RPi Low-level peripherals

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Revision as of 13:04, 14 June 2012 by Grumpy Mike (talk | contribs) (Removed the phrase "On the production board," because this is a function of the chip not the board.)
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Hardware & Peripherals:

Hardware - detailed information about the Raspberry Pi boards.

Hardware History - guide to the Raspberry Pi models.

Low-level Peripherals - using the GPIO and other connectors.

Expansion Boards - GPIO plug-in boards providing additional functionality.

Screens - attaching a screen to the Raspberry Pi.

Cases - lots of nice cases to protect the Raspberry Pi.

Other Peripherals - all sorts of peripherals used with the Raspberry Pi.


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" but 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 Rpi GPIO's, 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 Rpi allows peripherals and expansion boards (such as the upcoming Rpi Gertboard) to access the CPU by exposing the in and outputs.

For further general information about GPIOs, see:the wikipedia article.

The production Raspberry Pi board has a 26-pin 2.54mm (100mil)[1] expansion header, arranged in a 2x13 strip. They provide 8 GPIO pins plus access to I2C, SPI, UART), as well as +3V3, +5V and GND supply lines. Pin one is the pin in the first column and on the bottom row. [2]

GPIO voltage levels are 3v3 and are not 5v 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]

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 115200bps.

Header Pinout, top row:

Pin Name Pin Function Hardware Notes General Notes Alternative Functions
P1-02 5V0
P1-04 DNC
P1-06 GND
P1-12 BCM2835 GPIO18 GPIO18-ALT5 is PWM0
P1-14 DNC
P1-16 BCM2835 GPIO23 GPIO23-ALT4 is ARM_RTCK
P1-18 BCM2835 GPIO24 GPIO24-ALT4 is ARM_TDO
P1-20 DNC
P1-22 BCM2835 GPIO25 GPIO25-ALT4 is ARM_TCK
P1-24 SPI_CE0_N (BCM2835 GPIO8-ALT0) GPIO08
P1-26 SPI_CE1_N (BCM2835 GPIO7-ALT0) GPIO07

Header Pinout, bottom row:

Pin Name Pin Function Hardware Notes General Notes Alternative Functions
P1-01 3V3
P1-03 I2C0_SDA 1K8 pull up resistor (BCM2835 GPIO0-ALT0) GPIO00
P1-05 I2C0_SCL 1K8 pull up resistor (BCM2835 GPIO1-ALT0) GPIO01
P1-09 DNC
P1-13 BCM2835 GPIO21 GPIO21-ALT5 is GPCLK1
P1-15 BCM2835 GPIO22 GPIO22-ALT4 is ARM_TRST
P1-17 DNC
P1-25 DNC

Colour legend
Ground, 0V
Do not connect

KiCad symbol: File:Conn-raspberry.lib


Pin 3 (SDA0) and Pin 5 (SCL0) are preset to be used as I2C interface. So there are 1K8 pulls up resistors on the board for these pins.[8]

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.[9] 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 (hardware mod may be required[10]) or PCM interface.[11] 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.

A note about GPIO vs the schematic

You may notice that the GPIO connector as documented in the schematic does NOT match with what is on this wiki page. Do NOT update this wiki page. The pins which are marked as DNC should not be used in order to be compatible with possible future designs. The plan is that if a new design comes along and if the layout permits it we will connect additional GPIO pins to those DNC pins. (Gert's first vote is for GPIO 19 and 20, That gives us the second PWM, second SPI, I2S/PCM, slave I2C and slave SPI)

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

Maximum permitted current draw from the 3v3 pin is 50mA.

Maximum permitted current draw from the 5v pin is the USB input current (usually 1A) minus any current draw from the rest of the board.[12]

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

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

As noted above, GPIO00 and 01 (SDA0 and SCL0) have 1K8 pull-up resistors to 3v3.

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 3v3.

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. 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.

Driver support

The Foundation will not include a GPIO driver in the initial release, standard linux GPIO drivers should work with minimal modification.[13]

The community implemented SPI and I2C drivers [14], which will be integrated with the new Linux pinctrl concept in a later version of the kernel. A first compiled version as Linux modules is available to install on the 19/04/2012 Debian image, including 1-wire support[15]. 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[16] wrote a (mostly) Arduino compatible/style WiringPi library in C for controlling the GPIO pins.

Code examples

GPIO Driving Example (C)

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

// Access from ARM Running Linux

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

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <dirent.h>
#include <fcntl.h>
#include <assert.h>
#include <sys/mman.h>
#include <sys/types.h>
#include <sys/stat.h>

#include <unistd.h>

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

int  mem_fd;
char *gpio_mem, *gpio_map;
char *spi0_mem, *spi0_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");
      exit (-1);

   /* mmap GPIO */

   // Allocate MAP block
   if ((gpio_mem = malloc(BLOCK_SIZE + (PAGE_SIZE-1))) == NULL) {
      printf("allocation error \n");
      exit (-1);

   // Make sure pointer is on 4K boundary
   if ((unsigned long)gpio_mem % PAGE_SIZE)
     gpio_mem += PAGE_SIZE - ((unsigned long)gpio_mem % PAGE_SIZE);

   // Now map it
   gpio_map = (unsigned char *)mmap(

   if ((long)gpio_map < 0) {
      printf("mmap error %d\n", (int)gpio_map);
      exit (-1);

   // 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;

GPIO Driving Example (Python)

This uses the Python module available at http://pypi.python.org/pypi/RPi.GPIO 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)

GPIO Driving Example (Shell script)

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

sudo su -

# 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


On the production board[17], we bring out the MIPI CSI-2 (Camera Serial Interface[18]) to a 15-way flat flex connector S5, between the Ethernet and HDMI connectors. A compatible camera will be made available in due time[19].

is Sony sub-LVDS same as MIPI CSI-2? Sony IMX020 5Mbip module is available for $5-7 (SE K850i replacement camera).

Looks like Nokia N95 uses CSI-2 5Mpix camera module with autofocus. ~$15 replacement part.


On the production board, we bring out the DSI (Display Serial Interface[20]) 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[21].


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.

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.


  1. http://www.raspberrypi.org/forum/features-and-requests/easy-gpio-hardware-software/page-3/#p31907
  2. http://www.raspberrypi.org/archives/384
  3. http://www.raspberrypi.org/archives/384
  4. http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
  5. http://www.raspberrypi.org/archives/384#comment-5217
  6. http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
  7. http://www.raspberrypi.org/forum/projects-and-collaboration-general/gpio-header-pinout-clarification/page-2
  8. http://www.raspberrypi.org/forum/features-and-requests/easy-gpio-hardware-software/page-6/#p56480
  9. http://www.raspberrypi.org/forum?mingleforumaction=viewtopic&t=1288.1
  10. Forum:Sad about removal of I2S. Why was this change made?
  11. http://www.raspberrypi.org/forum?mingleforumaction=viewtopic&t=1288.2
  12. http://www.raspberrypi.org/forum?mingleforumaction=viewtopic&t=1536#postid-21841
  13. http://www.raspberrypi.org/forum?mingleforumaction=viewtopic&t=1278.0
  14. http://www.bootc.net/archives/2012/05/19/i2c-and-the-raspberry-pi/
  15. http://www.raspberrypi.org/phpBB3/viewtopic.php?p=86172#p86172
  16. http://www.raspberrypi.org/forum/general-discussion/wiring-for-the-raspberry-pis-gpio
  17. http://www.raspberrypi.org/wp-content/uploads/2012/04/Raspberry-Pi-Schematics-R1.0.pdf
  18. http://www.mipi.org/specifications/camera-interface
  19. http://www.raspberrypi.org/forum/projects-and-collaboration-general/complex-camera-peripherials#p72602
  20. http://www.mipi.org/specifications/display-interface
  21. http://en.wikipedia.org/wiki/Display_Serial_Interface