Mr RB
Well-Known Member
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The turnover temperature of a crystal is the temperature where the frequency is not going up or down. Crystals designed for a single temperature will have their turnover at that temperature. Watch crystals are designed like that for approximately wrist temperature.
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Temperature curves are not that critical for household clock applications. I've tested a number of typical microcontroller type xtals 4MHz to 20MHz range and their temperature drift is usually about 1 to 2 PPM max over 10'C temp change. For indoor clocks they can be easily calibrated to with a trim cap (or software) to give a clock accurate to average <2PPM or less than 1 minute error per year.
That's a big part of why I like using a software calibrated "second" like these methods;
https://www.romanblack.com/one_sec.htm
As you can just run the clock for a month (which will include household temperature variations), see how many seconds error, and adjust the one constant value in software. Then reprogram the PIC and have a clock that is extremely accurate after that.