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Steve Bertrand


RPi::WiringPi::FAQ - FAQ and Tutorial for RPi::WiringPi


This document will hopefully provide enough information in a sane way to get you well on your way with manipulating your Raspberry Pi with the RPi::WiringPi and related distributions.

In this document we use constants that are provided with the :all tag in RPi::WiringPi::Constant module.


    HIGH -          connected to 3.3v (positive)
    LOW  -          connected to 0v (ground)
    floating -      state where a pin is not stable at HIGH or LOW
    PWM -           Pulse Width Modulation (potentiometer-like)
    INPUT -         pin is listening only
    OUTPUT -        pin is active in turning things on/off
    PWM_OUT -       pin is OUTPUT, but PWM capable
    GPIO_CLOCK -    pin is used for timing
    PUD  -          internal pull up/down resistor
    PUD_UP -        PUD resistor pulled to HIGH
    PUD_DOWN -      PUD resistor pulled to LOW
    EDGE_FALLING -  a state when a pin goes from HIGH to LOW
    EDGE_RISING  -  a state when a pin goes from LOW to HIGH
    EDGE_BOTH    -  both of the above states


Pi: Create a Raspberry Pi object

The first thing you need to do is call an appropriate setup method. We do that automatically. See new() to see how to use non-default setup routines.

    my $pi = RPi::WiringPi->new;

Pin: Creating and using a GPIO pin object

The RPi::WiringPi::Pin class provides you with objects that directly map to the Raspberry Pi's onboard GPIO pins.

    my $pin = $pi->pin(27);

    # set the mode to output, presumably to power an external device


    # by default, pins are set to LOW (ie. 0 voltage). Turn it on...


    # get the current status of a pin (HIGH or LOW)


    # get a pin's pin number

    my $num = $pin->num;

Pin: Setting a pin's internal pull up/down resistor

All GPIO pins on the Raspberry Pi have built-in pull up/down resistors to prevent pins being in a "floating" state when not connected to either ground or power. This is very important in many situations, particularly when using interrupts.

    # HIGH when not in use


    # LOW when not in use


Pin: Using Pulse Width Modulation (PWM)

Pulse Width Modulation kind of acts like a potentiometer (or a variable switch... like a light dimmer). They are used to send pulses of electricity to a device across time. It is required for things like stepper motors, or dimming an LED.

    # set the pin to PWM_OUT mode


    # values are 0-1023 which represent 0% to 100% power

    $pin->pwm(512); # pin output is ~50%

    # make pin go from off to bright gradually...
    # requires Time::HiRes qw(usleep);

    my $pin = $pi->pin(27);

    for (0..1023){

Pin: Using interrupts

Built in is the ability to have code you define executed (interrupt handler) when a pin's edge changes. The Interrupt Service Request is handled in a separate C thread than your application. You can have a different handler for HIGH and LOW on the same pin at the same time.

    sub handler {
        print "in handler\n";
        # do other stuff, perhaps turning on/off other pins

    # set an interrupt handler for when the pin goes from
    # LOW to HIGH. The second parameter is the string name
    # of the previously written perl sub

    $pin->interrupt_set(EDGE_RISING, 'handler');

    # HIGH to LOW

    $pin->interrupt_set(EDGE_LOW, 'handler');

    # HIGH and LOW (interrupt will be triggered on both changes)

    $pin->interrupt_set(EDGE_BOTH, 'handler');

    # remove an interrupt


Interrupt: Overview

Although we've already used interrupts from within a pin object, you can use them separately as well.

Interrupt: Setting and unsetting

    my $interrupt = $pi->interrupt;

    # set an EDGE_RISING interrupt on pin 27

    # callback is the string name of your handler
    # subroutine that'll be called when an interrupt
    # has occurred

    $interrupt->set(27, EDGE_RISING, 'callback');

    # unset the interrupt


    sub callback {

Board: Get your Pi's board revision

    my $board = $pi->board;
    my $revision = $board->rev;

LCD: Initialize

Typical 16-pin, 2-4 row and 16-20 column LCD screens work here. You can use 4-bit or 8-bit mode (4-bit requires 6 GPIO pins, 8-bit requires 10). If you need a higher rate of data transmission to the LCD, use 8-bit mode. Typically, 4-bit has always worked well enough for me.

Before an LCD can be used, it must be initialized. This may look like a lot, but you only need to do it once. Essentially, you're configuring all pins up front.

NOTE: When in 4-bit mode, although you're setting d0 through d3 pins up and leaving d4 through d7 as 0, the wiring must connect through 4 to 7. Look at LCD pin 4-7 as LCD pin 0-3 when in 4-bit mode.

    my $lcd = $pi->lcd;

    my %lcd_args = (
        rows  => 2,     # number of display rows, 2 or 4
        cols  => 16,    # number of display columns 16 or 20
        bits  => 4,     # data width in bits, 4 or 8
        rs    => 1,     # GPIO pin for the LCD RS pin
        strb  => 2,     # GPIO pin for the LCD strobe (E) pin
        d0    => 3,     #
        ...             # d0-d3 GPIO pinout numbers
        d3    =>6,      #
        d4    => 7,     # set d4-d7 to all 0 (zero) if in 4-bit mode
        ...             # otherwise, set them to their respective
        d7    => 11     # GPIO pins


LCD: Display operations

Now that we've initialized the LCD, we're ready to use it.

    # turn the display on/off

    $lcd->display(ON);  # or OFF

    # put the cursor at col 0, row 0


    # clear the display and move cursor to home


LCD: Cursor operations

    # move the cursor to a position

    $lcd->position(0, 0);   # col 0, row 0 (top row)
    $lcd->position(0, 1);   # col 0, row 1 (bottom row on 2 row LCD)
    $lcd->positon(5, 1);    # col 6, row 1

    # turn on/off cursor

    $lcd->cursor(OFF);  # or ON

    # make the cursor blink

    $lcd->cursor_blink(ON); # or OFF

LCD: Output operations

By default, output starts at col 0 and row 0 of the display. Use position() to move it around before outputting.

    # print out a string

    $lcd->print("My name is stevieb");

LCD: Putting it all together

Here's a trivial script that outputs information to specific LCD positions (we'll start right after an LCD init()).

    my $perl_ver = '5.24.0';
    my $name = 'stevieb';


    $lcd->print("${name}'s RPi, on");
    $lcd->position(0, 1);
    $lcd->print("Perl $perl_ver...");

Util: Overview

The included RPi::WiringPi::Util module contains a few helper-type methods for internal and external use. Most of these you won't need, but others are very helpful when writing your own scripts that go beyond trivial.

You can transform pin numbers from one scheme to another, get full pin number maps/translation hashes, manually export and unexport GPIO pins etc.

It's worth having a look at...


Steve Bertrand, <steveb@cpan.org>


Copyright (C) 2016 by Steve Bertrand

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.18.2 or, at your option, any later version of Perl 5 you may have available.