PWM

Introduction

Face-RK3399 has two PWM outputs:

PWM1: LCD_BL_PWM

PWM2: VDDLOG power supply

This chapter describes how to configure PWM.

The PWM driver of Face-RK3399 is "kernel/drivers/pwm/pwm-rockchip.c"

Configurtion

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

DTS Configuration

Add the PWM DTS configuration in
"kernel/arch/arm64/boot/dts/rockchip/rk3399-firefly-demo.dtsi":

   pwm_demo: pwm_demo {
      status = "okay";
      compatible = "firefly,rk3399-pwm";
      pwm_id = <1>;
      min_period = <0>;
      max_period = <10000>;
      duty_ns = <5000>;
   };

pwm_id : Number of PWM channels to be applied.

min_period : Minimum period length.

max_period : Maximum period length.

duty_ns : Duty cycle activation time of PWM, unit ns.

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  struct pwm_device *pwm_request(int pwm_id, const char
*label);  Function to apply for PWM, for example:  struct pwm_device *
pwm1 = NULL;pwm0 = pwm_request(1, “firefly-pwm”);

(3) Configue PWM  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: Request a PWM  struct pwm_device *pwm_request(int pwm_id,
const char *label);

function: Release the PWM requested  void pwm_free(struct pwm_device
*pwm);

function: 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: Disable PWM  void pwm_disable(struct pwm_device *pwm);

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

Debugging

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

The interface name and register address will be returned if PWM
registration is successful.

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.