USART transmitter failure

[ASeyo]

I'm using stm32f103ze to send some data over the serial peripheral, but it didn't work. The settings are correctly set but when I try to write to the data register it always remains zero. Furthermore, none of the Serial peripherals seem to work. What have I missed? Thanks.

Here is the code:

#include "stm32f10x.h"         // Device header

#include "stdio.h"

#include "stdarg.h"

void USART1_Init(void);

void USART1_write(char ch);

void printMsg(char* buffer, char *msg, ...);

int main(void)

{

USART1_Init();

char buffer[100];

printMsg(buffer, "Today is %s", "awesome");

while(1)

{

}

}

void USART1_Init(void)

{

//Enable clock for USART1 and Port A

RCC->APB2ENR |= RCC_APB2ENR_USART1EN;

RCC->APB2ENR |= RCC_APB2ENR_IOPAEN;

/* Reset USART1*/

RCC->APB2RSTR |= RCC_APB2RSTR_USART1RST;

/* Clear reset */

RCC->APB2RSTR &= ~RCC_APB2RSTR_USART1RST;

// PA9 as alternate function push pull

GPIOA->CRH |= GPIO_CRH_MODE9 | GPIO_CRH_CNF9_1;

GPIOA->CRH &= ~GPIO_CRH_CNF9_0;

// Set you Baud rate 

USART1->BRR = 0xEA6; // 9600bps @ 36MHz

USART1->CR1 |= USART_CR1_TE; // TE

USART1->CR1 |= USART_CR1_UE; // UE

}

void USART1_write(char ch)

{

while((USART1->SR & USART_SR_TXE) == RESET) {}

USART1->DR = ch;

}

void printMsg(char* buffer, char *msg, ...)

{

//char buffer[100];

va_list args;

va_start(args, msg);

vsprintf(buffer, msg, args);

for (int i = 0; i < buffer[i] != '\0'; i++)

{

USART1_write(buffer[i]);

}

}

[Tesla DeLorean · Accepted solution]

It is a register, not a memory cell. The values read/written to USART->DR refer to TWO DIFFERENT internal registers. The write goes to a transmit holding register and the read from a receive holding register.

Don't peer into the register file in the debugger, it will clear bits in the status registers.

[waclawek.jan]

+1

Also, check if clock is set as you expect, i.e. UART's clock is 36MHz.

Observe the Tx pin using oscilloscope/LA.

Try to wiggle it as a GPIO output to exclude hardware problem.

JW

[ASeyo]

I have checked the clock settings and everything is as I expected. The BRR, CR1 settings are set correctly. Even the TXE flag is cleared when I write to the DR(TDR) although no data is written in effect. I used a logic analyzer in USART1 TX (PA9) pin , but there is nothing shown.