The STM32 USART_Irq example program shows how to configure and use the USART1 of STMicroelectronics STM32F103xx microcontroller in interrupt driven mode.

The configuration of USART1 is 9600 Baud, 8 data bits, 1 stop bit, no parity and no flow control. Retargetting is used to read a character over the serial input and to print out a string to the serial output. A buffer handling is implemented for transmit / receive data.

The configuration of the clocks and USART is done using the Configuration Wizard in file STM32_Init.c.

 *----------------------------------------------------------------------------
 * Name:    Usart.c
 * Purpose: USART usage for STM32
 * Version: V1.00
 *----------------------------------------------------------------------------
 * This file is part of the uVision/ARM development tools.
 * This software may only be used under the terms of a valid, current,
 * end user licence from KEIL for a compatible version of KEIL software
 * development tools. Nothing else gives you the right to use this software.
 *
 * Copyright (c) 2005-2007 Keil Software. All rights reserved.
 *----------------------------------------------------------------------------*/
#include <stm32f10x_lib.h>                        // STM32F10x Library Definitions
#include <stdio.h>
#include "STM32_Init.h"                           // STM32 Initialization

/*----------------------------------------------------------------------------
  Notes:
  The length of the receive and transmit buffers must be a power of 2.
  Each buffer has a next_in and a next_out index.
  If next_in = next_out, the buffer is empty.
  (next_in - next_out) % buffer_size = the number of characters in the buffer.
 *----------------------------------------------------------------------------*/
#define TBUF_SIZE   256      /*** Must be a power of 2 (2,4,8,16,32,64,128,256,512,...) ***/
#define RBUF_SIZE   256      /*** Must be a power of 2 (2,4,8,16,32,64,128,256,512,...) ***/
/*----------------------------------------------------------------------------
 *----------------------------------------------------------------------------*/
#if TBUF_SIZE < 2
#error TBUF_SIZE is too small.  It must be larger than 1.
#elif ((TBUF_SIZE & (TBUF_SIZE-1)) != 0)
#error TBUF_SIZE must be a power of 2.
#endif
#if RBUF_SIZE < 2
#error RBUF_SIZE is too small.  It must be larger than 1.
#elif ((RBUF_SIZE & (RBUF_SIZE-1)) != 0)
#error RBUF_SIZE must be a power of 2.
#endif
/*----------------------------------------------------------------------------
 *----------------------------------------------------------------------------*/
struct buf_st {
  unsigned int in;                                // Next In Index
  unsigned int out;                               // Next Out Index
  char buf [RBUF_SIZE];                           // Buffer
};
static struct buf_st rbuf = { 0, 0, };
#define SIO_RBUFLEN ((unsigned short)(rbuf.in - rbuf.out))
static struct buf_st tbuf = { 0, 0, };
#define SIO_TBUFLEN ((unsigned short)(tbuf.in - tbuf.out))
static unsigned int tx_restart = 1;               // NZ if TX restart is required
/*----------------------------------------------------------------------------
  USART1_IRQHandler
  Handles USART1 global interrupt request.
 *----------------------------------------------------------------------------*/
void USART1_IRQHandler (void) {
  volatile unsigned int IIR;
  struct buf_st *p;
    IIR = USART1->SR;
    if (IIR & USART_FLAG_RXNE) {                  // read interrupt
      USART1->SR &= ~USART_FLAG_RXNE;           // clear interrupt
      p = &rbuf;
      if (((p->in - p->out) & ~(RBUF_SIZE-1)) == 0) {
        p->buf [p->in & (RBUF_SIZE-1)] = (USART1->DR & 0x1FF);
        p->in++;
      }
    }
    if (IIR & USART_FLAG_TXE) {
      USART1->SR &= ~USART_FLAG_TXE;           // clear interrupt
      p = &tbuf;
      if (p->in != p->out) {
        USART1->DR = (p->buf [p->out & (TBUF_SIZE-1)] & 0x1FF);
        p->out++;
        tx_restart = 0;
      }
      else {
        tx_restart = 1;
  USART1->CR1 &= ~USART_FLAG_TXE;        // disable TX interrupt if nothing to send
      }
    }
}
/*------------------------------------------------------------------------------
  buffer_Init
  initialize the buffers
 *------------------------------------------------------------------------------*/
void buffer_Init (void) {
  tbuf.in = 0;                                    // Clear com buffer indexes
  tbuf.out = 0;
  tx_restart = 1;
  rbuf.in = 0;
  rbuf.out = 0;
}
/*------------------------------------------------------------------------------
  SenChar
  transmit a character
 *------------------------------------------------------------------------------*/
int SendChar (int c) {
  struct buf_st *p = &tbuf;
                                                  // If the buffer is full, return an error value
  if (SIO_TBUFLEN >= TBUF_SIZE)
    return (-1);
                                                  
  p->buf [p->in & (TBUF_SIZE - 1)] = c;           // Add data to the transmit buffer.
  p->in++;
  if (tx_restart) {                               // If transmit interrupt is disabled, enable it
    tx_restart = 0;
 USART1->CR1 |= USART_FLAG_TXE;            // enable TX interrupt
  }
  return (0);
}
/*------------------------------------------------------------------------------
  GetKey
  receive a character
 *------------------------------------------------------------------------------*/
int GetKey (void) {
  struct buf_st *p = &rbuf;
  if (SIO_RBUFLEN == 0)
    return (-1);
  return (p->buf [(p->out++) & (RBUF_SIZE - 1)]);
}

/*----------------------------------------------------------------------------
  MAIN function
 *----------------------------------------------------------------------------*/
int main (void) {
  buffer_Init();                                  // init RX / TX buffers
  stm32_Init ();                                  // STM32 setup
  printf ("Interrupt driven Serial I/O Example\r\n\r\n");
  while (1) {                                     // Loop forever
    unsigned char c;
    printf ("Press a key. ");
    c = getchar ();
    printf ("\r\n");
    printf ("You pressed '%c'.\r\n\r\n", c);
  } // end while
} // end main