Pwm led high at boot with DTS

General Discussion
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1

Hello everyone, I’m working with beaglebone black kernel 4.19-ti (debian 10). I want a led controlled by pwm to light up at boot time. I first wrote a DTS, to practice, turning on the led from boot to mode 7 (gpio) and it worked well, I switched to mode 4 and there it’s more complicated can somebody look at just these few lines?

 fragment@3 {
target-path="/";
__overlay__ {

leds {
pinctrl-names = "default";
pinctrl-0 = <&bb_pwm_led_pins>; /*not sure*/

compatible = "pwm-ehrpwm"; /*not sure*/

P8_13 {
label = "P8_13";
pwms = <&ehrpwm2 1 500000 0>; /*not sure*/
pwm-dutycycle = <500000>;  /* 100% duty cycle */
default-state = "on"; /*not sure*/
status = "okay"; /*not sure*/
};
};
};
};

I can put the whole file if needed.
Thanks a lot in advance,
Jean

2

Maybe it’s better but I don’t really know what I’m doing, see following all the code :

/*EHRPWM2B, P8_13*/
/dts-v1/;
/plugin/;

#include <dt-bindings/pinctrl/am33xx.h>
#include <dt-bindings/pwm/pwm.h>
#include <dt-bindings/board/am335x-bbw-bbb-base.h> /*???*/

/ {
	compatible = "ti,beaglebone", "ti,beaglebone-black";

	fragment@0 {
		target = <&am33xx_pinmux>;
		__overlay__ {
			bb_pwm_led_pins: pinmux_bb_pwm_led_pins {
				pinctrl-single,pins = <
					AM33XX_IOPAD(0x9a4, PIN_OUTPUT_PULLDOWN | MUX_MODE0)  /* OR BONE_P8_13 (PIN_OUTPUT_PULLUP | MUX_MODE4) */
				>;
			};
		};
	};

	fragment@1 {
		target = <&ocp>;
		__overlay__ {
			pwm_test_P8_13: pwm_test_P8_13 {
				compatible = "pwm-ehrpwm";  /* For eHRPWM or maybe it is gpio-pwms ???? */
				status = "okay";  /* Enable the PWM device */
				pwms = <&ehrpwm2 1 500000 0>;  /* PWM2B, period = 500000ns (2kHz), polarity = normal */
				pwm-dutycycle = <250000>;  /* 50% duty cycle */
				pinctrl-names = "default";
				pinctrl-0 = <&bb_pwm_led_pins>;
			};
		};
	};
};
1 Like
3

Where do you think I can find documentation on this subject? What is the difference between:
AM33XX_IOPAD(0x9a4, PIN_OUTPUT_PULLDOWN | MUX_MODE0) AND
BONE_P8_13 (PIN_OUTPUT_PULLUP | MUX_MODE4)
It is
compatible = "pwm-ehrpwm";
OR
compatible = "gpio-pwms"; or maybe gpio-pwm?

4

if you have an idea, even a link, I’d love to hear from you.

5

At a guess you need leds-pwm.

Check the kernel devictree documentation Documentation/devicetree/bindings/leds/leds-pwm.txt

Now how it all goes together I have no idea. I would start by searching the dts files for “pwm-leds” to see if they have been implemented for any other device and maybe get some idea from that what you need to set.

1 Like