nRF52833 LFRC calibration during system ON mode

[hardikp]

Hello,

1. What is the PPM of LFRC?
2. In our case, the device will remain in low power mode (System ON, wake up from RTC). The device will wake up at every one hour. Which would be a good option for the RTC clock? Internal RC or external crystal?
3. Will Soft device calibrate LFRC at every 8 seconds during low power mode? Can we disable it?
4. Shall we calibrate LFRC at every 1 hour when the device will be wakeup? Our goal is to know about consequences if not use external 32.768 kHz crystal.

[Hung Bui]

Hi Hardik, 

1. It's ±500ppm after you have it calibrated. And ±5% when it's uncalibrated. Note that it will drift after you calibrated. So ±500ppm is only applied right after you calibrate.

2. If you want to save power and be more accurate you should use the external crystal. But using the RC crystal you can save the BOM. 

3. Yes if you leave the softdevice running. You can reduce the rate that the softdevice calibrates if there is no change in temperature using NRF_SDH_CLOCK_LF_RC_TEMP_CTIV . If you don't want the softdevice to calibrate when you sleep, you can disable the softdevice. And enable it again when you wake up. I think for 1 hours I don't see much point keeping the LFCLK running at high correct rate. Unless you have some timing requirement on the sleeping time of the device. 

4. Calibrating every one hour wouldn't help much. The clock already drift a lot after a few minutes. But in the end the consequence is that your clock can be drifted up to 5%. For non critical application 1 hour or 1 hour 3 minutes wouldn't make any difference.