PWM

1. PWM 简介

PWM，全称为脉冲宽度调制（Pulse Width Modulation），是一种通过控制信号的脉冲宽度来实现模拟信号输出的技术。它常用于嵌入式系统和电子设备中，用于控制电机速度、LED 亮度、音频信号生成等应用。在 Linux 系统中，PWM 设备通常通过 sysfs 文件系统进行管理和配置，其设备目录通常位于 /sys/class/pwm/ 目录下。

2. 控制PWM（Shell）

2.1 引脚分布

• Luckfox Lyra Ultra/Ultra W 引脚图：

  

2.2 设备目录和设备属性

1. 查看 PWM 接口，pwmchip0（注意：此时的 pwmchip0 是 ff932000） 为屏幕的背光，系统默认开启：

   root@luckfox:~# ls -l /sys/class/pwm
   total 0
   lrwxrwxrwx    1 root     root  0 Jan  1 00:00 pwmchip0 -> ../../devices/platform/ff932000.pwm/pwm/pwmchip0

2. 当开启多个 PWM 设备树插件时，PWM 控制器值越小，系统分配的 pwmchip 越小。

3. 在 /sys/class/pwm/pwmchipN/pwm0/ 设备目录下有多种属性，可以通过这些属性控制，从而实现对 PWM 的控制。

   root@luckfox:/# ls /sys/class/pwm/pwmchip0/pwm0/
   capture  duty_cycle  enable  period  polarity  power  uevent

   • enable：用于启用或禁用PWM通道
   • period：用于设置PWM信号的周期时间
   • duty_cycle：用于设置PWM信号的占空比
   • polarity：用于配置PWM信号的极性
   • power/control：用于启用或禁用PWM通道的电源管理（通常用于省电模式）

2.3 控制 PWM

1. 导出到用户空间或者取消用户空间的导出。

   echo 0 > /sys/class/pwm/pwmchip0/export
   echo 0 > /sys/class/pwm/pwmchip0/unexport

2. 设置 PWM 周期单位为ns，比如 1KHz 频率的周期就是 1000000ns（注意，在任何的情况下都得保证 period 的值大于等于 duty_cycle 的值）。

   echo 1000000 > /sys/class/pwm/pwmchip10/pwm0/period

3. 设置PWM极性正常或翻转。

   echo "normal" > /sys/class/pwm/pwmchip0/pwm0/polarity
   echo "inversed" > /sys/class/pwm/pwmchip0/pwm0/polarity

4. 使能和关闭PWM。

   echo 1 > /sys/class/pwm/pwmchip1/pwm0/enable
   echo 0 > /sys/class/pwm/pwmchip1/pwm0/enable

5. 设置占空比（小于等于 period ）

   echo 100000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 200000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 300000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 400000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 500000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 600000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 700000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 800000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 900000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 1000000 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle
   echo 0 > /sys/class/pwm/pwmchip0/pwm0/duty_cycle

3. 使用python-periphery控制 PWM

1. 使用 python-periphery库进行PWM输出的完整示例代码如下：

   #!/usr/bin/python3
   
   from periphery import PWM
   import time
   
   pwm = PWM(1, 0)
   
   try:
       pwm.frequency = 1000
       pwm.duty_cycle = 0
       pwm.polarity = "normal"
       pwm.enable()
   
       direction = 1
   
       while True:
           pwm.duty_cycle += 0.01 * direction
           pwm.duty_cycle = round(pwm.duty_cycle, 2)
           if pwm.duty_cycle == 1.0:
               direction = -1
           elif pwm.duty_cycle == 0.0:
               direction = 1
   
           time.sleep(0.05)
   
   except KeyboardInterrupt:
       pass
   
   finally:
       pwm.close()

2. 程序解析。

   pwm = PWM(1, 0)         # 开启 PWM chip 0, channel 0
   pwm.frequency = 1000     # 设置1000Hz频率
   pwm.polarity = "normal" # 设置 PWM 极性
   pwm.duty_cycle = 0      # 初始占空比
   pwm.enable()            # 使能 PWM

   • 这个代码段实现了一个简单的 PWM 输出。通过循环不断调整 PWM 的占空比，以实现呼吸灯效果。pwm.duty_cycle 每次增加 0.01（可以根据需要调整），并使用 round 函数将其保留两位小数。在达到最大或最小值时，反转方向。并在每次更新 PWM 值后，通过 time.sleep 控制呼吸灯的速度。

   while True:
       pwm.duty_cycle += 0.01 * direction
       pwm.duty_cycle = round(pwm.duty_cycle, 2)
       if pwm.duty_cycle == 1.0:
           direction = -1
       elif pwm.duty_cycle == 0.0:
           direction = 1
   
       print(pwm.duty_cycle)
       time.sleep(0.05)

3. 运行程序：

   chmod +x pwm.py
   ./pwm.py

4. 可以看到对应 Pin12 号管脚的数据（呼吸灯）。

   

4. 控制PWM（C程序）

除了可以通过 shell 脚本和 Python 库操作 sysfs 控制 PWM 引脚外，还可以使用 C 语言操作 sysfs 来实现同样的功能。

1. 导出用户空间。

   int pwm_export(int channel) {
       FILE *pwm_export = fopen(PWM_PATH "/export", "w");
       if (!pwm_export) {
           perror("Failed to open PWM export");
           return 1;
       }
       fprintf(pwm_export, "%d", channel);
       fclose(pwm_export);
       return 0;
   }

2. 取消用户空间导出。

   int pwm_unexport(int channel) {
       FILE *pwm_unexport = fopen(PWM_PATH "/unexport", "w");
       if (!pwm_unexport) {
           perror("Failed to open PWM unexport");
           return 1;
       }
       fprintf(pwm_unexport, "%d", channel);
       fclose(pwm_unexport);
       return 0;
   }

3. 设置 PWM 周期。

   int pwm_set_period(int channel, int period_ns) {
       char period_path[128];
       snprintf(period_path, sizeof(period_path), PWM_PATH "/pwm%d/period", channel);
   
       FILE *period_file = fopen(period_path, "w");
       if (!period_file) {
           perror("Failed to open PWM period");
           return 1;
       }
       fprintf(period_file, "%d", period_ns);
       fclose(period_file);
       return 0;
   }

4. 设置占空比。

   int pwm_set_duty_cycle(int channel, int duty_cycle_ns) {
       char duty_cycle_path[128];
       snprintf(duty_cycle_path, sizeof(duty_cycle_path), PWM_PATH "/pwm%d/duty_cycle", channel);
   
       FILE *duty_cycle_file = fopen(duty_cycle_path, "w");
       if (!duty_cycle_file) {
           perror("Failed to open PWM duty cycle");
           return 1;
       }
       fprintf(duty_cycle_file, "%d", duty_cycle_ns);
       fclose(duty_cycle_file);
       return 0;
   }

5. 使能 PWM 。

   int pwm_enable(int channel, int enable) {
       char enable_path[128];
       snprintf(enable_path, sizeof(enable_path), PWM_PATH "/pwm%d/enable", channel);
   
       FILE *enable_file = fopen(enable_path, "w");
       if (!enable_file) {
           perror("Failed to open PWM enable");
           return 1;
       }
       fprintf(enable_file, "%d", enable);
       fclose(enable_file);
       return 0;
   }

6. 完整示例程序。

   #include <stdio.h>
   #include <stdlib.h>
   #include <string.h>
   #include <unistd.h>
   #include <math.h>  // 用于 sin() 函数
   
   #define PWM_PATH "/sys/class/pwm/pwmchip1"
   #define PERIOD_NS 1000000
   #define MAX_DUTY_CYCLE_NS 1000000
   #define MIN_DUTY_CYCLE_NS 0
   
   #define PI 3.141592653589793
   
   // 导出 PWM 通道
   int pwm_export(int channel) {
       FILE *pwm_export = fopen(PWM_PATH "/export", "w");
       if (!pwm_export) {
           perror("Failed to open PWM export");
           return 1;
       }
       fprintf(pwm_export, "%d", channel);
       fclose(pwm_export);
       return 0;
   }
   
   // 取消导出 PWM 通道
   int pwm_unexport(int channel) {
       FILE *pwm_unexport = fopen(PWM_PATH "/unexport", "w");
       if (!pwm_unexport) {
           perror("Failed to open PWM unexport");
           return 1;
       }
       fprintf(pwm_unexport, "%d", channel);
       fclose(pwm_unexport);
       return 0;
   }
   
   // 设置 PWM 周期
   int pwm_set_period(int channel, int period_ns) {
       char period_path[128];
       snprintf(period_path, sizeof(period_path), PWM_PATH "/pwm%d/period", channel);
   
       FILE *period_file = fopen(period_path, "w");
       if (!period_file) {
           perror("Failed to open PWM period");
           return 1;
       }
       fprintf(period_file, "%d", period_ns);
       fclose(period_file);
       return 0;
   }
   
   // 设置占空比
   int pwm_set_duty_cycle(int channel, int duty_cycle_ns) {
       char duty_cycle_path[128];
       snprintf(duty_cycle_path, sizeof(duty_cycle_path), PWM_PATH "/pwm%d/duty_cycle", channel);
   
       FILE *duty_cycle_file = fopen(duty_cycle_path, "w");
       if (!duty_cycle_file) {
           perror("Failed to open PWM duty cycle");
           return 1;
       }
       fprintf(duty_cycle_file, "%d", duty_cycle_ns);
       fclose(duty_cycle_file);
       return 0;
   }
   
   // 启用/禁用 PWM
   int pwm_enable(int channel, int enable) {
       char enable_path[128];
       snprintf(enable_path, sizeof(enable_path), PWM_PATH "/pwm%d/enable", channel);
   
       FILE *enable_file = fopen(enable_path, "w");
       if (!enable_file) {
           perror("Failed to open PWM enable");
           return 1;
       }
       fprintf(enable_file, "%d", enable);
       fclose(enable_file);
       return 0;
   }
   
   int main() {
       int channel = 0;  // 使用通道 0
   
       // 导出 PWM
       if (pwm_export(channel)) {
           return 1;
       }
   
       // 设置周期
       if (pwm_set_period(channel, PERIOD_NS)) {
           return 1;
       }
   
       // 启用 PWM
       if (pwm_enable(channel, 1)) {
           return 1;
       }
   
       // 使用 sin(x) 函数生成占空比的平滑变化
       double step_size = (2 * PI) / 100.0;
       while (1) {
           for (double i = 0; i <= 2 * PI; i += step_size) {
               // 使用 sin(x) 生成从 0 到 1 的占空比
               double duty_cycle_ratio = (sin(i) + 1) / 2;
               int duty_cycle_ns = (int)(duty_cycle_ratio * MAX_DUTY_CYCLE_NS);
   
               // 设置占空比
               if (pwm_set_duty_cycle(channel, duty_cycle_ns)) {
                   return 1;
               }
   
               usleep(20000);  // 控制呼吸灯速度，20毫秒更新一次
           }
       }
   
       // 禁用 PWM
       if (pwm_enable(channel, 0)) {
           return 1;
       }
   
       // 取消导出 PWM
       if (pwm_unexport(channel)) {
           return 1;
       }
   
       return 0;
   }

7. 交叉编译运行程序，搭建交叉编译环境请参考《程序编译》或《GPIO》部分。

   arm-none-linux-gnueabihf-gcc pwm.c -o pwm -lm

5. 设备树简介

1. 设备文件路径位于kernel-6.1/arch/arm/boot/dts/rk3506g-luckfox-lyra.dts，开启pwm的代码片段如下：

   /{
       pwm_rockchip_test: pwm-rockchip-test {
           compatible = "pwm-rockchip-test";
           pwms = <&pwm0_4ch_0 0 25000 0>,
                  <&pwm0_4ch_1 0 25000 0>,
                  <&pwm0_4ch_2 0 25000 0>,
                  <&pwm0_4ch_3 0 25000 0>,
                  <&pwm1_8ch_0 0 25000 0>,
                  <&pwm1_8ch_1 0 25000 0>,
                  <&pwm1_8ch_2 0 25000 0>,
                  <&pwm1_8ch_3 0 25000 0>,
                  <&pwm1_8ch_4 0 25000 0>,
                  <&pwm1_8ch_5 0 25000 0>;
           pwm-names = "pwm0_0",
                   "pwm0_1",
                   "pwm0_2",
                   "pwm0_3",
                   "pwm1_0",
                   "pwm1_1",
                   "pwm1_2",
                   "pwm1_3",
                   "pwm1_4",
                   "pwm1_5";
       };
   };
   
   
   /***********************PWM********************/
   &pwm0_4ch_0 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io12_pwm0_ch0>;
       assigned-clocks = <&cru CLK_PWM0>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm0_4ch_1 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io13_pwm0_ch1>;
       assigned-clocks = <&cru CLK_PWM0>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm0_4ch_2 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io9_pwm0_ch2>;
       assigned-clocks = <&cru CLK_PWM0>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm0_4ch_3 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io8_pwm0_ch3>;
       assigned-clocks = <&cru CLK_PWM0>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm1_8ch_0 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io4_pwm1_ch0>;
       assigned-clocks = <&cru CLK_PWM1>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm1_8ch_1 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io3_pwm1_ch1>;
       assigned-clocks = <&cru CLK_PWM1>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm1_8ch_2 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io2_pwm1_ch2>;
       assigned-clocks = <&cru CLK_PWM1>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm1_8ch_3 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io27_pwm1_ch3>;
       assigned-clocks = <&cru CLK_PWM1>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm1_8ch_4 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io26_pwm1_ch4>;
       assigned-clocks = <&cru CLK_PWM1>;
       assigned-clock-rates = <100000000>;
   };
   
   &pwm1_8ch_5 {
       status = "okay";
       pinctrl-names = "active";
       pinctrl-0 = <&rm_io25_pwm1_ch5>;
       assigned-clocks = <&cru CLK_PWM1>;
       assigned-clock-rates = <100000000>;
   };