Go Down

Topic: Adafruit Feather M0 - Crystal Capacitors (Read 107 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.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.


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.

Go Up