shiftOut()

Abstract

The shiftOut() sends a byte of data bit by bit.

Source Code

The shiftOut() is defined in hardware/arduino/avr/cores/arduino/wiring_shift.c as below.

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void shiftOut(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder, uint8_t val)
{
        uint8_t i;

        for (i = 0; i < 8; i++)  {
                if (bitOrder == LSBFIRST) {
                        digitalWrite(dataPin, val & 1);
                        val >>= 1;
                } else {
                        digitalWrite(dataPin, (val & 128) != 0);
                        val <<= 1;
                }

                digitalWrite(clockPin, HIGH);
                digitalWrite(clockPin, LOW);
        }
}

The inputs are dataPin, clockPin, bitOrder and val, they all are uint8_t, returns nothing.

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void shiftOut(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder, uint8_t val)
{
    uint8_t i;

It loops to send 8 bits of data.

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        for (i = 0; i < 8; i++)  {
                if (bitOrder == LSBFIRST) {
                        digitalWrite(dataPin, val & 1);
                        val >>= 1;
                } else {
                        digitalWrite(dataPin, (val & 128) != 0);
                        val <<= 1;
                }

When the bitOrder is LSBFIRST, this function sends data from LSB. It sends the 0th bit of val by calculating bitwise AND of val and 1. Then shifts right by 1 bit to make next bit to be the 0th bit of val.

When the bitOrder is MSBFIRST, this function sends data from MSB. It sends the 7th bit of val by calculating bitwise AND of val and 128(= 0B10000000). Then shifts left by 1 bit to make next bit to be the 7th bit of val.

It uses digitalWrite() to send data.

Finally it sends a pulse making clockPin HIGH then LOW.

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        digitalWrite(clockPin, HIGH);
        digitalWrite(clockPin, LOW);        
    }
}

Version

Arduino AVR Boards 1.8.6

License

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/*
  Copyright (c) 2016 garretlab.
  Added source code explanation by garretlab.
*/

/*
  wiring_shift.c - shiftOut() function
  Part of Arduino - http://www.arduino.cc/

  Copyright (c) 2005-2006 David A. Mellis

  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., 59 Temple Place, Suite 330,
  Boston, MA  02111-1307  USA
*/

Last Update

July 31, 2025