Memory Mapped GPIO

Created Wednesday 11 January 2017

// Memory Mapped GPIO example
// mmgpio.cpp
http:elinux.org/RPi_GPIO_Code_Samples
//
// How to access GPIO registers from C-code on the Raspberry-Pi
// Example program
// 15-January-2012
// Dom and Gert
// Revised: 15-Feb-2013

// Raspberry Pi 1, Raspberry Pi 2, or Raspberry Pi 3

//#define BCM2708_PERI_BASE 0x20000000 /* Pi 1 */
#define BCM2708_PERI_BASE 0x3F000000 /* Pi 2 & Pi 3 */

#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

#define GET_GPIO(g) (*(gpio+13)&(1<<g)) // 0 if LOW, (1<<g) if HIGH

#define GPIO_PULL *(gpio+37) // Pull up/pull down
#define GPIO_PULLCLK0 *(gpio+38) // Pull up/pull down clock

#define MY_GPIO 17

void setup_io();

void printButton(int g)
{
if (GET_GPIO(g)) // !=0 <-> bit is 1 <- port is HIGH=3.3V

printf("Button pressed!\n");

else // port is LOW=0V

printf("Button released!\n");

}

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! *
\************************************************************************/
// Jim's version - gpio 17 only
INP_GPIO(MY_GPIO); // must use INP_GPIO before we can use OUT_GPIO
OUT_GPIO(MY_GPIO);

for (rep=0; rep<10; rep++)
{

GPIO_SET = 1<<MY_GPIO;
sleep(1);
GPIO_CLR = 1<<MY_GPIO;
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 */
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!
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;
*/