ESP01 sleep_duration not accurate?

Hi.
I have a ESP01 device, modified so it can run on battery and wake up from deep sleep.
I specified the sleep_duration to 60min and run_duration to 45s.
Bute the device seems to wake up every 55min. It’s not really an issue, just curious why.

deep_sleep:
  run_duration: 45s
  sleep_duration: 60min

Don’t think the EPSs have a real-time clock, so they must count cpu cycles or flip flops or sing a long song.
With out a real-time clock they can drift. Once the WiFi come back and they wake, they should set the time correct again.

Stable power helps, does it do it if mains powered ?

Havent tried stable power with the sleep function. Just wanted to know if someone else have seen this.
Its not really a problem, just an observation.

I was tempted to build a battery powered esp8266 and have a solar panel to charge it, BMS , deep sleep etc. But then I found a 3 meter usb cable and just used that to power the unit
:grinning:

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