reset_loop.c

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>

#define BCM2708_PERI_BASE   0x20000000
#define GPIO_BASE           (BCM2708_PERI_BASE + 0x200000)

int  mem_fd;
void *gpio_map;
 
// I/O access
volatile unsigned *gpio;

#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

void setup_io();

int main(int argc, char *argv[])
{
  setup_io();
  INP_GPIO(26);
  OUT_GPIO(26);

  while (true) {

    GPIO_SET = 1<<26;
    usleep(150000);
    GPIO_CLR = 1<<26;

    sleep(1);
  }
  
  return EXIT_SUCCESS;
}

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
  );

  // no need to keep mem_fd open after mmap
  close(mem_fd);

  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;
}