RPi Low-level peripherals

From eLinux.org
Revision as of 10:29, 26 April 2012 by Meltwater (Talk | contribs) (General Purpose Input/Output (GPIO): - added note about GPIO-Pads-Control (thanks to Gert))

Jump to: navigation, search


Back to the Hub.


Hardware & Peripherals:

Hardware and Hardware History.

Low-level Peripherals and Expansion Boards.

Screens, Cases and Other Peripherals.


Introduction

[TODO]

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.

On the production board, all the UART, SPI and I2C pins can be reconfigured as GPIO pins, 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]

The availabile 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.

The source and sink ability is configurable 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.

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
P1-08 UART0_TXD (BCM2835 GPIO14-ALT0) GPIO14-ALT5 is UART1_TXD, GPIO14
P1-10 UART0_RXD (BCM2835 GPIO15-ALT0) GPIO15-ALT5 is UART1_RXD, GPIO15
P1-12 BCM2835 GPIO18 GPIO18-ALT5 is PWM0
P1-14 DNC
P1-16 BCM2835 GPIO23
P1-18 BCM2835 GPIO24
P1-20 DNC
P1-22 BCM2835 GPIO25
P1-24 SPI_CE0_N (BCM2835 GPIO8-ALT0) GPIO08
P1-26 SPI_CE1_N (BCM2835 GPIO7-ALT0) GPIO08


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
P1-07 GPCLK0 (BCM2835 GPIO4-ALT0) GPIO04-ALT5 is ARM_TDI, GPIO4
P1-09 DNC
P1-11 BCM2835 GPIO17 GPIO17_ALT3 is UART0_RTS
P1-13 BCM2835 GPIO21
P1-15 BCM2835 GPIO22
P1-17 DNC
P1-19 SPI_MOSI (BCM2835 GPIO10-ALT0) GPIO10
P1-21 SPI_MISO (BCM2835 GPIO09-ALT0) GPIO09
P1-23 SPI_SCLK (BCM2835 GPIO11-ALT0) GPIO11
P1-25 DNC


Colour legend
+5V
+3.3V
Do not connect
UART
GPIO
SPI
I2C

KiCad symbol: File:Conn-raspberry.lib

[7]


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 and I2S)

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 Foundation will not include an SPI driver in the initial release, we hope the community might write one.[14] The Foundation will not include an I2C driver in the initial release, we hope the community might provide one, standard linux I2C drivers should work with minimal modification.[15]

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
  setup_io();

  // 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
    OUT_GPIO(g);
  }

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

  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(
      (caddr_t)gpio_mem,
      BLOCK_SIZE,
      PROT_READ|PROT_WRITE,
      MAP_SHARED|MAP_FIXED,
      mem_fd,
      GPIO_BASE
   );

   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;
   short_wait();
   // clock on GPIO 24 & 25 (bit 24 & 25 set)
   GPIO_PULLCLK0 = 0x03000000;
   short_wait();
   GPIO_PULL = 0;
   GPIO_PULLCLK0 = 0;

GPIO Driving Example (Python)

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

  • Disclaimer: Untested !
import RPi.GPIO as GPIO

# set up the GPIO channels - channel 0 (input) and channel 1 (output)
GPIO.setup(0, GPIO.IN)
GPIO.setup(1, GPIO.OUT)

# input from channel 0 - note that input_value will be a boolean
input_value = GPIO.input(0)

# output True to channel 1.  NB you can use '1', 1 or anything that evaluates to True/False
GPIO.output(1, True)

GPIO Driving Example (Shell script)

  • Disclaimer: Untested !
#!/bin/sh

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

# 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

MIPI CSI-2

On the production board, we bring out the MIPI CSI-2 interface to a 15-way flat flex connector

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.

DSI

On the production board, we bring out the DSI interface to a 15-way flat flex connector.

CEC

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.

References

  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.raspberrypi.org/forum?mingleforumaction=viewtopic&t=1278.0
  15. http://www.raspberrypi.org/forum?mingleforumaction=viewtopic&t=1202