RTC runs slower 2 second / 5 minutes after disconnect power

[Dat Tran]

We are using LSE 32,768. RTC looks like runs correctly if power is on. But if we disconnected power, RTC still hold data but run slower, about 2 seconds / 5 minutes.

We changed new battery, even used power supply to feed vbat pin, same issue.

I tried ST example on Nucleo STM32H743ZI too (enabled RTC_CLOCK_SOURCE_LSE), with / without removing SB156, same issue.

Thank for any suggestion.

[TDK]

2s per 5min is about 7000ppm, which is way outside of what any crystal will promise.

Are you sure you're using the LSE and not the LSI? Check the relevant register values after configuring and post them.

[waclawek.jan]

Cube/HAL "RTC initialization" bug?

https://community.st.com/s/question/0D50X00009XkgBWSAZ/stm32-rtc-loses-one-second-after-each-reset

Does the delay increase with number of powerdowns? Does it increase with the power off time, with only one powerdown?

JW

[Dat Tran]

it was that. The forum is really useful.

Thanks a lot.