esp32 RC oscillator hiccups

[kjm1102]

I've got an esp32 that wakes up from deepsleep approximately once per hour and posts some timestamped metrics. Although the actual deepsleep time is usually less than an hour (because the 150kHz RC oscillator is invariably running fast) the timestamps are accurate because the chip syncs via NTP or similar before timestamping.

A by-product of the accurate time stamping is I get to see something I've long suspected, the RC oscillator does not show the relatively smooth, usually temperature dependent, drift typical of most RCs. In my case actual deepsleep seems to fluctuate randomly between 3600 & 3588s for a maximum variation of -12s. What intrigues me though is once a day or so it will randomly throw a long nap of up to 3616s.

Curious if anyone else has noticed this & has a hunch on the possible cause? It looks to me like the long sleeps are due to an accumulating register occasionally missing an update in deepsleep?