No announcement yet.

Can't calibrate 7 inch display

  • Filter
  • Time
  • Show
Clear All
new posts

  • Can't calibrate 7 inch display

    I'm running the latest image of debian on a Rev C Beaglebone Black, and I just hooked up a 7 inch touchscreen for the first time. It looks nice, and seems to work in most regards, but it needs to be calibrated or something. The cursor is jumpy, and it won't go near the of the screen. The datasheet makes it sound like some kind of calibration program should start automatically when the BBB detects the cape, but that never happened, and I can't find anything on how to invoke the calibration process directly.

    The datasheet does say you can force a recalibration by deleting the file /etc/pointercal.xinput, but there is no such file.

    Also, the datasheet says that you can adjust the brightness by changing the value in the file /sys/class/backlight/backlight.10/backlight. I see no such file. In fact, there's no backlight.10 directroy at all.

    Do I need to install some kind of package that used to be part of Angstrom, but isnt' any longer? The datasheet assumes that the BBB has the Angstrom distribution, but that hasn't been the standard installation for the BBB for a very long time.

  • #2
    Hi rsfairman,

    For images like Debian 8.6 2016-11-06 4GB SD LXQT, you can try checking if your screen is detected by the BBB. Then you should be able to see the directory /sys/class/backlight/backlight/brightness. You can then try to change this value after the cape has been placed.

    Related forum post:
    4D 7.0" LCD CAPE on BBB running latest dabian linux

    In the similar image, if you are able to click the start button(by touch or mouse) at the lowest left corner, under the Preference tab, there should be a 'Calibrate Touch screen' application which you can use to force calibrate the 4D cape.

    For older images of debian like the 2015 releases, /etc/pointercal.xinput can be found, it seems they relocated this one for newer images.

    Hope this helps,
    Last edited by Joseph Herrera; 9 February 2017, 03:35 PM.


    • #3
      Thanks. Here's what worked.

      It turns out that the critical thing here is that you must log in as root for the calibration program to start. Once that happens you can (re)calibrate from the menu. I don't think the menu item is visible until then (but maybe it was).

      Once you have access to that menu item, run it and a shell window will pop up showing the calibration parameters. Ignore most of what that window says, but note the line

      Option "Calibration" "1 2 3 4"

      where "1 2 3 4" will be four numbers generated by the calibration process.

      To make the calibration settings permanent, edit the file


      Scroll down to the block that says

      Identifier "evdev touchscreen catchall"

      and add the line noted earlier to that block. It should be added at the end of the block, just before the "EndSection".

      Also, the information in the datasheet is not quite right on how to adjust the brightness. To do this, edit the file (as root)


      It holds a value in the range 0-100 (in ASCII). Lower values make the screen dimmer, and the value persists across boots (so don't change it to '0' unless you have some other way of changing it back).