STM32U5 LSE/PC13 Errata and LSE bypass mode

[Terry Greeniaus]

The STM32U5 Errata Sheet (ES0499 rev 5 is the one I am looking at) has errata 2.2.1 which says: "The LSE frequency can be incorrect when PC13 is used as output for RTC_OUT1 or TAMP_OUT2 function". The workaround: "The RTC_OUT2 function must be used on PB2...".

We are using the 48-pin STM32U575CIT6Q package which does not have pin PB2 (it appears to have been removed to make space for the SMPS pins on the Q parts). Our design plans to use an external oscillator rather than a crystal, so the LSE would be placed in bypass mode. Obviously, the external oscillator frequency is not going to change no matter what happens with RTC_OUT1/PC13 since it is independent of the MCU.

Does this errata apply when the LSE is in bypass mode? I have been assuming that the errata is that RTC_OUT1/PC13 can interfere with properly driving an external crystal, but I suppose it could instead be interfering with an edge detector/counter or something else on the input side. Unfortunately there are very few details in the errata sheet.

Alternatively, is there some other workaround for packages that don't have PB2?

Thanks,

TG

[Ruchit N · Accepted solution]

I understand that you are using the LSE in the bypass mode and utilizing the sensor's external oscillator. The issue mentioned in the errata sheet (2.2.1) ES0499/Rev 5 should not be present if LSE bypass is used instead of a crystal.

The issue mentioned in the document is due to the internal analog oscillator which might be affected by the PC13 when the pin is used as RTC/TAM OUT function. In case of external source (i.e., LSE bypass) the internal analog oscillator is not used, and the external clock is proceeded as any input signal. Hence, LSE in the bypass mode can be a good workaround for the issue.