blink_mmap.c

/*
 * Copyright (c) 2013 Erlang Solutions Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

/*
 * File    : blink_mmap.c
 * Author  : Omer Kilic  <[email protected]>
 * Purpose : An example of using mmap'ed peripheral access on the Raspberry Pi.
 *
 * This example toggles the state of pin 17 in one second intervals. This change
 * can be observed by looking at the LED connected to Pin 17 on the demo board.
 *
 * Original idea from: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=33&t=8476
 */

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


// GPIO registers, as detailed in page 90 of the RPi "datasheet":
// http://www.raspberrypi.org/wp-content/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
#define	GPIO_BASE	0x20200000

// offsets
#define	GPIO_GPSEL1	1
#define	GPIO_GPSET0	7
#define	GPIO_GPCLR0	10

// globals
static volatile uint32_t *gpio;
int fd;


void
mem_open ()
{
	if ((fd = open ("/dev/mem", O_RDWR | O_SYNC) ) < 0) {
		printf("[fail] can't open /dev/mem: %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	} else {
		printf("[ok] /dev/mem opened\n");
	}
}

void
mem_map ()
{

	// Map a page of memory to access the GPIO peripheral at address 0x20200000
	// NULL = Delegate mmap to allocate memory (no malloc necessary)
	// 1 = length in bytes from offset
	// PROT_READ | PROT_WRITE = what we want to do to it
	// MAP_SHARED = allow other processes to access the mapped memory too 
	gpio = (uint32_t *)mmap(NULL, getpagesize(), PROT_READ|PROT_WRITE, MAP_SHARED, fd, GPIO_BASE);

	if (gpio == MAP_FAILED){
		printf("[fail] mmap error: %s\n", strerror(errno));
		exit(EXIT_FAILURE);
	} else {
		printf("[ok] mmap success\n");
	}

}

void
pin_out ()
{
	// Set GPIO17 as an output
	// Increment the pointer to 0x20200004 (GPFSEL1)
	// Set the value through a little bit twiddling where we only modify the bits 21-23 in the register
	*(gpio + GPIO_GPSEL1) = (*(gpio + GPIO_GPSEL1) & ~(7 << 21)) | (1 << 21);
}

void
pin_on ()
{
	// Set the pin high
	// Increment the pointer to 0x2020001C (GPSET0)
	*(gpio + GPIO_GPSET0) = 1 << 17;
	printf("pin on\n");
}

void
pin_off ()
{
	// Set the pin low
	// Increment the pointer to 0x20200028 (GPCLR0)
	*(gpio + GPIO_GPCLR0) = 1 << 17;
	printf("pin off\n");
}


int
main (int argc, char **argv)
{
	mem_open();

	mem_map();

	pin_out();

	for(;;){
		pin_on();
		sleep(1);

		pin_off();
		sleep(1);
	}

}