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7. PWM

7.1. Introduction​

This chapter describes how to configure PWM.

The PWM driver of ROC-RK3568-PC-SE is kernel/drivers/pwm/pwm-rockchip.c

7.2. DTS configure​

There are three main steps to configure PWM: configure PWM DTS node, configure PWM kernel driver and control PWM device.

7.2.1. Configure PWM DTS node​

Add the PWM DTS configuration in the DTS source file kernel/arch/arm64/boot/dts/rockchip/rk3399-firefly-demo.dtsi, as shown below:

pwm_demo: pwm_demo {
        status = "disabled";
        compatible = "firefly,rk356x-pwm";
        pwms = <&pwm1 0 10000 1>;   //pwm1:PWM number   0 10000:PWM period in nanoseconds  1:polarity
        duty_ns = <5000>;   //pwm duty cycle activation time, unit ns
    &#125;;
  • pwms : number of PWM channels to be applied.
  • duty_ns : duty cycle activation time of PWM, unit ns.

7.3. Interface specification​

Users can use the PWM nodes generated by the above steps in other driver files. The specific methods are as follows:

  • (1) The following header files are included in the device driver files that need to be controlled by PWM :

    #include <linux/pwm.h>

    This header file mainly contains the PWM function interface.

  • (2) Apply PWM

    Using:

    struct pwm_device *pwm_request(int pwm_id, const char *label);

    Function to apply for PWM. Such as:

    struct pwm_device * pwm1 = NULL;pwm0 = pwm_request(1, “firefly-pwm”);

  • (3) Configue PWM

    Using:

    int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);

    Configure the duty cycle of PWM, for example:

    pwm_config(pwm0, 500000, 1000000);

  • (4) Enable the PWM function

    int pwm_enable(struct pwm_device *pwm);

    Used to enable PWM, for example:

    pwm_enable(pwm0);

  • (5) Control PWM output mainly uses the following interface functions:

    function: Used to apply for PWM

    struct pwm_device *pwm_request(int pwm_id, const char *label);

    function: Used to release the PWM requested

    void pwm_free(struct pwm_device *pwm);

    function: Used to configure the duty cycle of PWM

    int pwm_config(struct pwm_device *pwm, int duty_ns, int period_ns);

    function: Enable PWM

    int pwm_enable(struct pwm_device *pwm);

    function: Ban PWM

    void pwm_disable(struct pwm_device *pwm);

Refer to the Demo: kernel/drivers/pwm/pwm-firefly.c.

7.4. Debug method​

Check PWM registration status through the rich debug interface of the kernel, adb shell or serial port to enter the android terminal and execute:

cat /sys/kernel/debug/ PWM

To see if the registration was successful, the interface name and register address are returned.

7.5. FAQs​

Q1: Pwm could not register successfully ?

A1:

  • whether the DTS configuration file opens the corresponding PWM.
  • whether the IO port where PWM is located is occupied by other resources, you can check the reason according to the error return value.