HardwareSerial::_tx_udr_empty_irq()

Abstract

The HardwareSerial::_tx_udr_empty_irq() copies data from Arduino serial communication transmit buffer to ATmega328P data transmit register.

This function is called from HardwareSerial::flush(), HardwareSerial::write() and USART_UDRE_vect.

Source Code

The HardwareSerial::_tx_udr_empty_irq() is defined in hardware/arduino/avr/cores/arduino/HardwareSerial.cpp as below.

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void HardwareSerial::_tx_udr_empty_irq(void)
{
  // If interrupts are enabled, there must be more data in the output
  // buffer. Send the next byte
  unsigned char c = _tx_buffer[_tx_buffer_tail];
  _tx_buffer_tail = (_tx_buffer_tail + 1) % SERIAL_TX_BUFFER_SIZE;

  *_udr = c;

  // clear the TXC bit -- "can be cleared by writing a one to its bit
  // location". This makes sure flush() won't return until the bytes
  // actually got written. Other r/w bits are preserved, and zeroes
  // written to the rest.

  *_ucsra = ((*_ucsra) & ((1 << U2X0) | (1 << MPCM0))) | (1 << TXC0);

  if (_tx_buffer_head == _tx_buffer_tail) {
    // Buffer empty, so disable interrupts
    cbi(*_ucsrb, UDRIE0);
  }
}

No inputs, no outputs.

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void HardwareSerial::_tx_udr_empty_irq(void)
{

Get the oldest data(_tx_buffer_tail-th data) from transmit buffer(_tx_buffer), substitutes to c.

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  // If interrupts are enabled, there must be more data in the output
  // buffer. Send the next byte
  unsigned char c = _tx_buffer[_tx_buffer_tail];
  _tx_buffer_tail = (_tx_buffer_tail + 1) % SERIAL_TX_BUFFER_SIZE;
 
  *_udr = c;

Because we got a data out of the buffer, increment _tx_buffer_tail. Note _tx_buffer is a ring buffer, correct the value by calculating division reminder.

Put the c to _udr(UDRn), which is the addres of transmit buffer.

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  // clear the TXC bit -- "can be cleared by writing a one to its bit
  // location". This makes sure flush() won't return until the bytes
  // actually got written. Other r/w bits are preserved, and zeroes
  // written to the rest.
 
  *_ucsra = ((*_ucsra) & ((1 << U2X0) | (1 << MPCM0))) | (1 << TXC0);

Set TXC0 bit of _ucsra(UCSR0A to clear TXC0 bit. This means unsent data exists.

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  if (_tx_buffer_head == _tx_buffer_tail) {
    // Buffer empty, so disable interrupts
    cbi(*_ucsrb, UDRIE0);
  }
}

If the _tx_buffer_head and _tx_buffer_tail is same, it means the transmit buffer of Arduino is empty, set UDRIE0 bit of _ucsrb(UCSR0B register to 0, using cbi(), to disable interrupts.

Version

Arduino AVR Boards 1.8.6

License

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/*
  Copyright (c) 2019 garretlab.
  Added source code explanation by garretlab.
*/

/*
  HardwareSerial.cpp - Hardware serial library for Wiring
  Copyright (c) 2006 Nicholas Zambetti.  All right reserved.

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

  Modified 23 November 2006 by David A. Mellis
  Modified 28 September 2010 by Mark Sproul
  Modified 14 August 2012 by Alarus
  Modified 3 December 2013 by Matthijs Kooijman
*/

Last Update

August 25, 2019