STM32H753-EVAL -- Use DB9 port (CN2) as usart 1

giorgio29 · ‎2026-04-27

Hie, according to the documentation of the board, USART1 can be used with db9 port by setting up JP7 and JP8 to 1 and 2 ( which I did). The second thing is to connect usart_tx and usart_rx to PB14 and PB15.
The problem is that nothing is showing up on the pc with DB3 port. I've check the tx port with an oscilloscope and I'm getting signal on data sending but only 1.1 V (in AC). I don't know if it's part of the issue but I hope someone has the solution

Thanks

giorgio29 · ‎2026-04-28

After activating the usart1_de pin, it is now working. I don't know why it was the issue but it has solved the problem.

Thank's you for the help !

Andrew Neil · ‎2026-04-28

I don't see "usart1_de" anywhere in the schematics or User Manual ?

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.

giorgio29 · ‎2026-04-28

It is in STM32CubeMx

mƎALLEm · ‎2026-04-28

Hello,

@giorgio29 wrote:

After activating the usart1_de pin, it is now working. I don't know why it was the issue but it has solved the problem.

What do you mean by activating USART1_DE pin? Set? Reset?

But USART1_DE alternate function is available only on PA12 but on the board PA12 is connected to FDCAN and USB, no relation with USART1/RS232

I'm not sure USART1_DE pin is the root cause!

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giorgio29 · ‎2026-04-28

Oh sorry, that's the usart clock that I activated on PA8

Andrew Neil · ‎2026-04-28

USART clock would only be relevant in synchronous mode?

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.

giorgio29 · ‎2026-04-28

Sincerely I don't why it solved my issue

Andrew Neil · ‎2026-04-28

As @mƎALLEm said, it looks like this is just a coincidence - not the actual solution to the problem.

The board only uses PA8 as MCO to optionally clock the Ethernet.

But, hey - it's working now!

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.

mƎALLEm · ‎2026-04-28

@giorgio29 wrote:

Oh sorry, that's the usart clock that I activated on PA8

USART1_CK is used in synchronous mode while you are using asynchronous mode.. I don't see the relevance of activating USART1 clock output here.. Moreover PA8 is floating (MCO output to a board connector) on the board no relation to USART1/RS232 hardware!

To me the root cause is not identified yet..

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