Saturday, November 23, 2013

Control the on-board LED using C language

Last post show how to control Raspberry Pi on-board LED OK or ACT in command line. This post show the code to control the LED using C language, it have the same result of the exercise using Python.

Control the on-board LED using C language
Control the Raspberry Pi on-board LED using C language
#include <stdio.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <unistd.h>

#define BCM2708_PERI_BASE 0x20000000
#define GPIO_BASE   (BCM2708_PERI_BASE + 0x200000) /* GPIO controller */
#define LED_ACT    16

#define BLOCK_SIZE (4*1024)

int mem_fd;
void *gpio_map;
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
int init_io()
 /* open /dev/mem */
 if ((mem_fd = open("/dev/mem", O_RDWR|O_SYNC) ) < 0) {
  printf("can't open /dev/mem \n");
  printf("/dev/mem opened\n");
  /* 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 
  printf("/dev/mem closed\n");
  if (gpio_map == MAP_FAILED) {
   printf("mmap error %d\n", (int)gpio_map);//errno also set!
   printf("mmap Success.\n");
 // Always use volatile pointer!
   gpio = (volatile unsigned *)gpio_map;

   return 0;

int main(){
 if(init_io() == -1) 
  printf("Failed to map the physical GPIO registers into the virtual memory space.\n");
  return -1;
 // must use INP_GPIO before we can use OUT_GPIO
 int repeat;
 for (repeat=1; repeat<5; repeat++)
  GPIO_SET = 1 << LED_ACT;
  printf("LED OFF\n");

  GPIO_CLR = 1 << LED_ACT;
  printf("LED ON\n");
 printf("- finished -\n");
 return 0;

Download testLED.c

- You have to login as root to read /dev/mem and control the GPIO. Read the post "Set password of root".
- Disable the LED trigger from mmc0, read the post "Control the on-board LED on Raspberry Pi".

- RPi Low-level peripherals

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