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RPi Low-level peripherals

Revision as of 22:18, 14 March 2013 by BerryBerry (Talk | contribs)

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Hardware & Peripherals:

Hardware and Hardware History.

Low-level Peripherals and Expansion Boards.

Screens, Cases and Other Peripherals.


In addition to the familiar USB, Ethernet and HDMI ports, the R-Pi offers lower-level interfaces intended to connect more directly with chips and subsystem modules. These GPIO (general purpose I/O) signals on the 2x13 header pins include SPI, I2C, serial UART, 3V3 and 5V power. These interfaces are not "plug and play" and require care to avoid miswiring. The pins use a 3V3 logic level and are not tolerant of 5V levels, such as you might find on a 5V powered Arduino. Not yet software-enabled are the flex cable connectors with CSI (camera serial interface) and DSI (display serial interface), and a serial link inside the HDMI connector called CEC. (consumer electronics control)

General Purpose Input/Output (GPIO)

The layout of the Raspberry Pi Revision 1 P1 pin-header seen from the top, containing pins useable for general purpose I/O. Colour coded to the table. Source

General Purpose Input/Output (a.k.a. GPIO) is a generic pin on a chip whose behavior (including whether it is an input or output pin) can be controlled (programmed) through software.

The Raspberry Pi allows peripherals and expansion boards (such as the Rpi Gertboard) to access the CPU by exposing the inputs and outputs.

For further general information about GPIOs, see: the wikipedia article.
For further specific information about the Raspberry Pi's BCM2835 GPIOs, see: this wiki article.

The production Raspberry Pi board has a 26-pin 2.54 mm (100 mil)[1] expansion header, marked as P1, arranged in a 2x13 strip. They provide 8 GPIO pins plus access to I²C, SPI, UART), as well as +3.3 V, +5 V and GND supply lines. Pin one is the pin in the first column and on the bottom row. [2]

RPi P1 header.png

GPIO voltage levels are 3.3 V and are not 5 V tolerant. There is no over-voltage protection on the board - the intention is that people interested in serious interfacing will use an external board with buffers, level conversion and analog I/O rather than soldering directly onto the main board.

All the GPIO pins can be reconfigured to provide alternate functions, SPI, PWM, I²C and so. At reset only pins GPIO 14 & 15 are assigned to the alternate function UART, these two can be switched back to GPIO to provide a total of 17 GPIO pins[3]. Each of their functions and full details of how to access are detailed in the chipset datasheet [4].

Each GPIO can interrupt, high/low/rise/fall/change.[5][6] There is currently no support for GPIO interrupts in the official kernel, howewer a patch exists, requiring compilation of modified source tree.[7] The 'Raspbian "wheezy"' [8] version that is currently recommended for starters already includes GPIO interrupts.

GPIO input hysteresis (Schmitt trigger) can be on or off, output slew rate can be fast or limited, and source and sink current is configurable from 2 mA up to 16 mA. Note that chipset GPIO pins 0-27 are in the same block and these properties are set per block, not per pin. See GPIO Datasheet Addendum - GPIO Pads Control. Particular attention should be applied to the note regarding SSO (Simultaneous Switching Outputs): to avoid interference, driving currents should be kept as low as possible.

The available alternative functions and their corresponding pins are detailed below. These numbers are in reference to the chipset documentation and may not match the numbers exposed in Linux. Only fully usable functions are detailed, for some alternative functions not all the necessary pins are available for the funtionality to be actually used.

There is also some information on the Tutorial on Easy GPIO Hardware & Software.

Kernel boot messages go to the UART at 115200 bit/s.

R-Pi PCB Revision 2 UPDATE: According to Eben at [1] the R-Pi Rev.2 board being rolled out starting in September 2012 adds 4 more GPIO on a new connector called P5, and changes some of the existing P1 GPIO pinouts. On Rev2, GPIO_GEN2 [BCM2835/GPIO27] is routed to P1 pin 13, and changes what was SCL0/SDA0 to SCL1/SDA1: SCL1 [BCM2835/GPIO3] is routed to P1 pin 5, SDA1 [BCM2835/GPIO2] is routed to P1 pin 3. Also the power and ground connections previously marked "Do Not Connect" on P1 will remain as connected, specifically: P1-04:+5V0, P1-09:GND, P1-14:GND, P1-17:+3V3, P1-20:GND, P1-25:GND. According to this comment [2] (and confirmed in this post [3]) the P1 pinout is not expected to change in future beyond the current Rev.2 layout.

P1 Header Pinout, top row:

Pin Number Pin Name Rev1 Pin Name Rev2 Hardware Notes Alt 0 Function Other Alternative Functions
P1-02 5V0 5V0 Supply through input poly fuse
P1-04 5V0 5V0 Supply through input poly fuse
P1-08 GPIO 14 GPIO 14 Boot to Alt 0 -> UART0_TXD ALT5 = UART1_TXD
P1-10 GPIO 15 GPIO 15 Boot to Alt 0 -> UART0_RXD ALT5 = UART1_RXD
P1-24 GPIO08 GPIO08 SPI0_CE0_N
P1-26 GPIO07 GPIO07 SPI0_CE1_N

P1 Header Pinout, bottom row:

Pin Number Pin Name Rev1 Pin Name Rev2 Hardware Notes Alt 0 Function Other Alternative Functions
P1-01 3.3 V 3.3 V 50 mA max (01 & 17)
P1-03 GPIO 0 GPIO 2 1K8 pull up resistor I2C0_SDA / I2C1_SDA
P1-05 GPIO 1 GPIO 3 1K8 pull up resistor I2C0_SCL / I2C1_SCL
P1-13 GPIO21 GPIO27 PCM_DOUT / reserved ALT4 = SPI1_SCLK ALT5 = GPCLK1 / ALT3 = SD1_DAT3 ALT4 = ARM_TMS
P1-17 3.3 V 3.3 V 50 mA max (01 & 17)

Colour legend
+5 V
+3.3 V
Ground, 0V

KiCad symbol: File:Conn-raspberry.lib


Pin 3 (SDA0) and Pin 5 (SCL0) are preset to be used as an I²C interface. So there are 1.8 kilohm pulls up resistors on the board for these pins.[10]

Pin 12 supports PWM .

It is also possible to reconfigure GPIO connector pins P1-7, 15, 16, 18, 22 (chipset GPIOs 4 and 22 to 25) to provide an ARM JTAG interface.[11] However ARM_TMS isn't available on the GPIO connector (chipset pin 12 or 27 is needed). Chipset pin 27 is available on S5, the CSI camera interface however.

It is also possible to reconfigure GPIO connector pins P1-12 and 13 (chipset GPIO 18 and 21) to provide an I2S (a hardware modification may be required[12]) or PCM interface.[13] However, PCM_FS and PCM_DIN (chipset pins 19 and 20) are needed for I2S or PCM.

A second I²C interface (GPIO02_ALT0 is SDA1 and GPIO03_ALT0 is SCL1) and two further GPIOs (GPIO05_ALT0 is GPCLK1, and GPIO27) are available on S5, the CSI camera interface.

Referring to pins on the Expansion header

The header is referred to as "The GPIO Connector (P1)". To avoid nomenclature confusion between Broadcom signal names on the SoC and pin names on the expansion header, the following naming is highly recommended.

  • The expansion header is referred to as "Expansion Header" or "GPIO Connector (P1)"
  • Pins on the GPIO connector (P1) are referred to as P1-01, etc.
  • Names GPIO0, GPIO1, GPIOx-ALTy, etc. refer to the signal names on the SoC as enumerated in the Broadcom datasheet, where "x" matches BCM2835 number (without leading zero) and "y" is the alternate number column 0 to 5 on page 102-103 of the Broadcom document. For example, depending on what you are describing, use either "GPIO7" to refer to a row of the table, and "GPIO7-ALT0" would refer to a specific cell of the table.
  • When refering to signal names, you should modify the Broadcom name slightly to minimize confusion. The Broadcom SPI bus pin names are fine, such as "SPI0_*" and "SPI1_*", but they didn't do the same on the I²C and UART pins. Instead of using "SDA0" and "SCL0", you should use "I2C0_SDA" and "I2C0_SCL"; and instead of "TX" or "TXD" and "RX" or "RXD", you should use "UART0_TXD" and "UART0_RXD".

Power pins

The maximum permitted current draw from the 3.3 V pins is 50 mA.

Maximum permitted current draw from the 5 V pin is the USB input current (usually 1 A) minus any current draw from the rest of the board.[14]

  • Model A: 1000 mA - 500 mA -> max current draw: 500 mA
  • Model B: 1000 mA - 700 mA -> max current draw: 300 mA

Be very careful with the 5 V pins P1-02 and P1-04, because if you short 5 V to any other P1 pin you may permanently damage your RasPi. Before probing P1, it's a good idea to strip short pieces of insulation off a wire and push them over the 5 V pins so you don't accidentally short them with a probe.

GPIO hardware hacking

The complete list of chipset GPIO pins which are available on the GPIO connector is:
0, 1, 4, 7, 8, 9, 10, 11, 14, 15, 17, 18, 21, 22, 23, 24, 25

(on the Revision2.0 RaspberryPis, this list changes to: 2, 3, 4, 7, 8, 9, 10, 11, 14, 15, 17, 18, 22, 23, 24, 25, 27, with 28, 29, 30, 31 additionally available on the P5 header)

As noted above, P1-03 and P1-05 (SDA0 and SCL0 / SDA1 and SCL1) have 1.8 kilohm pull-up resistors to 3.3 V.

If 17 GPIOs aren't sufficient for your project, there are a few other signals potentially available, with varying levels of software and hardware (soldering iron) hackery skills:

GPIO02, 03, 05 and 27 are available on S5 (the CSI interface) when a camera peripheral is not connected to that socket, and are configured by default to provide the functions SDA1, SCL1, CAM_CLK and CAM_GPIO respectively. SDA1 and SCL1 have 1K6 pull-up resistors to 3.3 V.

GPIO06 is LAN_RUN and is available on pad 12 of the footprint for IC3 on the Model A. On Model B, it is in use for the Ethernet function.

There are a few other chipset GPIO pins accessible on the PCB but are in use:

  • GPIO16 drives status LED D5 (usually SD card access indicator)
  • GPIO28-31 are used by the board ID and are connected to resistors R3 to R10 (only on Rev1.0 boards).
  • GPIO40 and 45 are used by analogue audio and support PWM. They connect to the analogue audio circuitry via R21 and R27 respectively.
  • GPIO46 is HDMI hotplug detect (goes to pin 6 of IC1).
  • GPIO47 to 53 are used by the SD card interface. In particular, GPIO47 is SD card detect (this would seem to be a good candidate for re-use). GPIO47 is connected to the SD card interface card detect switch; GPIO48 to 53 are connected to the SD card interface via resistors R45 to R50.

P2 header

The P2 header is the VideoCore JTAG and used only during the production of the board. It cannot be used as the ARM JTAG [15]. This connector is unpopulated in Rev 2.0 boards.

RPi P2 header.png

Useful P2 pins:

  • Pin 1 - 3.3V (same as P1-01, 50 mA max current draw across both of them)
  • Pin 7 - GND
  • Pin 8 - GND

P3 header

The P3 header, unpopulated, is the LAN9512 JTAG [16].

RPi P3 header.png

Useful P3 pins:

  • Pin 7 - GND

P5 header

The P5 header was added with the release of the Revision 2.0 PCB design.

RPi P5 header.png

P5 Header Pinout (seen from the back of the board), top row:

Pin Number Pin Name Rev2 Hardware Notes Alt 0 Function Other Alternative Functions
P5-01 5V0 Supply through input poly fuse
P5-07 GND

P5 Header Pinout (seen from the back of the board), bottom row:

Pin Number Pin Name Rev2 Hardware Notes Alt 0 Function Other Alternative Functions
P5-02 3.3 V 50 mA max (combined with P1)
P5-08 GND

Note that the connector is intended to be mounted on the bottom of the PCB, so that for those who put the connector on the top side, the pin numbers are swapped. Pin 1 and pin 2 are swapped, pin 3 and 4, etc.

The new header can provide a second I2C channel (SDA + SCL) and handshake lines for the existing UART (TxD and RxD), or it can be used for an I2S (audio codec chip) interface using the PCM signals CLK, FS (Frame Sync), Din and Dout.

Note that the connector is placed JUST off-grid with respect to the P1 connector.

P6 header

The P6 header was added with the release of the Revision 2.0 PCB design.

RPi P6 header.png

P6 Pinout:

Pin Number Pin Name Rev2 Hardware Notes
P6-01 RUN Short to ground to reset the BCM2835
P6-02 GND

A reset button can be attached to the P6 header, with which the Pi can be reset. Momentarily shorting the two pins of P6 together will cause a soft reset of the CPU (which can also 'wake' the Pi from halt/shutdown state).

Driver support

The Foundation will not include a GPIO driver in the initial release, standard Linux GPIO drivers should work with minimal modification.[17]

The community implemented SPI and I²C drivers [18], which will be integrated with the new Linux pinctrl concept in a later version of the kernel. (On Oct. 14 2012, it was already included in the latest raspbian image.) A first compiled version as Linux modules is available to install on the 19/04/2012 Debian image, including 1-wire support[19]. The I²C and SPI driver uses the hardware modules of the microcontroller and interrupts for low CPU usage, the 1-wire support uses bitbanging on the GPIO ports, which results in higher CPU usage.

GordonH[20] wrote a (mostly) Arduino compatible/style WiringPi library in C for controlling the GPIO pins.

A useful tutorial on setting up I²C driver support can be found at Robot Electronics - look for the downloadable document rpi_i2c_setup.doc

Graphical User Interfaces


WebIOPi allows you to control each GPIO with a simple web interface that you can use with any browser. Available in PHP and Python, they both require root access, but Python version serves HTTP itself. You can setup each GPIO as input or output and change their states (LOW/HIGH). WebIOPi is fully customizable, so you can use it for home remote control. It also work over Internet. UART/SPI/I2C support will be added later. If you need some computing for your GPIO go to code examples below.

GPIO Code examples


Examples in different C-Languages.


Gert van Loo & Dom, has provided some tested code which accesses the GPIO pins through direct GPIO register manipulation in C-code. (Thanks to Dom for doing the difficult work of finding and testing the mapping.) Example GPIO code:

//  How to access GPIO registers from C-code on the Raspberry-Pi
//  Example program
//  15-January-2012
//  Dom and Gert
//  Revised: 15-Feb-2013

// Access from ARM Running Linux

#define BCM2708_PERI_BASE        0x20000000
#define GPIO_BASE                (BCM2708_PERI_BASE + 0x200000) /* GPIO controller */

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>

#define PAGE_SIZE (4*1024)
#define BLOCK_SIZE (4*1024)

int  mem_fd;
void *gpio_map;

// I/O access
volatile unsigned *gpio;

// GPIO setup macros. Always use INP_GPIO(x) before using OUT_GPIO(x) or SET_GPIO_ALT(x,y)
#define INP_GPIO(g) *(gpio+((g)/10)) &= ~(7<<(((g)%10)*3))
#define OUT_GPIO(g) *(gpio+((g)/10)) |=  (1<<(((g)%10)*3))
#define SET_GPIO_ALT(g,a) *(gpio+(((g)/10))) |= (((a)<=3?(a)+4:(a)==4?3:2)<<(((g)%10)*3))

#define GPIO_SET *(gpio+7)  // sets   bits which are 1 ignores bits which are 0
#define GPIO_CLR *(gpio+10) // clears bits which are 1 ignores bits which are 0

void setup_io();

int main(int argc, char **argv)
  int g,rep;

  // Set up gpi pointer for direct register access

  // Switch GPIO 7..11 to output mode

  * You are about to change the GPIO settings of your computer.          *
  * Mess this up and it will stop working!                               *
  * It might be a good idea to 'sync' before running this program        *
  * so at least you still have your code changes written to the SD-card! *

  // Set GPIO pins 7-11 to output
  for (g=7; g<=11; g++)
    INP_GPIO(g); // must use INP_GPIO before we can use OUT_GPIO

  for (rep=0; rep<10; rep++)
     for (g=7; g<=11; g++)
       GPIO_SET = 1<<g;
     for (g=7; g<=11; g++)
       GPIO_CLR = 1<<g;

  return 0;

} // main

// Set up a memory regions to access GPIO
void setup_io()
   /* open /dev/mem */
   if ((mem_fd = open("/dev/mem", O_RDWR|O_SYNC) ) < 0) {
      printf("can't open /dev/mem \n");

   /* mmap GPIO */
   gpio_map = mmap(
      NULL,             //Any adddress in our space will do
      BLOCK_SIZE,       //Map length
      PROT_READ|PROT_WRITE,// Enable reading & writting to mapped memory
      MAP_SHARED,       //Shared with other processes
      mem_fd,           //File to map
      GPIO_BASE         //Offset to GPIO peripheral

   close(mem_fd); //No need to keep mem_fd open after mmap

   if (gpio_map == MAP_FAILED) {
      printf("mmap error %d\n", (int)gpio_map);//errno also set!

   // Always use volatile pointer!
   gpio = (volatile unsigned *)gpio_map;

} // setup_io

GPIO Pull Up/Pull Down Register Example
   // enable pull-up on GPIO24&25
   GPIO_PULL = 2;
   // clock on GPIO 24 & 25 (bit 24 & 25 set)
   GPIO_PULLCLK0 = 0x03000000;
   GPIO_PULL = 0;

C + wiringPi

Get and install wiringPi:

Save this, and compile with:

  gcc -o blink blink.c -lwiringPi

and run with:

  sudo ./blink
 * blink.c:
 *      blinks the first LED
 *      Gordon Henderson,

#include <stdio.h>
#include <wiringPi.h>

int main (void)
  printf ("Raspberry Pi blink\n") ;

  if (wiringPiSetup () == -1)
    return 1 ;

  pinMode (0, OUTPUT) ;         // aka BCM_GPIO pin 17

  for (;;)
    digitalWrite (0, 1) ;       // On
    delay (500) ;               // mS
    digitalWrite (0, 0) ;       // Off
    delay (500) ;
  return 0 ;


This must be done as root. To change to the root user:

sudo -i

You must also get and install the bcm2835 library, which supports GPIO and SPI interfaces. Details and downloads from

// blink.c
// Example program for bcm2835 library
// Blinks a pin on an off every 0.5 secs
// After installing bcm2835, you can build this 
// with something like:
// gcc -o blink -l rt blink.c -l bcm2835
// sudo ./blink
// Or you can test it before installing with:
// gcc -o blink -l rt -I ../../src ../../src/bcm2835.c blink.c
// sudo ./blink
// Author: Mike McCauley (
// Copyright (C) 2011 Mike McCauley
// $Id: RF22.h,v 1.21 2012/05/30 01:51:25 mikem Exp $

#include <bcm2835.h>

// Blinks on RPi pin GPIO 11
#define PIN RPI_GPIO_P1_11

int main(int argc, char **argv)
    // If you call this, it will not actually access the GPIO
    // Use for testing
//    bcm2835_set_debug(1);

    if (!bcm2835_init())
	return 1;

    // Set the pin to be an output
    bcm2835_gpio_fsel(PIN, BCM2835_GPIO_FSEL_OUTP);

    // Blink
    while (1)
	// Turn it on
	bcm2835_gpio_write(PIN, HIGH);
	// wait a bit
	// turn it off
	bcm2835_gpio_write(PIN, LOW);
	// wait a bit

    return 0;


RaspberryPiDotNet library is available at The library includes a GPIOFile and GPIOMem class. The GPIOMem requires compiling Mike McCauley's bcm2835 library above in to a shared object.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using RaspberryPiDotNet;
using System.Threading;

namespace RaspPi
    class Program
        static void Main(string[] args)
            // Access the GPIO pin using a static method
            GPIOFile.Write(GPIO.GPIOPins.GPIO00, true);

            // Create a new GPIO object
            GPIOMem gpio = new GPIOMem(GPIO.GPIOPins.GPIO01);


This example uses the WiringPi Ruby Gem: which you can install on your Pi with "gem install wiringpi"

MY_PIN = 1

require 'wiringpi'
io =

Alternatively the Pi Piper Gem ( allows for event driven programming:

require 'pi_piper'
include PiPiper

watch :pin => 23 do
  puts "Pin changed from #{last_value} to #{value}"



This must be done as root. To change to the root user:

sudo su -

Supports GPIO and SPI interfaces. You must also get and install the bcm2835 library. Details and downloads from You must then get and install the Device::BCM2835 perl library from CPAN

use Device::BCM2835;
use strict;

# call set_debug(1) to do a non-destructive test on non-RPi hardware
 || die "Could not init library";

# Blink pin 11:
# Set RPi pin 11 to be an output

while (1)
    # Turn it on
    Device::BCM2835::gpio_write(&Device::BCM2835::RPI_GPIO_P1_11, 1);
    Device::BCM2835::delay(500); # Milliseconds
    # Turn it off
    Device::BCM2835::gpio_write(&Device::BCM2835::RPI_GPIO_P1_11, 0);
    Device::BCM2835::delay(500); # Milliseconds


The GPIO Python module is available at Any Python script that controls GPIO must be run as root.

import RPi.GPIO as GPIO

# Set up the GPIO channels - one input and one output
GPIO.setup(11, GPIO.IN)
GPIO.setup(12, GPIO.OUT)

# Input from pin 11
input_value = GPIO.input(11)

# Output to pin 12
GPIO.output(12, True)

# The same script as above but using BCM GPIO 00..nn numbers
GPIO.setup(17, GPIO.IN)
GPIO.setup(18, GPIO.OUT)
input_value = GPIO.input(17)
GPIO.output(18, True)

Also available is RPIO at RPIO extends RPi.GPIO with PWM, GPIO interrups, TCP socket interrupts, command line tools and more. Uniquely (as of 14 March 2013) RPIO offers a debounce_timeout_ms option for use with swiches and push buttons connected to GPIO pins..


Java using the Pi4J Library

This uses the Java library available at (Any Java application that controls GPIO must be run as root.)

Please note that the Pi4J library uses the WiringPi GPIO pin numbering scheme [21] [22]. Please see the usage documentation for more details:

public static void main(String[] args) {

    // create gpio controller
    GpioController gpio = GpioFactory.getInstance();
    // provision gpio pin #01 as an output pin and turn off
    GpioPinDigitalOutput outputPin = gpio.provisionDigitalOutputPin(RaspiPin.GPIO_01, "MyLED", PinState.LOW);
    // turn output to LOW/OFF state

    // turn output to HIGH/ON state

    // provision gpio pin #02 as an input pin with its internal pull down resistor enabled
    GpioPinDigitalInput inputPin = gpio.provisionDigitalInputPin(RaspiPin.GPIO_02, "MyButton", PinPullResistance.PULL_DOWN);

    // get input state from pin 2
    boolean input_value = inputPin.isHigh();

More complete and detailed examples are included on the Pi4J website at

The Pi4J library includes support for:

  • GPIO Control
  • GPIO Listeners
  • Serial Communication
  • I2C Communication
  • SPI Communication


This uses the Java library available at It does not depend on (or use) the wiringPi driver, but uses the same numbering scheme. Instead it uses the default driver under /sys/class/gpio that ships with the distro, so it works out of the box. Any Java application that controls GPIO must be run as root.

public static void main(String[] args) {
  // reading from an in pin
  InPin button = new InPin(8);
  boolean isButtonPressed = button.getValue();

  // writing to an out pin
  OutPin led = new Outpin(0);

Java Webapp GPIO web control via http

This uses the Java Webapp available at You can control your GPIO over the internet. Any Java application that controls GPIO must be run as root.

host:~ sb$ curl 'http://raspberrypi:8080/handle?g0=1&g1=0'

Bash shell script, using sysfs, part of the raspbian operating system

The export and unexport of pins must be done as root. To change to the root user see below: To change back, the word exit must be entered.

sudo -i

Export creates a new folder for the exported pin, and creates files for each of its control functions (i.e. active_low, direction, edge, power, subsystem, uevent, and value). Upon creation, the control files can be read by all users (not just root), but can only be written to by user root, the file's owner. Nevertheless, once created, it is possible to allow users other than root, to also write inputs to the control files, by changing the ownership or permissions of these files. Changes to the file's ownership or permissions must initially be done as root, as their owner and group is set to root upon creation. Typically you might change the owner to be the (non root) user controlling the GPIO, or you might add write permission, and change the group ownership to one of which the user controlling the GPIO is a member. By such means, using only packages provided in the recommended rasbian distribution, it is possible for Python CGI scripts, which are typically run as user nobody, to be used for control of the GPIO over the internet from a browser at a remote location.


# GPIO numbers should be from this list
# 0, 1, 4, 7, 8, 9, 10, 11, 14, 15, 17, 18, 21, 22, 23, 24, 25

# Note that the GPIO numbers that you program here refer to the pins
# of the BCM2835 and *not* the numbers on the pin header. 
# So, if you want to activate GPIO7 on the header you should be 
# using GPIO4 in this script. Likewise if you want to activate GPIO0
# on the header you should be using GPIO17 here.

# Set up GPIO 4 and set to output
echo "4" > /sys/class/gpio/export
echo "out" > /sys/class/gpio/gpio4/direction

# Set up GPIO 7 and set to input
echo "7" > /sys/class/gpio/export
echo "in" > /sys/class/gpio/gpio7/direction

# Write output
echo "1" > /sys/class/gpio/gpio4/value

# Read from input
cat /sys/class/gpio/gpio7/value 

# Clean up
echo "4" > /sys/class/gpio/unexport
echo "7" > /sys/class/gpio/unexport

Shell script - take 2

You need the wiringPi library from Once installed, there is a new command gpio which can be used as a non-root user to control the GPIO pins.

The man page
man gpio
has full details, but briefly:
  gpio -g mode 17 out
  gpio -g mode 18 pwm

  gpio -g write 17 1
  gpio -g pwm 18 512

The -g flag tells the gpio program to use the BCM GPIO pin numbering scheme (otherwise it will use the wiringPi numbering scheme by default).

The gpio command can also control the internal pull-up and pull-down resistors:

  gpio -g mode 17 up

This sets the pull-up resistor - however any change of mode, even setting a pin that's already set as an input to an input will remove the pull-up/pull-down resistors, so they may need to be reset.

Additionally, it can export/un-export the GPIO devices for use by other non-root programms - e.g. Python scripts. (Although you may need to drop the calls to GPIO.Setup() in the Python scripts, and do the setup separately in a little shell script, or call the gpio program from inside Python).

  gpio export 17 out
  gpio export 18 in

These exports GPIO-17 and sets it to output, and exports GPIO-18 and sets it to input.

And when done:

  gpio unexport 17

The export/unexport commands always use the BCM GPIO pin numbers regardless of the presence of the -g flag or not.

If you want to use the internal pull-up/down's with the /sys/class/gpio mechanisms, then you can set them after exporting them. So:

  gpio -g export 4 in
  gpio -g mode 4 up

You can then use GPIO-4 as an input in your Python, Shell, Java, etc. programs without the use of an external resistor to pull the pin high. (If that's what you were after - for example, a simple push button switch taking the pin to ground.)

A fully working example of a shell script using the GPIO pins can be found at

Lazarus / Free Pascal

A simple app for controlling GPIO pin 17 with Lazarus

The GPIO pins are accessible from Lazarus without any third-party software. This is performed by means of the BaseUnix unit that is part of every distribution of Lazarus and Free Pascal or by invoking Unix shell commands with fpsystem. The following example uses GPIO pin 17 as output port. It is assumed that you created a form with a TToggleBox named GPIO17ToggleBox and for logging purposes a TMemo with name LogMemo (optional). The program has to be executed with root privileges.

Unit for controlling the GPIO port:

unit Unit1;

{Demo application for GPIO on Raspberry Pi}
{Inspired by the Python input/output demo application by Gareth Halfacree}
{written for the Raspberry Pi User Guide, ISBN 978-1-118-46446-5}

{$mode objfpc}{$H+}


  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls,
  Unix, BaseUnix;


  { TForm1 }

  TForm1 = class(TForm)
    LogMemo: TMemo;
    GPIO17ToggleBox: TToggleBox;
    procedure FormActivate(Sender: TObject);
    procedure FormClose(Sender: TObject; var CloseAction: TCloseAction);
    procedure GPIO17ToggleBoxChange(Sender: TObject);
    { private declarations }
    { public declarations }

  PIN_17: PChar = '17';
  PIN_ON: PChar = '1';
  PIN_OFF: PChar = '0';
  OUT_DIRECTION: PChar = 'out';

  Form1: TForm1;
  gReturnCode: longint; {stores the result of the IO operation}


{$R *.lfm}

{ TForm1 }

procedure TForm1.FormActivate(Sender: TObject);
  fileDesc: integer;
  { Prepare SoC pin 17 (pin 11 on GPIO port) for access: }
    fileDesc := fpopen('/sys/class/gpio/export', O_WrOnly);
    gReturnCode := fpwrite(fileDesc, PIN_17[0], 2);
    gReturnCode := fpclose(fileDesc);
  { Set SoC pin 17 as output: }
    fileDesc := fpopen('/sys/class/gpio/gpio17/direction', O_WrOnly);
    gReturnCode := fpwrite(fileDesc, OUT_DIRECTION[0], 3);
    gReturnCode := fpclose(fileDesc);

procedure TForm1.FormClose(Sender: TObject; var CloseAction: TCloseAction);
  fileDesc: integer;
  { Free SoC pin 17: }
    fileDesc := fpopen('/sys/class/gpio/unexport', O_WrOnly);
    gReturnCode := fpwrite(fileDesc, PIN_17[0], 2);
    gReturnCode := fpclose(fileDesc);

procedure TForm1.GPIO17ToggleBoxChange(Sender: TObject);
  fileDesc: integer;
  if GPIO17ToggleBox.Checked then
    { Swith SoC pin 17 on: }
      fileDesc := fpopen('/sys/class/gpio/gpio17/value', O_WrOnly);
      gReturnCode := fpwrite(fileDesc, PIN_ON[0], 1);
      gReturnCode := fpclose(fileDesc);
    { Switch SoC pin 17 off: }
      fileDesc := fpopen('/sys/class/gpio/gpio17/value', O_WrOnly);
      gReturnCode := fpwrite(fileDesc, PIN_OFF[0], 1);
      gReturnCode := fpclose(fileDesc);


Main program:

program io_test;

{$mode objfpc}{$H+}

  {$IFDEF UNIX}{$IFDEF UseCThreads}
  Interfaces, // this includes the LCL widgetset
  Forms, Unit1
  { you can add units after this };

{$R *.res}

  Application.CreateForm(TForm1, Form1);

An alternative way to access the GPIO port with Lazarus / Free Pascal is by using Lazarus wrapper unit for Gordon Henderson's wiringPi C library.


BASIC - Return to BASIC

RTB or Return to Basic can be found here:

It's a new BASIC featuring modern looping constructs, switch statements, named procedures and functions as well as graphics (caresian and turtle), file handling and more. It also supports the Pi's on-board GPIO without needing to be run as root. (You don't need any special setup routines either)

Sample blink program:

// blink.rtb:
//    Blink program in Return to Basic
//    Gordon Henderson,
PinMode (0, 1) // Output
  DigitalWrite (0, 1) // Pin 0 ON
  WAIT (0.5) // 0.5 seconds
  DigitalWrite (0, 0)
  WAIT (0.5)


Bywater BASIC Interpreter

The Bywater BASIC Interpreter (bwBASIC) implements a large superset of the ANSI Standard for Minimal BASIC (X3.60-1978) and a significant subset of the ANSI Standard for Full BASIC (X3.113-1987) in C. It also offers shell programming facilities as an extension of BASIC. bwBASIC seeks to be as portable as possible. You can download it at.

BASIC programming of the I/O

Setting up a GPIO pin to be used for inputs or for outputs.

We cannot load the control words directly into the 32 bit ARM registers with 32 bit addresses, as bwBASIC has no POKE and PEEK commands and other versions of BASIC (as far as I know) only handle 8 bit registers with 16 bit addresses with these commands. So we need to export the GPIO pins, so that they exist in a file structure which we can access from basic with the OPEN command.(ref 2)

We need to do this in Linux root. We need to run BASIC in the root too. First we go to the root, then we load bwbasic into root.

sudo -1
sudo bwbasic

REM Now to export the no4 GPIO pin for example, using a Shell command.

echo “4” >  /sys/class/gpio/export

Whilst bwbasic can accommodate shell commands, and we can store a set of these commands (eg. to export a number of GPIO pins at the outset) as numbered statements in a file that can be loaded with the basic command LOAD “filename” and RUN (ref 2), the shell commands have to run as a separate file, as they cannot be run from within, as part of a basic programme.

Now we can access the file containing the pin direction setting from BASIC

We can set GPIO pin 4 to input or to output by OPENing its pin direction file for output and writing “in” or “out” with a PRINT# command. (ref 2 )

10 OPEN ”O”,#1, “/sys/devices/virtual/gpio/gpio4/direction”,2
20 PRINT #1,”out”
30 CLOSE #1

REM closes the open direction file, whereupon the system performs the action of setting the direction to “out”. NB the system only carries out the action as the file is closed.(ref 3)

We are now able to control the output of the gpio 4 pin from BASIC

We can set the GPIO 4 pin to 1 or to 0 by OPENing its pin value file for output and writing “1” or “0” with a PRINT# command.

40 OPEN ”O”,#4, “/sys/devices/virtual/gpio/gpio4/value”,1
50 PRINT #4,”160 CLOSE #4

REM turns on the output of GPIO pin 4.

REM similarly we can turn off the output of GPIO pin 4. OPEN ”O”,#4, “/sys/devices/virtual/gpio/gpio4/value”,1 PRINT #4,”0” CLOSE #4.

Example of an (unstructured) BASIC programme

To read the state of a switch and control the power to two LEDs connected to GPIO pins 8,7 and 4 respectively.

Programme to set 2 pins as outputs and 1 pin as input and to read the input turning on two different combinations of the two outputs (ie output 0,1 or 1,0) depending on the state of the input (1 or 0).

sudo –i
sudo bwbasic
LOAD “export.bas”
REM a set of Shell statements to export the three GPIO pins.
10 echo “4> /sys/class/gpio/export
20 echo “7> /sys/class/gpio/export
30 echo “8> /sys/class/gpio/export

NEW REM clears the export.bas programme from memory

LOAD “demo1.bas”
10 OPEN ”O”,#1, “/sys/devices/virtual/gpio/gpio4/direction”,2 
20 OPEN ”O”,#2, “/sys/devices/virtual/gpio/gpio7/direction”,2 
30 OPEN ”O”,#3, “/sys/devices/virtual/gpio/gpio8/direction”,2
REM opens the three pin direction files
40 PRINT #1, “out”
50 PRINT #2, “out”
60 PRINT #3, “in”
REM sets GPIO pins 4 and 7 as outputs and GPIO pin 8 as input.
70 CLOSE #1
80 CLOSE #2
90 CLOSE #3
REM closes all open files, allowing the system to perform the direction settings.
100 OPEN ”I”,#8, “/sys/devices/virtual/gpio/gpio8/value”,1
REM opens the GPIO pin 8 value file
110 INPUT #8,x
REM reads the value of the input pin and stores the value in numerical variable x
120 CLOSE #8
REM closes the open file, allowing the system to read the value of the input pin and store the value in numerical variable x.
130 OPEN “O”,#1, “/sys/devices/virtual/gpio/gpio4/value”,1
140 OPEN “O”,#2, “/sys/devices/virtual/gpio/gpio7/value”,1
REM opens the GPIO pins 4 and value files ready for outputting 1s and 0s.
150 IF x<1 THEN GOTO 160 ELSE GOTO 190
REM tests the state of the switch (1 or0) and directs the program to generate the appropriate outputs
160 PRINT #1,”1170 PRINT #2,”0180 GOTO 210
190  PRINT#1,"0"
200 PRINT #2,"1"
210 CLOSE #1
220 CLOSE #2
REM Closes the files and allows the outputs to light the LED
230 END.

When all is done, we should unexport the GPIO pins, to leave the R-Pi as we found it.(Ref 1.)

LOAD “unexport.bas”
REM a set of Shell statements to unexport the three GPIO pins.
10 echo “4> /sys/class/gpio/unexport
20 echo “7> /sys/class/gpio/unexport
30 echo “8> /sys/class/gpio/unexport

A simple circuit to provide the switched input and the two LED outputs.

Ancient Mariner. Dec. 2012


1. This paper RPi Low-level peripherals.

2. Ed Beynon.

3. Arthur Kaletzky. Private communication. 25/10/2012

4. bwbasic manual.

For the two original documents this example has been copied from see:



On the production board[23], the Raspberry Pi Foundation design brings out the MIPI CSI-2 (Camera Serial Interface[24]) to a 15-way flat flex connector S5, between the Ethernet and HDMI connectors. A compatible camera has been discussed as working in tests and is planned for release at a later date.[25].


On the production board, the Raspberry Pi Foundation design brings out the DSI (Display Serial Interface[26]) to a 15-way flat flex connector labelled S2, next to Raspberry Pi logo. It has two data lanes and a clock lane, to drive a possible future LCD screen device. Some smart phone screens use DSI[27].


HDMI-CEC (Consumer Electronics Control for HDMI) is supported by hardware but some driver work will be needed and currently isn't exposed into Linux userland. Eben notes that he has seen CEC demos on the Broadcom SoC they are using.

libCEC with Raspberry Pi support has been included in OpenELEC and will be included in Raspbmc RC4.[28]

For more information about HDMI-CEC and what you could do with it on the Raspberry Pi please see the CEC (Consumer Electronics Control) over HDMI article.


  12. Forum:Sad about removal of I2S. Why was this change made?