21. PWM application programming

  One 、PWM Operation introduction      

        PWM, Programming method and... Of pulse width modulation technology LED Class devices can also pass sysfs Virtual file system control . Go to directory /sys/class/pwm Under the table of contents ：

Each of them pwmchipx Corresponds to a pwm controller , Get into pwmchip0 file ：

npwm： This is a read-only property , Read the file to know the PWM There are several channels under the controller PWM Output , As follows
in ：

It can be seen that I.MX6ULL Every PWM The controller corresponds to only one output .export： And GPIO Control is the same , In the use of PWM Before , You also need to export it , adopt export Attribute to export .unexport： Will export PWM Delete . When used PWM after , We need to export PWM Delete .
enable： Can read but write , write in "0" It is forbidden to PWM; write in "1" To enable PWM. Read the file to get PWM Is the current status prohibited or enabled .
polarity： Used to set polarity , Can read but write , Writable values such as ："normal"： Ordinary ;"inversed"： reverse ;period： Used for configuration PWM cycle , Can read but write ; Write a string numeric value , With ns（ nanosecond ） In units of
duty_cycle： Used for configuration PWM Duty cycle of , Can read but write ; Write a string numeric value , Also with ns In units of .

Two 、 Write application operation

        

#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>


static char pwm_path[100];
static int pwm_config(const char *attr, const char *val)    // To configure PWM
{
    char file_path[100];
    int len;
    int fd;
    sprintf(file_path, "%s/%s", pwm_path, attr);
    if (0 > (fd = open(file_path, O_WRONLY))) {
        perror("open error");
        return fd;
    }
    len = strlen(val);
    if (len != write(fd, val, len)) {
        perror("write error");
        close(fd);
    return -1;
    }
    close(fd); // Close file 
    return 0;
}


int main(int argc, char *argv[])
{
    /*  Check the transmission parameters  */
    if (4 != argc) {
        fprintf(stderr, "usage: %s <id> <period> <duty>\n",
        argv[0]);
        exit(-1);
    }
    /*  Print configuration information  */
    printf("PWM config: id<%s>, period<%s>, duty<%s>\n",
    argv[1], argv[2],
    argv[3]);
    /*  export  pwm */
    sprintf(pwm_path, "/sys/class/pwm/pwmchip%s/pwm0", argv[1]);
    if (access(pwm_path, F_OK)) {// If  pwm0  directory does not exist ,  Then export 
    char temp[100];
    int fd;
    sprintf(temp, "/sys/class/pwm/pwmchip%s/export", argv[1]);
    if (0 > (fd = open(temp, O_WRONLY))) {
    perror("open error");
    exit(-1);
    }
    if (1 != write(fd, "0", 1)) {// export  pwm
        perror("write error");
        close(fd);
        exit(-1);
    }
    close(fd); // Close file 
    }
    /*  To configure  PWM  cycle  */
    if (pwm_config("period", argv[2]))
        exit(-1);
    /*  Configure the duty cycle  */
    if (pwm_config("duty_cycle", argv[3]))
        exit(-1);
    /*  Can make  pwm */
    pwm_config("enable", "1");
    /*  Exit procedure  */
    exit(0);
}