Accuracy of the 32.768kHz XTAL oscillator over temperature

[Peter]

Hello,

as I understand the accuracy of the LF Crystal oscillator for the nRF52832 development board DK_PCA10040 ist set within the sdk_config.h in the following way:

#define NRF_SDH_CLOCK_LF_SRC 1

#define NRF_SDH_CLOCK_LF_RC_CTIV 0

#define NRF_SDH_CLOCK_LF_RC_TEMP_CTIV 0

#define NRF_SDH_CLOCK_LF_ACCURACY 7   // => NRF_CLOCK_LF_ACCURACY_20_PPM

An Epson FC-135 crystal with an initial frequency tolerance of +/-20ppm @25°C is used.

My question is this:

is the significant frequency drift over temperature taken into account anyway (e.g. internal correction by temperature measuerent ) or must it be considered within the NRF_SDH_CLOCK_LF_ACCURACY define depending on the real application temperature? For example an ambient temperature of +70°C will lead to a frequency error of -81ppm in addition to the +/-20ppm initial tolerance and the aging.

Best regards

Peter   

[Andreas]

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

[Peter]

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