STM32 Vbat monitoring when Vbat pin is not present?

Question

Many of ST's STM32 chips have a Vbat pin that supplies power to backup modules (e.g. RTC, low speed oscillator, backup registers). Many units also allow "the application to measure the VBAT battery voltage using the internal ADC channel ADC_IN18."

My specific chip (STM32F042) doesn't have a Vbat pin in any of the packages available, but both the datasheet and STM32CubeIDE say there is a Vbat ADC channel. However, I can't find any document explicitly stating what the measured voltage is, as every application note from ST assumes it's the voltage on the Vbat pin that's being measured.

On chips that do not have a Vbat pin, what is the voltage measured by the Vbat ADC channel?

Answer 1

In the datasheet of the STM32G071 it is stated as (This is not contained in the F042 datasheet however.):

When this pin is not available on the package, VBAT bonding pad is internally bonded to the VDD/VDDA pin.

My guess is that is done so on other STM32 as well. You could try to measure the VBAT channel, it should read half of the ADC full scale range as there is a divider of 2 on your device (it's 3 for other devices).