PWM Usage

This document introduces how to use hardware PWM.

By default, the DTS configuration file specifies PWM_F as the pin used to control the backlight of a VBO screen.

The following code modification is required:

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diff --git a/arch/arm64/boot/dts/amlogic/kvim4.dts b/arch/arm64/boot/dts/amlogic/kvim4.dts
--- a/arch/arm64/boot/dts/amlogic/kvim4.dts
+++ b/arch/arm64/boot/dts/amlogic/kvim4.dts
@@ -1749,13 +1749,13 @@
                };
        };
 
-       bl1_pwm_off_pins:bl1_pwm_off_pin {
-               mux {
-                       groups = "GPIOY_8";
-                       function = "gpio_periphs";
-                       output-low;
-               };
-       };
 

diff --git a/arch/arm64/boot/dts/amlogic/mesont7_an400-panel.dtsi b/arch/arm64/boot/dts/amlogic/mesont7_an400-panel.dtsi
--- a/arch/arm64/boot/dts/amlogic/mesont7_an400-panel.dtsi
+++ b/arch/arm64/boot/dts/amlogic/mesont7_an400-panel.dtsi
@@ -1442,13 +1442,13 @@
 
        backlight1{
                compatible = "amlogic, backlight-t7";
-               status = "okay";
+               status = "disabled";
                index = <1>;
                key_valid = <0>;
                pinctrl-names = "pwm_on",
                                "pwm_off";
-               pinctrl-0 = <&pwm_f_pins>;
-               pinctrl-1 = <&bl1_pwm_off_pins>;
                pinctrl_version = <2>; /* for uboot */
                interrupts = <0 197 1

PWM-F Harware Interfaces

PWM Usage

  • Export PWM
    1
    root@Khadas:~# echo 1 > /sys/class/pwm/pwmchip4/export
  • Configure PWM period
    1
    root@Khadas:~# echo 1000000 > /sys/class/pwm/pwmchip4/pwm1/period
  • Configure PWM duty cycle
    1
    root@Khadas:~# echo 500000 > /sys/class/pwm/pwmchip4/pwm1/duty_cycle
  • Enable PWM
    1
    root@Khadas:~# echo 1 > /sys/class/pwm/pwmchip4/pwm1/enable
  • Disable PWM
    1
    root@Khadas:~# echo 0 > /sys/class/pwm/pwmchip4/pwm1/enable

If you successfully turn on the hardware pwm, you can see the waveform through the oscilloscope:

pwm-Oscilloscope

Note

If you use an oscilloscope to check the waveform, remember that the board and the oscilloscope need to share the same ground.

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