STM32U5 RTC calibration process

[Rashmikat]

I am currently working with an STM32U5 microcontroller and would like to request more information on the process of calibrating the RTC (Real-Time Clock). We are using an external 32.768 kHz LSE oscillator with 10pf burden capacitors, but we are observing a 6-second drift over 24 hours.

I have followed the calibration process outlined in the link below:
STM32 RTC Calibration Process[https://community.st.com/t5/stm32-mcus/how-to-calibrate-the-stm32-s-real-time-clock-rtc/ta-p/744958?search-action-id=38939113459&search-result-uid=744958].

However, I need more detailed information regarding the calibration procedure.

1. I'm not clear how to calculate the correct CALM[8:0] value for the smooth calibration register (RTC_CALR).
1. How to Verifying the RTC calibration?
2. Is it necessary to perform smooth calibration at regular intervals? If so, what are the steps to do this?

[Sarra.S]

Hello @Rashmikat, welcome to ST Community, 

you can check "Smooth calibration application example" section in AN4759 

[Rashmikat]

Thanks Sarra.S,

I've gone through the shared application note, but I'm not clear on how to verify the calibration clock. Do I need to manually verify the output clock at pin PC13 using an oscilloscope, or is there an internal process for verification?

Additionally, I've observed that after upgrading the MCU from STM32U575ZG (1M) to STM32U575ZI (2M) for development, the 6s over 24Hr RTC drift issue occurred with the STM32U575ZI (2M) MCU, but not with the STM32U575ZG (1M) MCU. 

Is there any specific limitation related to the RTC in the STM32U575ZI (2M) part?