STM32U0: Will the LSESYSEN bit of RTC domain control register (RCC_BDCR) reset upon power-on reset?

[Sha0S]

Greetings!

I have the following system:

• STM32U073CC
• VBAT pin is connected to a super-cap, and it's voltage was measured to be above the required minimum.

During system reset (NRST) it behaves according to my expectations, the RCC_BDCR remains unchanged, but if it is disconnected from Vcc, then upon connecting it back, the LSESYSEN bit is set to 0. This seems reasonable, and is generally not a issue to me, but the reference manual claims that this shouldn't happen? (Everything else works perfectly.)

Am I missing something obvious here?
Couldn't find any other mention of the bits behaviour.

[Sha0S]

Tested it with a STM32U83 Nucleo too, had the same behaviour.

• Removed SB2, and powered VBAT directly from a PSU.
• The HAL automatically enables LSESYSEN if LSE is present with HAL_RCC_OscConfig.
• Resetting the system with NRST: bit remains '1'.
• Power-on reset by disconnecting the USB: bit set to '0', nothing else changes. (Well: LSESYSRDY is also 0 ofc.)

Obviously HAL_RCC_OscConfig will set it back to '1' regardless.

[Sha0S]

This behaviour is the same as stated in the STM32U5 reference manual:

[KDJEM.1]

Hello @Sha0S and welcome to the community;

Thank you for bringing this issue to our attention.

I reported this issue internally for checking and fixing.

Internal ticket number: 224876 (This is an internal tracking number and is not accessible or usable by customers).

Thank you.

Kaouthar