AM625: Use GPIO0_33 as pwm for display backlight on Custom AM625 SOM

Aditya Tandulwadkar

Part Number: AM625
Other Parts Discussed in Thread: SYSCONFIG

Hello,
I'm using custom AM625 SOM and have connected RGB display which needs backlight to work.
I want to control the backlight for display using software.
The pin I've assigned to display backligt is L24, i.e. GPIO0_33
I want to use this pin as pwm to control the display brightness.
The changes I've did in k3-am62x-sk-common.dtsi are

        backlight {
                compatible = "pwm-backlight";

                led0 {
                        pwms = <&epwm0 0 1000 1>;
                        brightness-levels = <0 4 8 16 32 64 128 255>;
			default-brightness-level = <32>;
                        linux,default-trigger = "default-on";
			status = "okay";
                };
        };
        
&epwm0 {
	pinctrl-names = "default";
	pinctrl-0 = <&main_backlight_pins_default>;
	status = "okay";
};

    main_backlight_pins_default: main-backlight-pins-default {
		pinctrl-single,pins = <
			AM62X_IOPAD(0x088, PIN_OUTPUT, 7) /* (L24) USB1_DRVVBUS */
		>;
	};

and after these changes, in bootup logs I see is:

[    0.920737] pwm-backlight backlight: supply power not found, using dummy regulator
[    0.928640] of_pwm_get(): can't parse "pwms" property
[    0.933813] pwm-backlight backlight: unable to request PWM
[    0.939422] pwm-backlight: probe of backlight failed with error -2

What changes I'm missing?

Thanks & Regards,
Aditya T

over 1 year ago

0 Anshu Madwesh over 1 year ago

TI__Mastermind 18020 points

Hi Aditya,

Please see the SDK Documentation: https://software-dl.ti.com/processor-sdk-linux/esd/AM62X/09_02_01_09/exports/docs/linux/Foundational_Components/Kernel/Kernel_Drivers/PWM.html

Also review device tree: https://git.ti.com/cgit/ti-linux-kernel/ti-linux-kernel/tree/arch/arm64/boot/dts/ti/k3-am625-sk-rpi-hdr-ehrpwm.dtso?h=ti-linux-6.1.y

Best Regards,

Anshu

0 Aditya Tandulwadkar over 1 year ago in reply to Anshu Madwesh

Intellectual 550 points

Hello,

I've read it already and made those changes in device tree, but the display backlight isn't glowing.

Thanks & Regards,
Aditya T

0 Anshu Madwesh over 1 year ago in reply to Aditya Tandulwadkar

TI__Mastermind 18020 points

Hi Aditya,

Please see the Data Sheet Section 6.3.8 EPWM for the Pin Muxing options: https://www.ti.com/lit/ds/symlink/am625.pdf

L24 is not an option for an ePWM. If ePWM is what you want to use, then you'll have to change the pin that the device tree is calling.

If you want to use a GPIO to make a PWM signal, it technically possible by just turning it on/off but it may not be effective in your usecase.

Best Regards,

Anshu

0 Aditya Tandulwadkar over 1 year ago in reply to Anshu Madwesh

Intellectual 550 points

Hello,
Can you guide me, to which pin should I change in device tree. Because, keeping pin L24 as normal gpio, I've turned on and off the backlight of display, but I want to change the brightness level of display.

Thanks & Regards,
Aditya T

0 Anshu Madwesh over 1 year ago in reply to Aditya Tandulwadkar

TI__Mastermind 18020 points

Hi Aditya,

I will re-assign this thread to a team member. Please allow a day for a response.

Best Regards,

Anshu

+1 Nick Saulnier over 1 year ago in reply to Anshu Madwesh

TI__Guru** 100740 points

Hello Aditya,

It is going to be up to you to decide how you want to mux different processor signals to different pins. We do not know your custom board, so we cannot provide suggestions on pinmuxing.

The SysConfig Pinmux tool

The SysConfig Pinmux tool is a really useful tool to help you keep track of all of your pinmux settings. It can also be used to generate the pinmux settings to copy-paste into your Linux devicetree:

https://www.ti.com/tool/SYSCONFIG

You open a pinmux project like this:

If you used one of the TI EVMs as a template for your board design, we are in the process of publishing the default pinmux settings for the TI EVMs. You can find the settings for the AM62x EVMs attached to this e2e response:
https://e2e.ti.com/support/processors-group/processors/f/processors-forum/1348153/re-am625-how-to-set-pinmuxing-for-pru-subsystem/5139063#5139063

Regards,

Nick

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AM625: Use GPIO0_33 as pwm for display backlight on Custom AM625 SOM