Errata nRF52840 - RTC: Register values are invalid

This anomaly applies to IC Rev. Engineering B, build codes QIAA-BA0, QIAA-BB0, CKAA-AA0.

It was inherited from the previous IC revision Engineering A nRF52840.

Symptoms

RTC registers will not contain the correct/expected value if read.

The RTC has been idle.

RTC configuration cannot be determined by reading RTC registers.

Execute the below code before you use RTC.

 
NRF_CLOCK->EVENTS_LFCLKSTARTED  = 0;
NRF_CLOCK->TASKS_LFCLKSTART     = 1;
while (NRF_CLOCK->EVENTS_LFCLKSTARTED == 0) {}
NRF_RTC0->TASKS_STOP = 0;