Need help in making USART2 work on STM32F303RE

[SKhan.7]

Hi, I am doing a MOOC for learning STM32 programming, in the UART topic, we are supposed to implement UART. The course follows STM32F4 but I have an STM32F303RE and I tried to adapt it to my board but it is still not showing anything on the serial monitor (I tried CuteCom and Arduino Serial Monitors as I use a Mac). Can you please tell me what is wrong with the code below? I tried to input various values directly into the BRR register as I had doubts about the baud rate formula in the course.

#include <stdio.h>

#include <stdint.h>

#include "stm32f303xe.h"

#define IOPAEN (1U<<17)

#define UART2EN (1U<<17)

#define CR1_TE (1U<<3)

#define CR1_UE (1U<<13)

#define SR_TXE (1U<<7)

#define SYS_FREQ 16000000

#define APB1_CLK SYS_FREQ

#define UART_BAUDRATE 115200

static void uart_set_baudrate(USART_TypeDef *USARTx, uint32_t PeriphClk, uint32_t BaudRate);

static uint16_t compute_uart_bd(uint32_t PeriphClk, uint32_t BaudRate);

void uart2_tx_init(void);

void uart2_write (int ch);

int main (void){

uart2_tx_init();

while(1){

uart2_write('Y');

}

}

void uart2_tx_init(void){

//Configure UART GPIO pin

// Enable clock access to GPIOA

RCC->AHBENR |= IOPAEN;

// Set PA2 to Alternate Function Mode

GPIOA->MODER &= ~(1U<<4);

GPIOA->MODER |= (1U<<5);

// Set PA2 Alternate Function type to UART_TX (AF07)

GPIOA->AFR[0] |= (1U<<8);

GPIOA->AFR[0] |= (1U<<9);

GPIOA->AFR[0] |= (1U<<10);

GPIOA->AFR[0] &= ~(1U<<11);

//Configure UART Module

// Enable clock access to UART2

RCC->APB1ENR |= UART2EN;

// Configure baud rate

uart_set_baudrate(USART2, APB1_CLK, UART_BAUDRATE);

// Configure the transfer direction

USART2->CR1 = CR1_TE;

// Enable UART module

USART2->CR1 |= CR1_UE;

}

void uart2_write (int ch){

//Make sure transmit data register is empty

while (!(USART2->ISR & SR_TXE)){}

//Write to transmit data register

USART2->TDR = (ch & 0xFF);

}

static void uart_set_baudrate(USART_TypeDef *USARTx, uint32_t PeriphClk, uint32_t BaudRate){

USARTx->BRR = compute_uart_bd(PeriphClk,BaudRate);

}

static uint16_t compute_uart_bd(uint32_t PeriphClk, uint32_t BaudRate){

return ((PeriphClk + (BaudRate/2U))/BaudRate);

}

[SKhan.7]

So I found one mistake in my code, what an idiot I am. The CR1_UE is supposed to be (1U<<0). Now I am getting something on serial but it is a weird character instead of capital Y, I am getting ÿ repeatedly.

[Karl Yamashita]

Good catch. I've actually grabbed the first reference data sheet available and it so happen to be for a STM32F407, lol. Now that I grabbed the correct data sheet, the STM32F303RE CR1_UE bit is bit 0.

[Karl Yamashita]

You didn't turn on 9 bits did you?

[SKhan.7]

Ah finally, got it working. So the problems were:

#define CR1_UE (1U<<0) // This has been corrected from bit 13 to 0

#define SYS_FREQ 8000000 // This has been changed from 16000000 to 8000000 (But I don't know why, APB1 is supposed to be on 36Mhz?)

Thanks for the help, I guess I will continue with the course and "be careful".

[SKhan.7]

No no, it works on 8 bits so no need for 9 bits..

[SKhan.7]

Ah thanks a lot..!

[SKhan.7]

This is one of the things I like about embedded programming, it makes you go crazy but when it works, it feels sooooooo goooddd...!

[SKhan.7]

If someone can, please tell me why 8Mhz frequency worked rather than 16Mhz? I see in the reference manual that APB1 runs on 36Mhz.