cembeddedstm32usart

USART3 Initilization STM32F103RB


I'm a beginner when it comes to using STM chips, and I have a project where I have to use all three USART terminals in Uvision.

I am using an STM32F103RB chip, and I already got the first two USART_init functions working, but I can't get the third one to work for some reason. I would really appreciate any help Here are my USART_init functions:

/*----------------------------------------------------------------------------
  Initialize UART pins, Baudrate
 *----------------------------------------------------------------------------*/
void USART1_Init (void) {
  int i;

  RCC->APB2ENR |=  (   1UL <<  0);        /* enable clock Alternate Function  */
  AFIO->MAPR   &= ~(   1UL <<  2);        /* clear USART1 remap               */

  RCC->APB2ENR |=  (   1UL <<  2);        /* enable GPIOA clock               */
  GPIOA->CRH   &= ~(0xFFUL <<  4);        /* clear PA9, PA10                  */
  GPIOA->CRH   |=  (0x0BUL <<  4);        /* USART1 Tx (PA9) output push-pull */
  GPIOA->CRH   |=  (0x04UL <<  8);        /* USART1 Rx (PA10) input floating  */

  RCC->APB2ENR |=  (   1UL << 14);        /* enable USART#1 clock             */

  USART1->BRR   = 0x0271;                 /* 115200 baud @ PCLK2 72MHz        */
  USART1->CR1   = ((   1UL <<  2) |       /* enable RX                        */
                   (   1UL <<  3) |       /* enable TX                        */
                   (   0UL << 12) );      /* 1 start bit, 8 data bits         */
  USART1->CR2   = 0x0000;                 /* 1 stop bit                       */
  USART1->CR3   = 0x0000;                 /* no flow control                  */
  for (i = 0; i < 0x1000; i++) __NOP();   /* avoid unwanted output            */

  USART1->CR1  |= ((   1UL << 13) );      /* enable USART                     */
}

/*----------------------------------------------------------------------------
  Initialize UART2 pins, Baudrate
 *----------------------------------------------------------------------------*/
void USART2_Init (void) {

  RCC->APB2ENR |= 1;                                    // enable clock for AF
    AFIO->MAPR |= 1<<3;                                 // set USART2 remap
    RCC->APB2ENR |= 1<<5;                           // enable clock for GPIOD
    GPIOD->CRL &= ~(0xFF << 20);                // Clear PD5, PD6
    GPIOD->CRL |= (0x0B << 20);                 // USART2 Tx (PD5) output push-pull
    GPIOD->CRL |= (0x04 << 24);                 // USART2 Rx (PD6) input floating
    RCC->APB1ENR |= 1<<17;                          // enable clock for USART2
    USART2->BRR = 0x138;                                // set baudrate -115.2kB from 36MHz
    USART2->CR1 &= ~(1<<12);                        // force 8 data bits
    USART2->CR2 &= ~(3<<12);                        // force 1 stop bit
    USART2->CR3 &= ~(3<<8);                         // force no flow control
    USART2->CR1 &= ~(3<<9);                         // force no parity
    USART2->CR1 |= 3<<2;                                // RX, TX enable
    USART2->CR1 |= 1<<5;                                // Rx interrupts if required
    NVIC->ISER[1] = (1 << 6);                   // enable interrupts if required
    USART2->CR1 |= 1<<13;                           // USART enable
    
}

/*----------------------------------------------------------------------------
  Initialize UART3 pins, Baudrate
 *----------------------------------------------------------------------------*/
void USART3_Init (void) {

  RCC->APB2ENR |= 1;                                    // enable clock for AF
    AFIO->MAPR |= 1<<3;                                // set USART3 remap
    RCC->APB2ENR |= 1<<4;                               // enable clock for GPIOC
    
  GPIOC->CRH   &= ~(0xFFUL <<  4);        /* clear PC10, PC11                  */
    GPIOC->CRH   |=  (0x04UL <<  8);        /* USART3 Rx (PC10) input floating  */
  GPIOC->CRH   |=  (0x01UL <<  8);        /* USART3 Tx (PC11) output push-pull */
    RCC->APB1ENR |=  1<<20;                                 // enable clock for USART3
    
    USART3->BRR = 0x138;                                // set baudrate -115.2kB from 36MHz; USART3    
                                                    //should have the same baudrate as USART2
    USART3->CR1 &= ~(1<<12);                        // force 8 data bits
    USART3->CR2 &= ~(3<<12);                        // force 1 stop bit
    USART3->CR3 &= ~(3<<8);                         // force no flow control
    USART3->CR1 &= ~(3<<9);                         // force no parity
    USART3->CR1 |= 3<<2;                                // RX, TX enable
    USART3->CR1 |= 1<<5;                                // Rx interrupts if required
    NVIC->ISER[1] = (1 << 6);                   // enable interrupts if required
    USART3->CR1 |= 1<<13;                           // USART enable
    
}

Solution

  • After some research, I found there were more problems than just the clock not being enabled. Below is the complete code of the initialization for UART3 using an STM32F103RB chip

    /*----------------------------------------------------------------------------
      Initialize UART3 pins, Baudrate
     *----------------------------------------------------------------------------*/
    void USART3_Init (void) {
    
        RCC->APB2ENR |= 1;                                  // enable clock for AF
        AFIO->MAPR |= AFIO_MAPR_USART3_REMAP_PARTIALREMAP;  // set USART3 to partical remap to use PC10 and PC11
        RCC->APB2ENR |= 1<<4;                               // enable clock for GPIOC
        
        // since our pins are above 8, we use H instead of L
        // since we are using 10 & 11 we will be using bits 8-15 in the CRH register
        /* USART3 Tx (PC10) output push-pull */             
        GPIOC->CRH   &= ~(0xFFUL <<  8);        
      GPIOC->CRH   |=  (0x0BUL <<  8);       
        /* USART3 Rx (PC11) input floating  */
        GPIOC->CRH   |=  (0x04UL <<  12);     
        
        RCC->APB1ENR |=  RCC_APB1ENR_USART3EN; /* enable clock for USART3         */
        USART3->BRR = 0x138;                  /* set baudrate -115.2kB from 36MHz */
        
        USART3->CR1   = ((   1UL <<  2) |     /* enable RX                        */
                       (   1UL <<  3) |       /* enable TX                        */
                       (   0UL << 12) );      /* 1 start bit, 8 data bits         */
        
      USART3->CR2   = 0x0000;                 /* 1 stop bit                       */
      USART3->CR3   = 0x0000;                 /* no flow control                  */
        
        USART3->CR1 |= 1<<13;                 /* USART3 enable                    */
        
        //Configure and enable USART3 interrupt
        NVIC->ICPR[USART3_IRQn/32] = 1UL << (USART3_IRQn%32);   // clear any previous pending interrupt flag
        NVIC->IP[USART3_IRQn] = 0x80;                           // set priority to 0x80
        NVIC->ISER[USART3_IRQn/32] = 1UL << (USART3_IRQn%32);   // set interrupt enable bit
        USART3->CR1 |= USART_CR1_RXNEIE;                        // enable USART3 receiver not empty interrupt
    }
    

    and the interrupt I used to prove this works looks like this

    // UART3 Interupt Handler
    void USART3_IRQHandler (void) {
    
        uint8_t inKey3 = (int8_t) (USART3->DR & 0x1FF);
        SendCharTo3(inKey3);
        if(inKey3 == 0x0D) {
            SendCharTo3('\n');
        }
    }
    
    /*----------------------------------------------------------------------------
      SendChar
      Write character to Serial Port. for UART 3
     *----------------------------------------------------------------------------*/
    int SendCharTo3 (uint8_t ch)  {
    
      while (!(USART3->SR & USART_SR_TXE));
      USART3->DR = ((uint16_t)ch & 0x1FF);
    
      return (ch);
    }