digitalPinToPort()

Abstract

The digitalPinToPort() is a macro that returns the port of a specified pin.

Source Code

The digitalPinToPort() is defined in hardware/arduino/avr/cores/arduino/Arduino.h as below.

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#define digitalPinToPort(P) ( pgm_read_byte( digital_pin_to_port_PGM + (P) ) )

The input is P. The macro calls pgm_read_byte() with an argument digital_pin_to_port_PGM + (P).

The digital_pin_to_port_PGM is defined in hardware/arduino/variants/standard/pins_arduino.h(in case of Arduino Uno). It is an array shown below. As this array is declared as PROGMEM, it is placed on flash memory rather than SRAM.

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const uint8_t PROGMEM digital_pin_to_port_PGM[] = {
        PD, /* 0 */
        PD,
        PD,
        PD,
        PD,
        PD,
        PD,
        PD,
        PB, /* 8 */
        PB,
        PB,
        PB,
        PB,
        PB,
        PC, /* 14 */
        PC,
        PC,
        PC,
        PC,
        PC,
};

As the digital_pin_to_port_PGM is an array, adding (P) to the digital_pin_to_port_PGM means the (P+1)th element of the array. In C++ language, the element of array x[i] can be accessed by *(x + i). For example, if the P is 13, the forteenth element(note that the elements begins with 0) of the array, that is PB, is returned.

The pgm_read_byte() is a macro to read a valiable maked as PROGMEM and is defined in hardware/tools/avr/avr/include/avr/pgmspace.h.

As a result digitalPinToPort() returns the (P+1)th element of digital_pin_to_port_PGM which is placed on the flash memory.

The relation of pin, port and bit is shown below. For example, D0(digital pin number 0) is mapped to the 0th bit of the PORTD. Reading digitalPinToBitMask() would help to understand.

	PORTB(PB)	PORTC(PC)	PORTD(PD)
D0			0
D1			1
D2			2
D3			3
D4			4
D5			5
D6			6
D7			7
D8	0
D9	1
D10	2
D11	3
D12	4
D13	5
A0		0
A1		1
A2		2
A3		3
A4		4
A5		5

Version

Arduino AVR Boards 1.8.6

License

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

/*
  Arduino.h - Main include file for the Arduino SDK
  Copyright (c) 2005-2013 Arduino Team.  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
*/

Last Update

July 31, 2025