17. pwm控制

本章讲解Linux pwm相关应用层程序控制。

17.1. 开启pwm设备树插件

野火imx6ull提供了很多的设备树插件，用于控制板子上外设的开启，默认状态下pwm相关插件是没有开启的， 因此我们先需要开启pwm相关的设备树插件。修改 /boot/uEnv.txt 文件内容，使能pwm设备树相关插件内容， 并注释原来LED设备树插件

 #dtoverlay=/usr/lib/linux-image-4.19.35-imx6/overlays/imx-fire-led.dtbo
 dtoverlay=/usr/lib/linux-image-4.19.35-imx6/overlays/imx-fire-pwm.dtbo

野火提供的pwm设备树插件引脚为RGB控制所在的三盏灯。其引脚关系如下

pwm3	用户空间	对应LED
pwm3	/sys/class/pwm/pwmchip2	LED_R
pwm7	/sys/class/pwm/pwmchip6	LED_B
pwm8	/sys/class/pwm/pwmchip7	LED_G

开启pwm设备树插件之后RGB所在引脚为电平，也就是说RGB灯默认为点亮状态。

17.2. pwm控制方式(shell)

 #将pwm3导出到用户空间
 echo 0 > /sys/class/pwm/pwmchip2/export

 #设置pwm周期 单位为ns
 echo 1000000 > /sys/class/pwm/pwmchip2/pwm0/period

 #设置占空比
 echo 500000 > /sys/class/pwm/pwmchip2/pwm0/duty_cycle

 #设置pwm极性
 echo "normal" > /sys/class/pwm/pwmchip2/pwm0/polarity

 #使能pwm
 echo 1 > /sys/class/pwm/pwmchip2/pwm0/enable

 #取消将pwm3导出到用户空间
 echo 0 > /sys/class/pwm/pwmchip2/unexport

提示

当设置period与duty_cycle值的时候需要注意在任何的情况下都得保证period的值大于等于duty_cycle的值。

17.3. pwm控制方式(系统调用)

ebf_6ull_quick_start_code/Source/pwm/pwm.c

 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>

 #include <unistd.h>

 #define PWM_PATH "/sys/class/pwm/pwmchip2/"

 int main(int argc,char * argv[])
 {
     int fd;
     ssize_t ret;
     long period,duty_cycle;

     //判断参数合法性
     if( argc < 3)
     {
         printf("Please enter period and duty_cycle!\n ");
         exit(-1);
     }
     period = atol(argv[1]);
     duty_cycle = atol(argv[2]);
     if ( (period < duty_cycle)  ||
         ( strspn(argv[1], "0123456789")!= strlen(argv[1]) ) ||
         ( strspn(argv[2], "0123456789")!= strlen(argv[2]) )
         )
     {
         printf("Period or duty_cycle parameter error!\n ");
         exit(-1);
     }

     //导出用户空间
     fd = open(PWM_PATH"export",O_WRONLY);
     if( fd < 0 )
     {
         perror(PWM_PATH"export");
         goto err1;
     }

     ret = write( fd,"0",1);
     if( ret < 1 )
     {
         perror(PWM_PATH"export");
         goto err1;
     }
     close(fd);

     //设置pwm周期
     fd = open(PWM_PATH"/pwm0/period",O_RDWR);
     if( fd < 0 )
     {
         perror(PWM_PATH"/pwm0/period");
         goto err1;
     }
     ret = write( fd,argv[1],strlen(argv[1]));
     if( ret < strlen(argv[1]) )
     {
         perror(PWM_PATH"/pwm0/period");
         goto err1;
     }
     close(fd);

     //设置占空比
     fd = open(PWM_PATH"/pwm0/duty_cycle",O_RDWR);
     if( fd < 0 )
     {
         perror(PWM_PATH"/pwm0/duty_cycle");
         goto err1;
     }
     ret = write( fd,argv[2],strlen(argv[2]));
     if( ret < strlen(argv[2]) )
     {
         perror(PWM_PATH"/pwm0/duty_cycle");
         goto err1;
     }
     close(fd);

     //设置极性
     fd = open(PWM_PATH"/pwm0/polarity",O_RDWR);
     if( fd < 0 )
     {
         perror(PWM_PATH"/pwm0/polarity");
         goto err1;
     }
     ret = write( fd,"normal",strlen("normal"));
     if( ret < strlen("normal") )
     {
         perror(PWM_PATH"polarity");
         goto err1;
     }
     close(fd);

     //使能pwm
     fd = open(PWM_PATH"/pwm0/enable",O_RDWR);
     if( fd < 0 )
     {
         perror(PWM_PATH"/pwm0/enable");
         goto err1;
     }
     ret = write( fd,"1",1);
     if( ret < 1 )
     {
         perror(PWM_PATH"/pwm0/enable");
         goto err1;
     }
     close(fd);


     printf("Set pwm success! \n");

     //从用户空间删除
     fd = open(PWM_PATH"unexport",O_WRONLY);
     if( fd < 0 )
     {
         perror(PWM_PATH"unexport");
         goto err1;
     }

     ret = write( fd,"0",1);
     if( ret < 1 )
     {
         perror(PWM_PATH"unexport");
         goto err1;
     }
     close(fd);

     return 0;

 err1:

     close(fd);
     return -1;
 }

17.4. 修改pwm相关设备树

imx6ull提供了pwm1~pwm8总的8个，每个pwm能选择不同的IO引脚(具有PWM复用的引脚)作为pwm引脚。 imx-fire-pwm.dts源码如下所示

/arch/arm/boot/dts/overlays/ebf/imx-fire-pwm.dts

 #include "imx6ul-pinfunc.h"
 #include "imx6ull-pinfunc-snvs.h"
 #include "dt-bindings/input/linux-event-codes.h"
 #include "dt-bindings/gpio/gpio.h"
 #include <dt-bindings/clock/imx6ul-clock.h>
 #include "dt-bindings/interrupt-controller/irq.h"
 #include "dt-bindings/interrupt-controller/arm-gic.h"
 /dts-v1/;
 /plugin/;

 /{

     fragment@0 {
         target = <&iomuxc>;
         __overlay__ {
             pinctrl_pwm3: pwm3grp {
                 fsl,pins = <
                     MX6UL_PAD_GPIO1_IO04__pwm3_OUT   0x000010B1
                 >;
             };
             pinctrl_pwm7: pwm7grp {
                 fsl,pins = <
                     MX6UL_PAD_CSI_VSYNC__pwm7_OUT    0x000010B1
                 >;
             };
             pinctrl_pwm8: pwm8grp {
                 fsl,pins = <
                     MX6UL_PAD_CSI_HSYNC__pwm8_OUT    0x000010B1
                 >;
             };
         };
     };

     fragment@1 {
         target = <&pwm3>;
         __overlay__ {
             pinctrl-names = "default";
             pinctrl-0 = <&pinctrl_pwm3>;
             status = "okay";
         };
     };

     fragment@2 {
         target = <&pwm7>;
         __overlay__ {
             pinctrl-names = "default";
             pinctrl-0 = <&pinctrl_pwm7>;
             clocks = <&clks IMX6UL_CLK_pwm7>,
                             <&clks IMX6UL_CLK_pwm7>;
             status = "okay";
         };
     };


     fragment@3 {
         target = <&pwm8>;
         __overlay__ {
             pinctrl-names = "default";
             pinctrl-0 = <&pinctrl_pwm8>;
             clocks = <&clks IMX6UL_CLK_pwm8>,
                             <&clks IMX6UL_CLK_pwm8>;
             status = "okay";
         };
     };

 };

pwm设备树插件的源码也是相对简单，若想要修改PWM3的引脚只需要修改第18行的内容即可。 想要使能新的pwm也只需要根据代码黄色部分改写即可。