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Backlight flickering, PWM Frequency, Burn-In

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  • Backlight flickering, PWM Frequency, Burn-In

    I am having an issue where the backlight of my GEN4-4DCAPE-70CT-CLB connected to a BeagleBone Black is flickering after I change it to something other than 100%.

    I checked the signal with an o-scope at the screen pin and looks like a typical square wave, but I did notice it was 2khz and the 4D datasheet calls out a minimum of 5khz, so my first question is what should the PWM frequency be?

    Second, the 4D datasheet says there is a one-wire communication option that is more stable and that a driver is supplied by 4D for debian based systems.
    I am thinking that may be a better option so where might I find that driver?

    (Edit) And last question, is dimming the screen even needed to prevent burn-in?

    Thanks!

    (Update, End of Day) I found that the datasheet for the TPS61165 backlight driver specifies a recommended range of 6.5khz to 100khz for the PWM, so I think I will try a 50,000ns period or 20khz for the PWM frequency and see what happens.
    Last edited by Jeff Renga; 30 June 2022, 07:11 AM. Reason: Added Update

  • #2
    Hi Jeff,

    Welcome to the forum.

    Apologies for the late reply.
    Can you provide image of the display which shows the flickering?

    I suggest to check this link for more information on the display driver and settings.
    To prevent burn-in you can dim the display or show a screen saver.

    Best Regards,
    Ferdinand

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    • #3
      Hi Ferdinand,

      I am not sure I could show flickering in an image. The full screen changes the brightness rapidly as seen from the backlight at the rear of the screen.

      I looked at the device tree overlay at the link below and it seems like the PWM is the only option for the backlight control. Is that correct?
      https://github.com/beagleboard/bb.or...7C-01-00A1.dts

      I was able to edit the file and compile it to change the PWM from 2khz to 20khz and that did fix the backlight flickering but is that correct or should I be doing something different?

      Thanks,
      Jeff
      Last edited by Jeff Renga; 7 July 2022, 03:05 AM. Reason: Tried to fix link

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      • #4
        Hi Jeff,

        Thank your update.

        Good to know that you manage to change the PWM and fix the backlight flickering. As it was on the file, it seems that it is the only option for the backlight control. Also, if it fine with you to share the steps on how you change and compile the file to help others who will encounter this?

        Regards,
        Ferdinand

        Comment


        • #5
          The process I went through:

          The TPS61165 backlight controller on the GEN4-4DCAPE-70CT-CLB screen recommends a PWM frequency of 6.5khz to 100khz.

          I found which device tree file was being used by reading the cape/screen EEPROM over the i2c bus (assuming the dip switches are set to the default address of 0x54, both on):
          cat /sys/bus/i2c/devices/2-0054/eeprom (This command may be slightly different depending on the configuration of the BBB)

          Response: U3 014D 7.0 LCD CAPE - 4DCAPE-70CT00A14D SystemsBB-BONE-4D7C-011174D000017 r r r r 7

          Depending on which BBB image is used, the device tree overlay source may already exist at /opt/source/bb.org-overlays/ on the device or can be cloned with the following command:
          git clone https://github.com/beagleboard/bb.org-overlays

          I then found the matching device tree source file for the 4D 7" capacitive touch screen: BB-BONE-4D7C which is bb.org-overlays/src/arm/BB-BONE-4D7C-01-00A1.dts.

          I found the section for the backlight and changed the PWM value.
          It was 500,000ns or 2khz: pwms = <&ehrpwm1 0 500000 0>;
          I changed it to 50,000ns or 20khz: pwms = <&ehrpwm1 0 50000 0>;

          I then compiled and installed the BB-BONE-4D7C-01-00A1.dtbo device tree blob file by running the install script from the bb.org-overlays directory:
          ./install.sh

          This updated the BB-BONE-4D7C-01-00A1.dtbo in /lib/firmware/

          I then rebooted and confirmed the change with an oscilloscope after changing the backlight with the following command:
          sudo echo 50 > /sys/class/backlight/backlight/brightness

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          • #6
            Hi Jeff,

            Thank you for those information.
            This will be helpful to others.

            Best Regards,
            Ferdinand

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