What is the VBAT function?

[Kazuhiro]

If the PDR_ON connector is connected to the GND, the VBAT function may not be available.
What is the VBAT function?

To use RTC, a coin cell is connected to the VBAT connector. Is there a problem if the PDR_ON connector is connected to the GND?

I am using STM32F429ZI.

Please let me know.

[Andrew Neil]

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@Kazuhiro wrote:

What is the VBAT function?

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https://www.st.com/resource/en/datasheet/stm32f429zi.pdf#PAGE=34

https://www.st.com/resource/en/reference_manual/rm0090-stm32f405415-stm32f407417-stm32f427437-and-stm32f429439-advanced-armbased-32bit-mcus-stmicroelectronics.pdf#PAGE=177

via: https://www.st.com/en/microcontrollers-microprocessors/stm32f429zi.html#documentation

[Kazuhiro]

Thank you for your reply.

I understand that in order to use VBAT after turning off the VDD, the PDR_ON pin must be connected to the VDD, and if the PDR_ON pin is GND, turning off the VDD will not switch VBAT.

[Kazuhiro]

Please tell me one more thing.
When both VDD and VBAT are turned OFF, is the register of RTC kept?
Specifically, I want to know if the register of BKP0R is kept.

[STOne-32]

Dear @Kazuhiro ,

Do you know why you would like to connect PDR_On to ground ?  The only reason is when your external Power supply has a voltage below 1,8 Volts and you can go below down to 1,7 Volts . 
https://www.st.com/resource/en/datasheet/stm32f427vg.pdf#page95

 in that case you need an extra power supervisor externally and adds a BOM cost increase , as well as loosing VBAT function as explained above .

let us know 

STOne-32