Shutdown mode for RTC

[Chamo]

Hello, I'm using the demoboard B-U585I-IOT02A with a STM32U5 microcontroller. Today, I'm using a RTC with an external battery placed before VBAT. But in order to reduce current consumption, I'm currently working on using shutdown mode in order to just update RTC without using CPU.

My question is the following : If I lose power for Vdd, so only remains Vbat for the RTC (based on the external battery), does the microcontroller go into shutdown mode automatically? Or do I have to do it manually by detecting a power failure just before losing power for the CPU ?

Thanks in advance,

[TDK]

> If I lose power for Vdd, so only remains Vbat for the RTC (based on the external battery), does the microcontroller go into shutdown mode automatically?

It automatically goes into VBAT mode, which has lower power draw than any other low-power mode. The other low-power modes, including shutdown, require VDD to be present.

[Chamo]

Oh okay, I didn't understand that. To be sure, do you mean that shutdown mode is unavailable with only Vbat (so with no power on Vdd) ? In any case, thank you very much for your answer!

[TDK]

Correct. The datasheet should provide the details.