Class that takes care of GPIO (General-purpose input/output).

GPIO is a generic pin on an integrated circuit whose behavior, including whether it is an input or output pin, can be controlled by the user at run time.

GPIO pins have no special purpose defined, and go unused by default. The idea is that sometimes the system integrator building a full system that uses the chip might find it useful to have a handful of additional digital control lines, and having these available from the chip can save the hassle of having to arrange additional circuitry to provide them.

This class is divided into two files, header (GPIO.h) and source code (GPIO.cpp).


Link to the code: GPIO.h

Linux::LinuxGPIOclass defines the methods inherited from the AP_HAL::GPIO abstract class.Linux::LinuxDigitalSourceclass is also defined in this file and inherit fromAP_HAL::DigitalSource`.

For more clarity in the code: The GPIO files define C preproccesor Hexadecimal addresses and BeagleBone Black GPIO mappings in linux .

#ifndef __AP_HAL_LINUX_GPIO_H__
#define __AP_HAL_LINUX_GPIO_H__

#include <AP_HAL_Linux.h>

#define SYSFS_GPIO_DIR "/sys/class/gpio"

#define GPIO0_BASE 0x44E07000
#define GPIO1_BASE 0x4804C000
#define GPIO2_BASE 0x481AC000
#define GPIO3_BASE 0x481AE000

#define GPIO_SIZE  0x00000FFF

// OE: 0 is output, 1 is input
#define GPIO_OE    0x14d
#define GPIO_IN    0x14e
#define GPIO_OUT   0x14f

#define LED_AMBER       117
#define LED_BLUE        48
#define LED_SAFETY      61
#define SAFETY_SWITCH   116
#define LOW             0
#define HIGH            1

// disable GPIO

// BeagleBone Black GPIO mappings
#define BBB_USR0 53
#define BBB_USR1 54
#define BBB_USR2 55
#define BBB_USR3 56
#define BBB_P8_3 38
#define BBB_P8_4 39
#define BBB_P8_5 34
#define BBB_P9_42 7
  • Following the statement:#define identifier replacement, when the preprocessor encounters this directive, it replaces any occurrence of identifier in the rest of the code by replacement.This is what is done for the identifiers in the code above.
  • It also contains a or statement (||) in order to enable and disable the GPIO, depending on the specified board.

class Linux::LinuxGPIO : public AP_HAL::GPIO {
    struct GPIO {
        volatile uint32_t *base;
        volatile uint32_t *oe;
        volatile uint32_t *in;
        volatile uint32_t *out;
     } gpio_bank[LINUX_GPIO_NUM_BANKS];

    void    init();
    void    pinMode(uint8_t pin, uint8_t output);
    int8_t  analogPinToDigitalPin(uint8_t pin);
    uint8_t read(uint8_t pin);
    void    write(uint8_t pin, uint8_t value);
    void    toggle(uint8_t pin);

    /* Alternative interface: */
    AP_HAL::DigitalSource* channel(uint16_t n);

    /* Interrupt interface: */
    bool    attach_interrupt(uint8_t interrupt_num, AP_HAL::Proc p,
            uint8_t mode);

    /* return true if USB cable is connected */
    bool    usb_connected(void);
  • The class LinuxGPIO is defined. It inherits from AP_HAL::GPIO.
  • The volatile keyword is a type qualifier used to declare that an object can be modified in the program by something such as the operating system, the hardware, or a concurrently executing thread.Here are defined some volatile pointers.
  • It also defines a GPIO struct called gpio_bank[], as the name suggests, is used to group pins in banks.You can access a bank by index.
  • In the public fields: there is a init() method definition, read() and write() methods... (these functions are implemented in GPIO.cpp)
  • The methods defined inside GPIO.h in class Linux::LinuxGPIO like LinuxGPIO::pinMode(),LinuxGPIO::read(),LinuxGPIO::write() and so on, are implemented for handle the GPIO pin banks of the board.
class Linux::LinuxDigitalSource : public AP_HAL::DigitalSource {
    LinuxDigitalSource(uint8_t v);
    void    mode(uint8_t output);
    uint8_t read();
    void    write(uint8_t value);
    void    toggle();
    uint8_t _v;


#endif // __AP_HAL_LINUX_GPIO_H__
  • Defines the LinuxDigitalSourcethat inherits from DigitalSource, which is part of AP_HAL::GPIO
  • The methods defined inside gpio.h in Linux :: LinuxDigitalSource class, have been implemented here to manage digital external sources.
  • Some methods are defined here, for later implementation in GPIO.cpp.


GPIO.cpp implements the methods defined in GPIO.h.

The most remarkable is the init() method which enables all GPIO banks, open the export directory /sys/class/gpio/export, and map the GPIO banks in /dev/mem.

#include <AP_HAL.h>


#include "GPIO.h"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>
#include <poll.h>
#include <sys/mman.h>
#include <sys/stat.h>
  • In this piece of code AP_HAL.h and GPIO.h are included and the board is defined.

Some functions and libraries are included:

  • Deal with Input and Output operations: manage files,read and write... - (stdio.h)
  • This header defines several general purpose functions, including dynamic memory management, random number generation, communication with the environment, integer arithmetics, searching, sorting and converting.- (stdlib.h)
  • The string.h header file defines several functions to manipulate C strings and arrays. - (string.h)
  • C Header that defines the following macro: errno->Last error number (macro ); plus at least three additional macro constants: EDOM, ERANGE and EILSEQ.errnodelas with errors (see errno for more details). - (errno.h)
  • This header defines miscellaneous symbolic constants and types, and declares miscellaneous functions.It is provided by POSIX(Portable Operating System Interface-calls to the OS)-compatible systems.- (unistd.h)
  • The fcntl.h header shall define some requests and arguments for use by the functions fcntl() and open(). - (fcntl.h)
  • The poll.h header defines the pollfd structure that includes at least the following member: int fd(the following descriptor being polled), short int events (the input event flags) and short int revents ( the output event flags) - (poll.h)
  • The header sys/mman.h defines protection options when Advisory Information, Memory Mapped Files, or Shared Memory Objects options are supported.- (sys/mman.h)
  • The sys/stat.h header defines the structure of the data returned by the functions fstat(),lstat(), and stat(). - (sys/stat.h)
using namespace Linux;

static const AP_HAL::HAL& hal = AP_HAL_BOARD_DRIVER;
  • Begins a namespace definition, takes the AP_HAL::HAL& hal value and call LinuxGPIO() funtion that manage pin banks.
void LinuxGPIO::init()
    int mem_fd;
    // Enable all GPIO banks
    // Without this, access to deactivated banks (i.e. those with no clock source set up) will (logically) fail with SIGBUS
    // Idea taken from!msg/beagleboard/OYFp4EXawiI/Mq6s3sg14HoJ

    uint8_t bank_enable[3] = { 5, 65, 105 };
    int export_fd = open("/sys/class/gpio/export", O_WRONLY);
    if (export_fd == -1) {
        hal.scheduler->panic("unable to open /sys/class/gpio/export");
    for (uint8_t i=0; i<3; i++) {
        dprintf(export_fd, "%u\n", (unsigned)bank_enable[i]);

    /* 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 */
    for (uint8_t i=0; i<LINUX_GPIO_NUM_BANKS; i++) {
        gpio_bank[i].base = (volatile unsigned *)mmap(0, GPIO_SIZE, PROT_READ|PROT_WRITE, MAP_SHARED, mem_fd, offsets[i]);
        if ((char *)gpio_bank[i].base == MAP_FAILED) {
            hal.scheduler->panic("unable to map GPIO bank");
        gpio_bank[i].oe = gpio_bank[i].base + GPIO_OE;
        gpio_bank[i].in = gpio_bank[i].base + GPIO_IN;
        gpio_bank[i].out = gpio_bank[i].base + GPIO_OUT;


Enable all GPIO banks(setting up the banks):

  • First, try to open the file /sys/class/gpio/export: if fail print a message, if not print the content.
  • Open /dev/mem.
  • off_t is a type used to pass offset to various file related functions. With this we enable GPIO banks, mapping them in /dev/mem.


void LinuxGPIO::pinMode(uint8_t pin, uint8_t output)
    uint8_t bank = pin/32;
    uint8_t bankpin = pin & 0x1F;
    if (bank >= LINUX_GPIO_NUM_BANKS) {
    if (output == HAL_GPIO_INPUT) {
        *gpio_bank[bank].oe |= (1U<<bankpin);
    } else {
        *gpio_bank[bank].oe &= ~(1U<<bankpin);

int8_t LinuxGPIO::analogPinToDigitalPin(uint8_t pin)
    return -1;
  • Check the bank numbers and the output. Note the use of 1U for the left shift, to make it unsigned and the allocation using binary asigments.
  • The return -1 is like a return boolean=False.
uint8_t LinuxGPIO::read(uint8_t pin) {

    uint8_t bank = pin/32;
    uint8_t bankpin = pin & 0x1F;
    if (bank >= LINUX_GPIO_NUM_BANKS) {
        return 0;
    return *gpio_bank[bank].in & (1U<<bankpin) ? HIGH : LOW;


void LinuxGPIO::write(uint8_t pin, uint8_t value)
    uint8_t bank = pin/32;
    uint8_t bankpin = pin & 0x1F;
    if (bank >= LINUX_GPIO_NUM_BANKS) {
    if (value == LOW) {
        *gpio_bank[bank].out &= ~(1U<<bankpin);
    } else {
        *gpio_bank[bank].out |= 1U<<bankpin;
  • Similarly to pinmode() acts read() and write().
void LinuxGPIO::toggle(uint8_t pin)
    write(pin, !read(pin));

/* Alternative interface: */
AP_HAL::DigitalSource* LinuxGPIO::channel(uint16_t n) {
    return new LinuxDigitalSource(n);

/* Interrupt interface: */
bool LinuxGPIO::attach_interrupt(uint8_t interrupt_num, AP_HAL::Proc p, uint8_t mode)
    return true;

bool LinuxGPIO::usb_connected(void)
    return false;

LinuxDigitalSource::LinuxDigitalSource(uint8_t v) :


void LinuxDigitalSource::mode(uint8_t output)
    hal.gpio->pinMode(_v, output);

uint8_t LinuxDigitalSource::read()
    return hal.gpio->read(_v);

void LinuxDigitalSource::write(uint8_t value)
    return hal.gpio->write(_v,value);

void LinuxDigitalSource::toggle()

  • Here the missing functions are implemented.

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