M0 Oscillator Accuracy

[mlewus]

I’m trying to determine how often I will need to adjust a RTC based on the M0. So I need to determine the accuracy of the M0’s 32Khz oscillator over temperature and aging.

I searched github and this forum but I was unable to find a part number or a BOM for this very new product. If someone can post the crystal’s part number I will look up the specs and post them here.

Thanks!

[Felix]

It is this part: https://www.digikey.com/product-detail/en/fox-electronics/FK135EIHM0.032768-T3/631-1002-1-ND/1024707

[mlewus]

Thanks, Felix.

Crystal Specs:

• 32.768 Khz fundamental with a 12.5pf load.
• Initial: +- 20ppm @25°C
• Temp coefficient: 0.04ppm/°C²
• Aging: 3ppm per year
• For reference, 20 ppm is a bit less than a minute per month of error.

Note that the °C² term means the temperature coefficient is shaped like an inverted parabola. The vertex which represents maximum frequency is at 25°C. Temperatures above or below that causes the crystal to oscillate at a slower rate.

20ppm and 0.04ppm/°C² is about as good as you can do with a reasonably priced crystal.