Question about using RTC timer for time keeping

[AparnaAnand]

Hi,

We are designing a system using nrf52840, in which we intend to keep the wall clock time with battery support.

Reading about the internal RTC module, it gives an impression that the time will roll over after approximately 24 days.
Since with 125 ms resolution and 24 bit counter register, overflow will happen.

Can you please confirm this?
Also any suggestions, as to how to implement sytem time, without using an external RTC.

Thanks & Regards,

  Aparna

[haakonsh]

((2^24)ticks / (1/0.125)seconds) / (60s*60m*24h) = 24.27 days. 

Without an accurate clock source, you will need to synchronize the clock if you need an accurate clock. The internal RC oscillator has ~500ppm accuracy if it's calibrated at regular intervals and at changing temperatures, this yields a drift of 43.2 seconds every day.

 I suggest you check out an experimental Calendar example: https://github.com/NordicPlayground/nrf5-calendar-example.