HardwareSerial calls the transmit interrupt one more time than needed. [imported]

[cmaglie]

This is Issue 1008 moved from a Google Code project.
Added by 2012-08-17T23:16:20.000Z by blackwel...@gmail.com.
Please review that bug for more context and additional comments, but update this bug.

Original labels: Type-Enhancement, Priority-Medium, Component-Core

Original description

Each serial buffer transmission results in 1 extra interrupt to detect the TX
buffer being empty (verified with O-Scope).

Using USART3 as an example, here's an easy fix to remove this extra interrupt.

EXISTING:
ISR(USART3_UDRE_vect)
{
  if (tx_buffer3.head == tx_buffer3.tail) {
    // Buffer empty, so disable interrupts
    cbi(UCSR3B, UDRIE3);
  }
  else {
    // There is more data in the output buffer. Send the next byte
    unsigned char c = tx_buffer3.buffer[tx_buffer3.tail];
    tx_buffer3.tail = (tx_buffer3.tail + 1) % SERIAL_BUFFER_SIZE;

    UDR3 = c;
  }
}


NEW:
ISR(USART3_UDRE_vect)
{
  // There is more data in the output buffer. Send the next byte
  unsigned char c = tx_buffer3.buffer[tx_buffer3.tail];
  tx_buffer3.tail = (tx_buffer3.tail + 1) % SERIAL_BUFFER_SIZE;

  UDR3 = c;

  if (tx_buffer3.head == tx_buffer3.tail) {
    // Buffer empty, so disable interrupts
    cbi(UCSR3B, UDRIE3);
  }
}

[cmaglie]

Fixed in 1.5.6