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Topic: Adafruit Feather M0 - Crystal Capacitors (Read 240 times) previous topic - next topic



On the schematic of the Feather M0 board (https://cdn-learn.adafruit.com/assets/assets/000/028/801/original/adafruit_products_M0SCHEM.png?1448656357) between pinouts 1 and 2 there is a 32.768KHz crystal, after the crystal there are 2, 22pF capacitors and I was wondering what the purpose of these are and how sensitive the value of the capacitors is.

Thank you


Those capacitors provide a load for the oscillator, without them the oscillator won't oscillate.

The value is somewhat critical - there's a range in between which it should be. 22 pF sounds quite large for this frequency, I've encountered that value for 12/16 mHz crystals. Those are usually loaded with 18-22 pF. The 32.768 kHz (used for real time clocks) I've seen mostly with 9 pF crystals.

Very simply said: too small, not enough load to oscillate; too big, impeding oscillation too much. Just follow manufacturer's recommendations as per data sheet.
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32.768KHz crystals are usually used for timekeeping.
If so, then the cap values are very important.
With two 22pf caps (11pF from the crystal's point of view) they have assumed a ~12.5pf load.
If the values are wrong, frequency will be slightly off, and time will drift.

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