@Michael99 , according to design procedure formula on page 13 of AN2867, 22pF is probably too large a value for a crystal that specified CL=12.5pF. You suggested perhaps using 6pF - this tells me that your guessing at the value instead of calculating it. If you want to do it that way, test very possible value between 6pF-22pF and establish a range that works, then pick a value. Or:
- Do the calculation from page 13 and that value and values around it. The parasitic capacitance of the crystal to the surrounding board is generally unknown but usually on the order of a few pF, so some trial-and-error is required to dial in a value.
- Once you found a capacitance value (even better, a range of values), repeat the process for as many boards as you can justify, to make sure they ALL work - there's no point in making one board work and making others stop.
- If this fixes the issue, you've probably identified the cause.
Note that this only gives evidence of proper functioning under "typical" conditions, not "worst-case" conditions. Remember that capacitance varies with temperature (depends on dielectric), age, is specified only up to some tolerance, and other factors. So you should do at least at least wave a hair-dryer over it and see if it still starts up. Unless you intend to deploy your product in the Nordic countries, in which case you should place it on a block of frozen Lutefisk instead. And if you're developing a commercial product that has Max operating condition specified, you obviously have to closely test for those.
Peter correctly point out that PCB layout is important for proper crystal functioning, and that the appnote was written specifically to help you do that (and I know he is secretly heart-broken since everyone on the forum who reports oscillator issues always seems to have ignored the appnote's layout guidelines completely). But it's also important to note that, practically speaking, many designs violate some of the guideline and still work under typical conditions (and beyond). It's just more risky to do so. So I wouldn't immediately suspect the layout as being the cause unless you've done something completely horrible (though the layout may be the issue after all). You do ultimately want good layout because it reduces risk and mitigates *potential* issues, not because following every guideline is absolutely necessary for your product to function.
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