TS-4100 interrupts

The i.MX6UL CPU GPIO are also able to function as interrupts on rising and falling edges. This is accessible from the kernel as well as userspace. Userspace IRQs are exposed through the sysfs GPIO mechanism. This example will trigger on a falling edge for GPIO 48:

echo "48" > /sys/class/gpio/export
echo "in" > /sys/class/gpio/gpio48/direction
echo "falling" > /sys/class/gpio/gpio48/edge

From here, it's possible to use functions such as poll() or select() on the /sys/class/gpio/gpio48/value file to get information on when the input changes and matches the edge setting.

#include <stdio.h>
#include <fcntl.h>
#include <sys/select.h>
#include <sys/stat.h>
#include <unistd.h>

int main(int argc, char **argv)
{
	char gpio_irq[64];
	int ret, irqfd = 0, i = 0;
	fd_set fds;
	FD_ZERO(&fds);
 
	if(argc < 2) {
		printf("Usage: %s <gpio number>\n", argv[0]);
		return 1;
	}

	snprintf(gpio_irq, sizeof(gpio_irq), "/sys/class/gpio/gpio%d/value", atoi(argv[1]));
	irqfd = open(gpio_irq, O_RDONLY, S_IREAD);

	if(irqfd == -1) {
		printf("Could not open IRQ %s\n", argv[1]);
		printf("Make sure the GPIO is already exported", argv[1]);
		return 1;
	}

	while(1) {
		int buf; // Holds irq junk data
		FD_SET(irqfd, &fds); //add the fd to the set
		// See if the IRQ has any data available to read
		ret = select(irqfd + 1, NULL, NULL, &fds, NULL);
		if(ret == -1)
			continue;

		if(FD_ISSET(irqfd, &fds))
		{
			printf("IRQ detected %d\n", i);
			i++;
			// Clear the junk data in the IRQ file
			lseek(irqfd, 0, 0);
			read(irqfd, &buf, sizeof(buf));
		}
	}
 
	return 0;
}

This example can be run as "./irqtest 48" which will echo every time the pin changes, but will otherwise take no cpu time.