Accuracy of the 32.768kHz XTAL oscillator over temperature

RE: Accuracy of the 32.768kHz XTAL oscillator over temperature

Peter — Mon, 25 Feb 2019 15:05:56 GMT

Yes, that is also my understandig. A tuning fork crystal like the FC 135 type will always have a temperature drift in the range of (0,4*(T-T25)^2) other than AT cut crystals. So we will set the NRF_CLOCK_LF_ACCURACY to the worst case value (<250ppm) depending on our max/min application tempearture.

Thank you,

Peter

RE: Accuracy of the 32.768kHz XTAL oscillator over temperature

Andreas — Mon, 25 Feb 2019 14:59:08 GMT

Hi,

As AmbystomaLabs says, frequency drift due to temperature is not taken into account in the NRF_CLOCK_LF_ACCURACY. If you expect large temerature deviation, you might want to adjust the loading, the drift is indicated by the 'Parabolic coefficient' parameter in the datasheet.

Best regards,

Andreas

RE: Accuracy of the 32.768kHz XTAL oscillator over temperature

AmbystomaLabs — Mon, 25 Feb 2019 13:33:32 GMT

You did not specify the full part number for your crystal. Epson makes many versions of that product with different tolerance and temp stability specs.

Crystals are always tolerance plus stability plus aging plus loading/warping. Capacitive loading can move the nominal frequency, but if you followed the reference design your nominal is probably close to spec. Assuming yours is +/-20ppm tolerance and +/-20ppm stability then the final error is +/-40ppm.

There is no temp comp in the nRF. The field associated with NRF_CLOCK_LF_ACCURACY is just you telling the SD what the error of your solution is so it can adjust its internal timing to compensate.

The BLE spec allows for up to +/-250ppm for the LF clock.