ESP32 Forum

My esp32 is supposed to wake up at a certain time. It calculates the number of microseconds for esp_sleep_enable_timer_wakeup() by getting the current time from NTP and calculating the difference between now and the desired wakeup time. I turned in on around 10:30pm and set it to wake up at 8:30am. It woke up at 7:55am. I knew the RTC timer clock was not very accurate, but I was not expecting it to drift that far. Is this normal?

If you rely on the internal oscillator (a simple RC), it should have a relative error of up to 10% (if I recall well). Maybe ESP32 can be slightly better, but in general RC oscillator has a significant drift. So if it deep sleeps few hours, it can drift *minutes*.

In my tests on some devices, after entering the deep-sleep mode, the frequency of RTC_CAL_8MD256 is reduced by about 6%. By calibrating the error between the sleep mode and the actual time, before entering the sleep mode

const double clockRate = 1.06; // Each device needs to be calibrated independently
REG_WRITE(RTC_SLOW_CLK_CAL_REG, (uint32_t)(0.5 + rtc_clk_cal(RTC_CAL_8MD256, CONFIG_ESP32_RTC_CLK_CAL_CYCLES) * clockRate));

A clock accuracy of about 50ppm can be obtained. I recalibrated the RTC clock by waking up for about 40 milliseconds every about 1 minute in the deep-sleep mode. In the test of my two devices, I got an accuracy of 10ppm, which is very close to the accuracy of a 40M crystal oscillator. The error of one device after running for 3 days is 300 milliseconds, and the error of another 8 hours of testing is 200 milliseconds

If you need to avoid clock drift then you really need to use an external 32K crystal in your circuit. There are some dedicated APIs for this....