Bootloader Indication LED

shaling · ‎2026-04-07

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.

Uwe Bonnes · ‎2026-04-07

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.

View solution in original post

Andrew Neil · ‎2026-04-07

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.

shaling · ‎2026-04-07

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 · ‎2026-04-07

@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.

Uwe Bonnes · ‎2026-04-07

There are many nucleo boards, and I think LD3 may be used for different indicator purposes. What board do you refer to?

shaling · ‎2026-04-07

I am new to STM32 and the schematic in CAD link is useful, thanks.
But I do not know what the state of PB14 is when in the System Bootloader.
Can some sort of pseudo indicator for System Bootloader be made with any of the pins?

Andrew Neil · ‎2026-04-07

AFAIK, the System Bootloader only initialises the pins that it actually uses - so all others stay at their default reset state.

Check out Application note AN2606, Introduction to system memory boot mode on STM32 MCUs.

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.

Andrew Neil · ‎2026-04-07

@Uwe Bonnes wrote:
What board do you refer to?

OP states STM32H753ZI Nucleo board

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.

shaling · ‎2026-04-07

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 · ‎2026-04-07

No need to guess:

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.