Small changes to clock calibration value will cause increasing errors
the longer the device runs. Consider the case of deep sleep, assuming
that RTC counter is used for timekeeping:
- before sleep:
time_before = rtc_counter * calibration_val
- after sleep:
time_after = (rtc_counter + sleep_count) * (calibration_val + epsilon)
where 'epsilon' is a small estimation error of 'calibration_val'.
The apparent sleep duration thus will be:
time_after - time_before = sleep_count * (calibration_val + epsilon)
+ rtc_counter * epsilon
Second term on the right hand side is the error in time difference
estimation, it is proportional to the total system runtime (rtc_counter).
To avoid this issue, this change makes RTC_SLOW_CLK calibration value
persistent across restarts. This allows the calibration value update to
be preformed, while keeping time after update same as before the update.
8ccb2a4990