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I've seen questions such as this one and this one that talk about selecting capacitors that, in series, match up as closely as possible to the load capacitance of the crystal.

What effect does selecting the wrong capacitance have? Does it skew the frequency, alter the shape of the waveform produced, or something entirely different? Is there an obvious correlation between too low and too high capacitance? Is it a case of slight errors, or orders of magnitude?

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up vote 28 down vote accepted

May not oscillate.
May not oscillate reliably.
May have low to very low output.
May oscillate on wrong frequency (may be an undertone or an overtone).
For extra points may jump between modes (not usual)
May start up very slowly. And/or not always.
May produce less sinusoidal waveform than expected.
May shatter crystal (not usual in modern applications).

May work perfectly.

Factor of 2 in capacitance usually OK, but not always.
Note that some ceramic cap compositions are far worse in tempco, initial accuracy and general stability than others.

Depends on IC used, layout, how Murphy is feeling and phase of Moon.
Layout can be extremely important. Loop area enclosed by capacitor and crystal leads forma part of the oscillator circuit. Manufacturers often give quite specific advice on crystal and capacitor layout. Note it.

Crystal oscillators approach more closely to analog and to black magic than sane people want to get on a good day. In many cases they "just work" and the arcanery is well hidden. You MAY find that capacitance that is off by a factor or 2 or even 5x still works OK. Or, you may not. When they don't work as hoped they can be most obdurate obstinate obnoxious odious and generally overwhelmingly annoying.

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Are you saying that frequency accuracy is independent of the load capacitors? – jippie Dec 23 '13 at 10:06
@jippie - No, that would be covered by "May oscillate on wrong frequency (may be an undertone or an overtone)." - with undertone or overtone being extreme cases of "wrong frequency", but for day to day microcontroller work that does not involve longer term 'real time' timekeeping or high precision timing, frequency pulling by wrong caps is liable to of relatively minor importance compared to some of the other listed effects - if they do occur. – Russell McMahon Dec 23 '13 at 12:01

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