Transmitting characters with the interrupt sub system

isr_xmit.c

#define BUF_SIZE  16
char ring_buf[BUF_SIZE];
uint8_t cur_char, nxt_char = 0;

void _write_char(char *c)
{
  /* ring buffer is full, wait for isr to drain a character out of it */
  while (((cur_char + 1) % BUF_SIZE) == nxt_char) ;
  
  /* Is the USART idle? Then just write the character */
  if ((USART_TXE == 1) && (cur_char == nxt_char)) {
    USART_DR = c;
    return;
  }
  /* Else stick it into the ring buffer, ISR will get it */
  ring_buf[nxt_char++] = c;  // store char
  nxt_char = nxt_char % BUF_SIZE;  // update ring buffer
  return;
}

usart_isr(void)
{
  /* Is the transmitter buffer empty? */
  if (USART_TXE) {
    /* yes: Is there a character in the ring buffer that hasn't been sent? */
    if (cur_char != nxt_char) {
      USART_DR = ring_buf[cur_char++]; // write char
      cur_char = cur_char & BUF_SIZE;  // update ring buffer
    }
  }
}