跳到主要内容

PWM - 脉冲宽度调制

脉冲宽度调制（PWM）是一种对模拟信号电平进行数字编码的方法。 通过高分辨率计数器的使用，方波的占空比被调制用来对一个具体模拟信号的电平进行编码。

PWM模块属于Linux PWM子系统，会调用PWM子系统的相关接口。

不同平台上拥有不同个数的PWM通道，其中两个为一个PWM对。其中PWM具有以下特点：

  • 支持脉冲，周期和互补对输出；
  • 支出捕捉输入；
  • 带可编程死区发生器，死区时间可控；
  • 0-100M输出频率范围。0%-100%占空比可调，最小分辨率1/65536；
  • 支持PWM输出和捕捉输入产生中断；
  • 组模式下各通道输出波形相对相位可配置。

模块配置​

驱动配置​

驱动位于

Allwinner BSP  --->
    Device Drivers  --->
        PWM Drivers ->
            <*> PWM Support for Allwinner SoCs
            < > Soft-PWM Support for Allwinner SoCs

image-20250701101503807

设备树配置​

PWM 控制器节点​

这里以 PWM0 控制器作为示例说明

pwm0: pwm@42000c00 {
    #pwm-cells = <0x3>;
    compatible = "allwinner,sunxi-pwm-v205";
    reg = <0x0 0x42000c00 0x0 0x400>;
    clocks = <&ccu CLK_PWM>, <&aon_ccu CLK_DCXO>;
    clock-names = "bus", "clk_hosc";
    resets = <&ccu RST_BUS_PWM>;
    interrupts-extended = <&plic0 19 IRQ_TYPE_LEVEL_HIGH>;
    pwm-number = <12>;
    pwm-base = <0x0>;
    sunxi-pwms = <&pwm0_0>, <&pwm0_1>, <&pwm0_2>, <&pwm0_3>, <&pwm0_4>,
            <&pwm0_5>, <&pwm0_6>, <&pwm0_7>, <&pwm0_8>,
            <&pwm0_9>, <&pwm0_10>, <&pwm0_11>;
    status = "okay";
};
  • compatible表示用哪套设备和驱动绑定
  • reg 寄存器的基地址
  • clocks时钟配置
  • interrupts中断号的配置
  • resets复位配置
  • pwm-number控制器的pwm通道个数
  • pwm-basepwm的起始基数
  • sunxi-pwms 控制器的具体通道

PWM 通道配置​

pwm0_0: pwm0@42000c10 {
    compatible = "allwinner,sunxi-pwm0";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c10 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_1: pwm0@42000c11 {
    compatible = "allwinner,sunxi-pwm1";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c11 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_2: pwm0@42000c12 {
    compatible = "allwinner,sunxi-pwm2";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c12 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_3: pwm0@42000c13 {
    compatible = "allwinner,sunxi-pwm3";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c13 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_4: pwm0@42000c14 {
    compatible = "allwinner,sunxi-pwm4";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c14 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_5: pwm0@42000c15 {
    compatible = "allwinner,sunxi-pwm5";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c15 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_6: pwm0@42000c16 {
    compatible = "allwinner,sunxi-pwm6";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c16 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_7: pwm0@42000c17 {
    compatible = "allwinner,sunxi-pwm7";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c17 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_8: pwm0@42000c18 {
    compatible = "allwinner,sunxi-pwm8";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c18 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_9: pwm0@42000c19 {
    compatible = "allwinner,sunxi-pwm9";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c19 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_10: pwm0@42000c1a {
    compatible = "allwinner,sunxi-pwm10";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c1a 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

pwm0_11: pwm0@42000c1b {
    compatible = "allwinner,sunxi-pwm11";
    pinctrl-names = "active", "sleep";
    reg = <0x0 0x42000c1b 0x0 0x4>;
    reg_base = <0x42000c00>;
    status = "disabled";
};

具体通道配置按照需求进行配置。

  • pinctrl-names：分别表示pwm的io口的两种工作状态；
  • pinctrl-0、pinctrl-1：pwm两种工作状态的引脚参数设置；
  • status:模块设备的打开或关闭，当编译该模块时，status="okay";不编译时，status="disabled";。

设备端配置​

设备端主要配置 PWM 的引脚，配置每个 PWM 是否启用功能。

示例配置，具体引脚复用请参照芯片手册

&pio {
    pwm8_pins_active: pwm8@0 {
        pins = "PD18";
        function = "pwm0_8";
    };
    pwm8_pins_sleep: pwm8@1 {
        pins = "PD18";
        function = "gpio_in";
        bias-pull-down;
    };
}；

&pwm0_8 {
    pinctrl-names = "active", "sleep";
    pinctrl-0 = <&pwm8_pins_active>;
    pinctrl-1 = <&pwm8_pins_sleep>;
    status = "okay";
};

模块驱动​

PWM驱动的源代码位于BSP独立仓库的 drivers/pwm 目录下，具体的路径如下所示：

.
├── Kconfig
├── Makefile
├── pwm-sunxi.c
├── pwm-sunxi.h
└── sunxi-soft-pwm.c

用户层接口​

可以直接在linux内核中调试pwm模块，具体如下：

  • 进入 /sys/class/pwm 目录。该目录是 linux 内核为 pwm 子系统提供的类目录，遍历该目录。可看到很多的 pwmchipX，其中X表示很多能够被控制的通道，下面以 pwmchip0 为例进行说明。
/sys/class/pwm # ls
pwmchip0
  • 可以看到，上述 pwmchip0 就是我们注册的 pwm 控制器，进入该目录，然后遍历该目录。
/sys/class/pwm # cd pwmchip0/
/pwm/pwmchip0 # ls
device     export     npwm       subsystem  uevent     unexport
  • 其中 npwm 文件储存了该pwm控制器的pwm个数，而export和unexport是导出和删除某个pwm设备的文件，下面演示导出pwm1。
/pwm/pwmchip0 # cat npwm
2
/pwm/pwmchip0 # echo 1 > export
/pwm/pwmchip0 # ls
device     export     npwm       pwm1      subsystem  uevent     unexport

可以看到目录中多出pwm1目录，进入该目录显示出不同的节点。不同节点的参数如下：

  • enable：使能pwm，其中1代表使能，0代表不使能；
  • duty_cycle：pwm信号的占空比，单位为（ns）；
  • period：pwm信号的频率，单位为（ns）；
  • polarity：是否翻转极性，其中1表示翻转极性，0表示不翻转极性。
/pwm/pwmchip0 # cd pwm1/
/pwm/pwmchip0/pwm1 # ls
capture     duty_cycle  enable      period      polarity    uevent

可通过以上节点来对pwm的状态进行改变：

/pwm/pwmchip0/pwm1 # echo 1000000000 > period
/pwm/pwmchip0/pwm1 # echo 500000000 > duty_cycle
/pwm/pwmchip0/pwm1 # echo normal > polarity
/pwm/pwmchip0/pwm1 # echo 1 > enable

如果相关引脚接上了示波器等，可以看到波形。最后返回上层目录，删除该pwm设备。

/pwm/pwmchip0/pwm1 # cd ..
/pwm/pwmchip0 # ls
device     export     npwm       pwm1       subsystem  uevent     unexport
/pwm/pwmchip0 # echo 1 > unexport
/pwm/pwmchip0 # ls
device     export     npwm       subsystem  uevent     unexport

内核层接口​

内核调用的接口如下

devm_pwm_get​

  • 函数原型:
struct pwm_device *devm_pwm_get(struct device *dev, const char *con_id)。

  • 作用: 请求pwm设备。

  • 参数:

    • dev: 指向当前PWM设备的实例；
    • con_id: 使用方绑定的名称；

  • 返回值:

    • 成功：返回请求成功的pwm设备;
    • 失败：返回错误码

pwm_config​

  • 函数原型
static inline int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns)

  • 作用: 更改PWM设备配置。

  • 参数:

    • dev: 指向当前PWM设备的实例；
    • duty_ns: 占空比；
    • period_ns：周期

  • 返回值:

    • 成功：0;
    • 失败：负值错误码

pwm_enable​

  • 函数原型：
static inline int pwm_enable(struct pwm_device *pwm)
  • 作用: 启动PWM输出。
    • 参数: pwm: 指向当前PWM设备的实例；
  • 返回值:
    • 成功：0；
    • 失败：负值错误码

模块 DEMO​

内核驱动 DEMO​

编写一个内核驱动，调用 PWM

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/platform_device.h>
#include <linux/pwm.h>

static int sunxi_pwm_test_probe(struct platform_device *pdev)
{
    int ret;
    struct pwm_device *chip;
    struct device *dev = &pdev->dev;

    chip = devm_kzalloc(dev, sizeof(*chip), GFP_KERNEL);
    if (!chip)
        return -ENOMEM;

    chip= devm_pwm_get(dev, "pwm-test");
    if (IS_ERR(chip)) {
        dev_err(dev, "Cannot request PWM channel\n");
        return PTR_ERR(chip);
    }

    /* set pwm period and duty_cycle */
    ret = pwm_config(chip, 5000, 10000);
    if (ret) {
        dev_err(dev, "Cannot configure PWM\n");
        return ret;
    }

    /* Enable PWM */
    ret = pwm_enable(chip);
    if (ret) {
        dev_err(dev, "Cannot enable PWM\n");
        return ret;
    }

    platform_set_drvdata(pdev, chip);

    return 0;
}

static int sunxi_pwm_test_remove(struct platform_device *pdev)
{
    struct pwm_device *chip = platform_get_drvdata(pdev);

    pwm_disable(chip);
    pwm_free(chip);

    return 0;
}

static const struct of_device_id sunxi_pwm_test_of_match[] = {
    { .compatible = "sunxi-pwm-test", },
    {},
};
MODULE_DEVICE_TABLE(of, sunxi_pwm_test_of_match);

static struct platform_driver sunxi_pwm_test_driver = {
    .driver = {
        .name = "sunxi-pwm-test",
        .owner = THIS_MODULE,
        .of_match_table = sunxi_pwm_test_of_match,
    },
    .probe = sunxi_pwm_test_probe,
    .remove = sunxi_pwm_test_remove,
};

module_platform_driver(sunxi_pwm_test_driver);

MODULE_LICENSE("GPL v2");
MODULE_AUTHOR("yourname <yourname@yourname.com>");
MODULE_DESCRIPTION("sunxi-pwm test driver");
MODULE_VERSION("1.0.0");

增加一个设备树节点，注册这个设备

示例配置，具体引脚复用请参照芯片手册

sunxi_pwm_test: sunxi-pwm-test {
    compatible = "sunxi-pwm-test";
    #pwm-cells = <3>;
    pwms = <&pwm0 0 500000 1000000 0>;
    pwm-names = "pwm-test";
    status = "oaky";
};
  • pwms = <&pwm0 0 500000 1000000 0>：**&pwm0表示要引用的PWM的控制器（根据需求可引用对应的引脚）；**0 表示对应 pwm0_0 下的具体通道；500000表示占空比；1000000表示周期；0表示极性。
  • pwm-names = "pwm-test"; 表示索引具体的pwm通道名称。其名称与devm_pwm_get(dev, "pwm-test")的第二个参数保持一致。

配置 PWM0_0 通道的引脚

示例配置，具体引脚复用请参照芯片手册

&pio {
    pwm0_0_pin_active: pwm0_0@0 {
        pins = "PD23";
        function = "pwm0_0";
    };
    
    pwm0_0_pin_sleep: pwm0_0@1 {
        pins = "PD23";
        function = "gpio_in";
        bias-pull-down
    };
};

&pwm0_0 {
    pinctrl-names = "active", "sleep";
    pinctrl-0 = <&pwm0_0_pin_active>;
    pinctrl-1 = <&pwm0_0_pin_sleep>;
    status = "okay";
};

编译内后，当系统启动即可启用 PWM0_0 通道输出波形

用户层调用 DEMO​

以下是一个使用 C 语言编写的操作 PWM0 的示例代码：

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

#define PWM_PATH "/sys/class/pwm/pwmchip0/"
#define PWM_EXPORT_PATH PWM_PATH "export"
#define PWM_UNEXPORT_PATH PWM_PATH "unexport"
#define PWM0_PATH PWM_PATH "pwm0/"

int main() {
    // 导出 PWM0
    FILE *export_file = fopen(PWM_EXPORT_PATH, "w");
    if (export_file == NULL) {
        perror("Error opening export file");
        return 1;
    }
    fprintf(export_file, "0");
    fclose(export_file);

    // 设置 PWM0 的周期和占空比
    char period_path[256];
    strcpy(period_path, PWM0_PATH);
    strcat(period_path, "period");
    FILE *period_file = fopen(period_path, "w");
    if (period_file == NULL) {
        perror("Error opening period file");
        return 1;
    }
    fprintf(period_file, "1000000000"); // 设置周期为 1s
    fclose(period_file);

    char duty_cycle_path[256];
    strcpy(duty_cycle_path, PWM0_PATH);
    strcat(duty_cycle_path, "duty_cycle");
    FILE *duty_cycle_file = fopen(duty_cycle_path, "w");
    if (duty_cycle_file == NULL) {
        perror("Error opening duty cycle file");
        return 1;
    }
    fprintf(duty_cycle_file, "500000000"); // 设置占空比为 50%
    fclose(duty_cycle_file);

    // 设置极性为正常并使能 PWM
    FILE *polarity_file = fopen(PWM0_PATH "polarity", "w");
    if (polarity_file == NULL) {
        perror("Error opening polarity file");
        return 1;
    }
    fprintf(polarity_file, "normal");
    fclose(polarity_file);

    FILE *enable_file = fopen(PWM0_PATH "enable", "w");
    if (enable_file == NULL) {
        perror("Error opening enable file");
        return 1;
    }
    fprintf(enable_file, "1");
    fclose(enable_file);

    // 删除 PWM0
    FILE *unexport_file = fopen(PWM_UNEXPORT_PATH, "w");
    if (unexport_file == NULL) {
        perror("Error opening unexport file");
        return 1;
    }
    fprintf(unexport_file, "0");
    fclose(unexport_file);

    return 0;
}

这段代码将会打开相应的文件，并向其中写入所需的数值来配置 PWM0 的周期、占空比、极性和使能状态。完成后，会关闭文件并最终删除 PWM0。

PWM 捕获模式 DEMO​

捕获模式原理：将两个引脚连接起来，对A引脚设置参数，用B引脚去捕获A的参数。

测试方法：用杜邦线将两个具有PWM功能的引脚连接起来，在一个引脚上设置需要的频率及占空比,在另一个引脚的下读取capture。

  • 根据普通调试接口的方法,对其中一个引脚设置频率及占空比。

示例配置，具体引脚复用请参照芯片手册

/pwm/pwmchip0 # echo 0 > export
/pwm/pwmchip0 # cd pwm0/
/pwm/pwmchip0/pwm0 # echo 20000 > period
/pwm/pwmchip0/pwm0 # echo 5000 > duty_cycle
/pwm/pwmchip0/pwm0 # echo normal > polarity
/pwm/pwmchip0/pwm0 # echo 1 > enable
  • 采用capture模式在另一个引脚上捕获频率及占空比。

示例配置，具体引脚复用请参照芯片手册

/pwm/pwmchip0 # echo 3 > export
/pwm/pwmchip0 # cd pwm3/
/pwm/pwmchip0/pwm3 # cat capture