect
on the crystal.

You are unlikely to see any variation in frequency with the different values
of capacitor unless you use a frequency counter to measure some output
frequency. You are more likely to see erratic starting behaviour of the
oscillator if you go to too large or too small value of capacitance. Hence
to achieve around 20-30pF of load capacitance for the crystal, I would be
expecting to use a pair of capacitors of 40-60pF each, and typically use
about 47pF. It may well be that a pair of 22pF capacitors is enough to make
the crystal operate reliably.

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