Gap with GPIO pin

Question

I want to blink a simple led, here's the code :

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

#define BASE 0x7e200000

int main() {
        int mem_fd;
        void *gpio_map;
        volatile unsigned int *gpio;
        mem_fd = open("/dev/gpiomem", O_RDWR | O_SYNC);
        if (mem_fd < 0) {
                perror("Error");
                exit(-1);
        }
        gpio_map = mmap(NULL, 0x1000, PROT_READ | PROT_WRITE, MAP_SHARED, mem_fd, BASE);
        if (gpio_map == MAP_FAILED) {
                perror("map failed");
                exit(-1);
        }
        gpio = (volatile unsigned int *)gpio_map;
        // gpio[2] = address of GPFSEL2
        // setting 21st bit to 1 set GPIO27 as an output
        gpio[2] |= (1 << 21);
        while (1) {
                // gpio[10] = address of GPCLR0
                gpio[10] |= (1 << 27);
                sleep(1);
                // gpio[7] = address of GPSET0
                gpio[7] |= (1 << 27);
                sleep(1);
        }

}

This code work but not with the given port, if I wire the LED on 27 nothing happens, but if I wire to 22 (the pin just below 27 in the GPIO schema) it will work, and it's the same with other port, I tried with port 17 and it will work with 27. I check the physical connection and it's seems good. I use a raspberry pi 4B. (Documentation)

Answer 1

From your description, my best guess is that your extension board connector is off by one row.

The code looks ok to me, and I don't see how a coding error could produce the 27->22 and 17->27 mis-mappings you describe, but a 1 pin misalignment on the connector could.

FWIW, if you want to test your hardware setup you can use the libgpiod v2 tools to blink the lead for you:

gpioset -t1s,1s GPIO27=1

Answer 2

If you want to write your own code I suggest you avoid the magic numbers (which are error prone and almost impossible for anyone to read).

I have listed a fragment of my code which may help. (NOTE this is not intended to be complete or executable code.)

#define FSEL_OFFSET 0          // 0x0000
#define SET_OFFSET 7           // 0x001c / 4
#define CLR_OFFSET 10          // 0x0028 / 4

void setup_gpio(int gpio, int direction, int pud) {
  // Set gpio as an input or an output
  //        direction: 0=IN, 1=OUT
  //        pud: 0=None 1=Up 2=Down
  int offset = FSEL_OFFSET + (gpio / 10);
  int shift = (gpio % 10) * 3;
  set_pullupdn(gpio, pud);
  if (direction) { //  OUTPUT
    *(gpio_map + offset) =
        (*(gpio_map + offset) & ~(7 << shift)) | (1 << shift);
  } else { // INPUT
    *(gpio_map + offset) = (*(gpio_map + offset) & ~(7 << shift));
  }
}

void output_gpio(int gpio, int value) {
  //        Output to a GPIO channel
  //        value - 0/1 or False/True or LOW/HIGH
  int offset, shift;

  if (value) // value == HIGH
    offset = SET_OFFSET + (gpio / 32);
  else // value == LOW
    offset = CLR_OFFSET + (gpio / 32);

  shift = (gpio % 32);

  *(gpio_map + offset) = 1 << shift;
}