USART display error on STM32 NUCLEO H533RE

Rav3nPho3nix · ‎2026-06-04

Hi, i'm currently beginning to create my own RTOS (on bare metal) for my NUCLEO H533RE.

My source code is available here : https://github.com/Rav3nPho3nix/Rav3nTOS

Here is my issue : i'm trying to implement USART 2 for my board to have a "print" tool. I can connect with "minicom" to see the output from the ST LINK.

My main function (the main function on my repo is not up to date) is here :

#include <stdint.h>
#include "platform.h"
#include "usart.h"

int main(void) {
    platform_init();
    while (1) {
        usart_write_string("Test\n\r");
        for (volatile uint32_t i = 0; i < 1000000; i++);
    }
}

It just call 'platform_init' then print with usart 2.

My 'platform_init' just initialize usart by setting the baud rate to 115200 :

usart_init(115200U);

And to finish, here is the code for usart : https://github.com/Rav3nPho3nix/Rav3nTOS/blob/main/bsp/stm32h533/usart.c

I think my problem come from this line when is calculate the brr value :

    // Set up BBR
    // 64 Mhz of frequency
    // BBR = (64,000,000) / BaudRate
    USART2->BRR = 64000000U / baudrate;

Here is supposed that the frequency is 64 Mhz, but i'm not sure. I have read the reference manual and datasheet of the STM32H533 but i didn't found what is the correct frequency value.

As a result, when i use minicom to display my usart 'print', these characters appears : ��怆, wich correspond to my `Test\r\n', so it's really just a brr value calculation error for me.

Do you know the correct value ?
Thx in advance.