GPIO reading for external value

rrsuj

Hi,
The GPIO blink program using write operation works for me, which is described here - https://up-community.org/forum/public-ubilinux/2141-tutorial-gpio-i2c-spi-access-without-root-permissions#5724

But now I want to read signal using pin 16 (UP GPIO 18, linux GPIO 23), so I change the above mentioned program like this -

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <getopt.h>
#include <sys/ioctl.h>
#include <linux/gpio.h>
#include <time.h>
#include <mraa.h>

#define HEADERPIN 16

void sleepMillis(uint32_t millis) {
  struct timespec sleep;
  sleep.tv_sec = millis / 1000;
  sleep.tv_nsec = (millis % 1000) * 1000000L;
  while(clock_nanosleep(CLOCK_MONOTONIC, 0, &sleep, &sleep) && errno == EINTR);
}

int main(int argc, char **argv)
{
	mraa_init();
	mraa_gpio_context gpio;

	if (!(gpio = mraa_gpio_init(HEADERPIN))) {
		fprintf(stderr, "Error exporting pin %d!\n", HEADERPIN);
		mraa_deinit();
		exit(1);
	}

	/* Check if the binary has root-permissions: if not,
	sleep for 100ms to give udev time to set the GPIO-permissions
	correctly for us to use the pin we just initialized above.
	!IMPORTANT! */
	/* Try uncommenting this or changing the amount of time
	we sleep and see what happens. */
	if(geteuid()) sleepMillis(100);

	if(mraa_gpio_dir(gpio, MRAA_GPIO_IN) != MRAA_SUCCESS){
		fprintf(stderr, "Error setting pin-direction!\n");
		mraa_gpio_close(gpio);
		mraa_deinit();
		exit(1);
	}
	int test;
        while(1){
	       test = mraa_gpio_read(gpio);
	       printf("value: %d\n", test);
               sleepMillis(1000); //Sleep one second
        }
	
	mraa_gpio_close(gpio);
	mraa_deinit();
	exit(0);
}

When I execute the program the value is always 1. I think after boot the gpio becomes high, so it reads value 1 always.
So I change the active_low to 1 to make the reading value to zero (I did: echo 1 > /sys/class/gpio/gpio23/active_low)
After that when I execute the program the value is always 0.
Now I connect pin1 to pin16, so that pin16 gets 3.3v, but there is no change in the readings of the program.
It still reads 0.

How can I read external signal using the GPIO in the correct way?
Please help me in this issue.

Nicola Lunghi

Hi rrsuj
The active low property doesn't change the external behaviour it only change the reported value.
So To read a 1 you have to put the pin at 0 (GND, pin 6 of the hat https://up-community.org/wiki/Pinout)

Regards
Nicola Lunghi

rrsuj

Hi Nicola,
Sorry I am not getting it, I have connected pin16 to pin1 just for test.
In real environment, I have the plan to connect this input gpio to an external circuit, and the external circuit sends 0v or 1v. I need to read the 0/1 volt of the external circuit correctly through this gpio using the program running inside UP board.
So I am confused how connecting to ground can help me in real environment where the gpio will be connected to another circuit (no ground line).
How can I work in the real environment then?