My 60-cent 32678 Hz crystal at 12.5pf is specified at +/- 20ppm (aka: 0.002% accuracy). But with a caveat: I need to load the crystal with exactly 12.5pf capaci

My 60-cent 32678 Hz crystal at 12.5pf is specified at +/- 20ppm (aka: 0.002% accuracy). But with a caveat: I need to load the crystal with exactly 12.5pf capacitance. So sure, I can go grab 2x 25pF capacitors and load the crystal to ground (series capacitors half the capacitance, so 2x 25pF in series == 12.5pf). Except... what? Trace-capacitance and on-chip capacitance hasn't been factored in. Hmmmmm... okay. Does anyone have a picoFarad accurate multimeter? Oh snap, those are expensive. No, the easiest way to tune a crystal oscillator to its appropriate accuracy is through the use of a ground truth clock, and comparing that ground-truth clock vs the clock. So I make my guess with 2x 9pF capacitors (5pF internal capacitance on the chip x2 pins + 2pF estimated trace capacitance) and measure the result against the ground truth. Well, to calibrate a 20ppm crystal oscillator means I need a time sync service that has 20ppm accuracy or better. That's ya know: +/- 20 microseconds per second accuracy. ----------- That's my cheap 60-cent crystal by the way. If you go with a temperature-controlled oven crystal oscillator for better accuracy, you'll need more accuracy to tune that.