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shaling
Associate
April 7, 2026
Solved

Bootloader Indication LED

  • April 7, 2026
  • 3 replies
  • 394 views

Hi,

I have the STM32H753ZI Nucleo board and have developed firmware around this MCU with this board as my reference platform.
On our product, firmware updates will be uploaded via the UART interfaces.
So the user will need to enter Bootloader.
On the Nucleo board, the LD3 Red LED stays illuminated when in Bootloader.
I think this is because the GPIO pin is being pulled up, and this is not intentional as an indicator?
An indication to the user that they have successfully reset the MCU into Bootloader is useful.
How can you recommend we can do this in a reliable way in the future?
Thanks.

Best answer by Uwe Bonnes

According to AN2606 PB14 is USART1 TX (alternate) and set to push pull during system bootloader. TX inactive level is high and so Q3 will conduct and LD3 light up.

3 replies

Andrew Neil
Andrew Neil
Super User
April 7, 2026

You're talking about ST's System Bootloader in ROM - Yes?

AIUI, that does not provide any indication itself.

But your application could provide an indication when it is running - so you just invert that!

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.
    S
    shalingAuthor
    Associate
    April 7, 2026

    I have a heartbeat LED when the the program is running.
    This is not the problem.
    I found LD3 staying on when in System Bootloader useful.
    I would imagine I am not the only one.
    Can you explain why LD3 is constant ON when in Bootloader?
    Thanks.

    Andrew Neil
    Andrew Neil
    Super User
    April 7, 2026
    @shaling wrote:

    Can you explain why LD3 is constant ON when in Bootloader?

    Have you looked at the board schematic?

    See the 'CAD Resources' tab on the boards Product Page:

    https://www.st.com/en/evaluation-tools/nucleo-h753zi.html#cad-resources

    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.
    S
    shalingAuthor
    Associate
    April 7, 2026

    Reading between the lines, I would guess the default reset state is pull-up, so this (just) switches on Q3 NPN transistor. Maybe through an opamp and NPN, some logic can be made to display an LED when reset.

    Thanks.

    Andrew Neil
    Andrew Neil
    Super User
    April 7, 2026

    No need to guess:

    Image1.png

    https://www.st.com/resource/en/datasheet/stm32h753zi.pdf#page=31

    via: https://www.st.com/en/microcontrollers-microprocessors/stm32h753zi.html

    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.
    Uwe Bonnes
    Uwe BonnesBest answer
    Chief
    April 7, 2026

    According to AN2606 PB14 is USART1 TX (alternate) and set to push pull during system bootloader. TX inactive level is high and so Q3 will conduct and LD3 light up.

      S
      shalingAuthor
      Associate
      April 7, 2026

      That explains it. Thanks very much. I can use this :).

      Much appreciated.

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