What happens if VBAT is higher than VDD?

[Pahul_Singh]

Hello STM32 Folks,

I was hoping to get some information regarding VBAT on STM32U5 MCU. I am currently powering the STM32U5 MCU with 1.8V (VDD supply). I need to have external clock oscillator for the Real Time Clock that is powered by VBAT (Coin Cell battery 3.0V). The external clock oscillator can be powered by 1.65V to 3V.

I didn't find any information regarding VBAT being higher than VDD. Will there be any issue with 3.0 V levels for the external clock oscillator for RTC? Or do I need to regulate the VBAT voltage to 1.8V for external clock oscillator? 

Thanks,

[SofLit]

Hello @Pahul_Singh ,

According to the datasheet there is no constraint regarding this point VBAT vs VDD:

VBAT max = 3.6V and not VDD.

[Pahul_Singh]

Thanks for the fast response.
Pin PC14 on the STM32U5 device is the 32.768 KHz Oscillator input pin. This pin has the external oscillator powered by 3.0V connected to it. 

If the STM32 device is powered by 1.8V (VDD power supply), the logic voltage levels that the PC14 will accept are 0 to 1.8V. 
Is my assumption correct for this? 

Or the voltage levels that PC14 accepts are based on the VBAT pin.

[Uwe Bonnes]

PC13/14/15 are powered by VBAT. If you have an external oscilator to PC14, you should power the oscillator from VBAT.