Paul B. Webster VK2BZC [paulb@MIDCOAST.COM.AU] says:
OK, the crystal has a rated load capacitance, which is what it wants to see for exact frequency. What it sees is this:Osc1 o------+--------+ Where: C1 = loading cap on Osc1 | | C2 = pin capacity of Osc1 input === C1 === C2 C3 = loading cap on Osc2 | | C2 = pin capacity of Osc2 output Crystal +--------+-- Ground | | === C3 === C4 | | Osc2 o------+--------+
This is actually pretty simple because we make C1 = C3 and assume that C2 = C4 although for some peculiar reason, these are not specified in the datasheets. 10pF sounds a round guess however (implied from other CMOS spec sheets).
Let's say your crystal was specified at 15pF. This would be achieved with each of the pairs C1+C2 = C3+C4 = 30pf, i.e., two 30pF in parallel. If C2=C4 is approximately 10pF, then C1 and C3 would each be 20pF or in practice, a 22pF (nominal) capacitor.
If you really want accuracy, then C1 becomes a 8.2pF in series with a 3-30pF trimmer. Also, as the internal capacities C2 and C4 may vary with temperature and voltage, it is somewhat preferable if the loading capacitors are bigger rather than smaller. (This obviously influences crystal selection, but only for high accuracy.)
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<A HREF="http://www.piclist.com/techref/oscpinc.htm"> Including pin capacity in calculating crystal loading caps.</A>
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