RE: Any issues without external crystal besides power ?

ShannonPahl — Tue, 11 Apr 2017 16:30:04 GMT

great, thanks for the help

RE: Any issues without external crystal besides power ?

Stian Røed Hafskjold — Mon, 10 Apr 2017 15:04:05 GMT

The calibration is not blocking, so it can run while the CPU is doing other stuff. You can use PPI with the calibration timer to start calibration without any CPU interaction. This is handled by the SoftDevice btw. As long as the accuracy (ppm) of the LFRC is given to the SoftDevice while you enable it, the stability is as good as when you use LFXO, since the SD will increase the window widening on the RX event corresponding to the accuracy given.

Regarding current consumption using the LFRC vs the LFXO, you can have a look at this post.

For the nRF51 you had the choice to only calibrate if the temperature had changed. For nRF52 the SD will calibrate the clock even though the temp is stable. Please see the notes in the SD spec here.

For nRF52, the application must ensure calibration at least once every 8 seconds to ensure +/-250 ppm clock stability. The recommended configuration for NRF_CLOCK_LF_SRC_RC on nRF52 is rc_ctiv=16 and rc_temp_ctiv=2. This will ensure calibration at least once every 8 seconds and for temperature changes of 0.5 degrees Celsius every 4 seconds.