CODEWITHSUNDEEP
linuxinterruptangstrom-linuxbeagleboneblack
mrbubz
mrbubz

Reputation: 537

Poll() to monitor a pin on BeagleBone Black. Continuous output even though pin not connected.

I want to trigger an event whenever there is a rising edge on one of the pins of Beaglebone Black. Problem is, even though I havent connected that pin to anything, the output just goes on printing, interrupt occured, interrupt occured. I came across question Interrupts in Beaglebone on stackoverflow and tried to follow the steps. There was a link to a Program which implements the functionality. I read about poll() and I made slight changes in the program since I want to monitor just one pin. The changed code is : 

int main(int argc, char **argv, char **envp)
{
struct pollfd fdset[1];  // fdset[2] changed to fdset[1] since I will monitor just 1 pin
int nfds = 1;            // nfds changed from 2 to 1
int gpio_fd, timeout, rc;
char *buf[MAX_BUF];
unsigned int gpio;
int len;



if (argc < 2) {
    printf("Usage: gpio-int <gpio-pin>\n\n");
    printf("Waits for a change in the GPIO pin voltage level or input on stdin\n");
    exit(-1);
}

gpio = atoi(argv[1]);

gpio_export(gpio);
gpio_set_dir(gpio, 0);
gpio_set_edge(gpio, "rising");
gpio_fd = gpio_fd_open(gpio);

timeout = POLL_TIMEOUT;

while (1) {
    memset((void*)fdset, 0, sizeof(fdset));

    fdset[0].fd = gpio_fd;                 // This is the pin to be monitored
    fdset[0].events = POLLIN;

    //fdset[1].fd = gpio_fd;               // commented since I do not need this
    //fdset[1].events = POLLPRI;

    rc = poll(fdset, nfds, timeout);      

    if (rc < 0) {
        printf("\npoll() failed!\n");
        return -1;
    }

    if (rc == 0) {
        printf(".");
    }

    if (fdset[0].revents & POLLIN) {
        len = read(fdset[0].fd, buf, MAX_BUF);
        printf("\npoll() GPIO %d interrupt occurred\n", gpio);
    }

            // ****Commented block****
    //if (fdset[0].revents & POLLIN) {
    //  (void)read(fdset[0].fd, buf, 1);
    //  printf("\npoll() stdin read 0x%2.2X\n", (unsigned int) buf[0]);
    //}

    fflush(stdout);
}

gpio_fd_close(gpio_fd);
return 0;
}

Running Angstrom on Beaglebone black.

Upvotes: 1

Views: 3737

Answers (2)

tweej
tweej

Reputation: 842

https://www.kernel.org/doc/Documentation/gpio/sysfs.txt

If the pin can be configured as interrupt-generating interrupt and if it has been configured to generate interrupts (see the description of "edge"), you can poll(2) on that file and poll(2) will return whenever the interrupt was triggered. If you use poll(2), set the events POLLPRI and POLLERR. If you use select(2), set the file descriptor in exceptfds. After poll(2) returns, either lseek(2) to the beginning of the sysfs file and read the new value or close the file and re-open it to read the value.

You have not set events POLLPRI and POLLERR.

struct pollfd fdset[1];
memset((void*)fdset, 0, sizeof(fdset));
fdset[0].fd = fd;
fdset[0].events = POLLPRI | POLLERR;
...
poll()
...
lseek(fdset[0].fd, 0, SEEK_SET);
const ssize_t rc = read(fdset[0].fd, buf, MAX_BUF);

The above works on a BeagleBone Black Rev. C running Debian with linux 3.8.13-bone47.

Upvotes: 3

RG_
RG_

Reputation: 21

Since you "havent connected that pin to anything", if it isn't internally tied low or high then it's just a floating input, which could cause the apparent interrupts you're seeing.

Upvotes: 1

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