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]

One thing I have found is that APB1 frequency is 36Mhz, I tried various values with the formula 36000000/(16*115200) but even that does not work with the BRR register.

[Tesla DeLorean]

Perhaps send a stream of U characters and use a scope?

BRR = 36000000 / 115200 is probably a better formula.

S​ome of the code is also very inefficient. The compiler can't optimize or fold them.

I​ would enable the clocks first, so you have a slight dwell time before using the registers they clock.

[SKhan.7]

Unfortunately I don't have a scope so checking it on a serial monitor on my laptop. I used the formula you suggested and things still don't work. Optimisation is turned off for the compiler. Yes, the clocks are enabled in the init function.

[S.Ma]

Go to debug mode and manually plau woth jart registere

[SKhan.7]

Unfortunately I find the debug mode quite complicated but I will try..

[SKhan.7]

Another thing I am suspecting is that the course made us configure word length as 8 but my microcontroller's Transmit Data Register has space for 9 bits.. I will try changing the code to adjust for a 9 bit word length.

[Karl Yamashita]

The CR1 register is 0 on POR. You've only enabled CR1_UE and CR1_TE bits. The M bit should be zero so your word length is 8 bits.

[SKhan.7]

Yes, but the thing is it 8 bits word length alright or should I change it to 9 bits as the TDR has a space for 9 bits.

[Karl Yamashita]

Unless the course says to change to 9 bits, leave it as 8 bits. How is the UART communicating to the serial program? USB to Serial adapter (TTL level) and hopefully not a USB to RS232 adapter?