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K
Kazuhiro
Associate
April 8, 2025
Question

What is the VBAT function?

  • April 8, 2025
  • 2 replies
  • 623 views

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.

2 replies

Andrew Neil
Andrew Neil
Super User
April 8, 2025
@Kazuhiro wrote:

What is the VBAT function?

AndrewNeil_0-1744104354090.png

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

 

AndrewNeil_1-1744104453523.png

AndrewNeil_2-1744104523220.png

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

 

A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
    K
    KazuhiroAuthor
    Associate
    April 8, 2025

    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.

     

    K
    KazuhiroAuthor
    Associate
    April 8, 2025

    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
    STOne-32
    Technical Moderator
    April 9, 2025

    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

    IMG_2360.jpeg

    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

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