Color banding with 24 bit parallel RGB LCD

Technical Support
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1

Hello.

I’m testing hardware project with Colibri iMX6DL module and have some issue with image displayed on LCD (Winstar WF70YTIAGDNG0#).

Files i’ve used for test are (“gradient-*.jpg”): link text

And on our display we get this: link text

There are visble color banding on the display. Is this correct for 24-bit parallel RGB and 32 bpp framebuffer? If not then should I look for solution in different OS configuration?

My current OS config:

  • Uboot env variable “vidargs” is set to ‘video=mxcfb0:dev=lcd,WINSTAR-WVGA,if=RGB24,bpp=32 video=mxcfb1:off fbmem=8M’.

  • Custom Device-Tree with enabled all 24 RGB data output pins.

  • Custom RGB timings in kernel source.

Output from “fbset” command is:

mode "800x480-60"
    # D: 33.333 MHz, H: 31.566 kHz, V: 60.125 Hz
    geometry 800 480 800 480 32
    timings 30000 26 210 13 22 20 10
    rgba 8/16,8/8,8/0,8/24
endmode
2

Hi @user622190

Could you provide your exact software version ( uname -a )? Have you done any changes to the device tree and kernel? If yes, could you please share them?

Thanks and best regards, Jaski

3

uname -a:

Linux (***) 4.1.44-00008-gf3637519a6aa-dirty #4 SMP Wed Feb 6 11:36:26 CET 2019 armv7l GNU/Linux"

My custom Device-Tree: link text

Changes in the kernel:

  • used “colibri_imx6_defconfig” configuration;

  • enabled IPC POSIX option;

  • enabled EDT-FT5x06 driver;

  • changed “mxc_lcdif.c” file, added display timings:

    {
    “WINSTAR-WVGA”,
    60,
    800, 480,
    30000,
    .left_margin = 26,
    .right_margin = 210,
    .hsync_len = 20,
    .upper_margin = 13,
    .lower_margin = 22,
    .vsync_len = 10,
    .sync = FB_SYNC_CLK_LAT_FALL,
    .vmode = FB_VMODE_NONINTERLACED,
    .flag = 0,
    },

4

I would suggest for you to have a look at Apalis iMX6 where unlike the Colibri iMX6 parallel RGB actually defaults to using 24-bit rather than 18-bit. Please also note that using 24-bit on Colibri won’t be compatible across other modules.

5

Thank you for the suggestion, but I’ve already managed to reconfigure my Colibri module to 24-bit RGB interface, and also lack of compatibility is not a problem for me.

Do you think that switching form Colibri to Apalis will solve my “color banding” issue (assuming that my display works correctly)? Do you know anything about some hardware/software problem with Colibri iMX6 module, that could be the cause of the issue?

6

Hi @jaski.tx

Do you have any new suggestion to my problem?

Best regards.

7

hi @user622190
Did you try the suggestion of marcel.tx?

8

Do you think that switching from Colibri to Apalis module will solve my issue? If yes, why? Are there any hw/sw problem with Colibri module and 24-bit RGB interface?

9

Thank you for the suggestion, but I’ve already managed to reconfigure my Colibri module to 24-bit RGB interface, and also lack of compatibility is not a problem for me.

You are very welcome.

Do you think that switching form Colibri to Apalis will solve my “color banding” issue (assuming that my display works correctly)?

While it should really work fine both on Apalis as well as Colibri iMX6 I am just saying that the Apalis iMX6 is configured for 24-bit out-of-the-box.

Do you know anything about some hardware/software problem with Colibri iMX6 module, that could be the cause of the issue?

No, we are not aware of any such.

10

Ok, thx.

We would prefer to stay with Colibri module, so that is why we try to pinpoint the source of our issue. I’m also tring to get any answer from Winstar, about this color issue, and will write an update here when I got some.

11

I have just received new information about Winstar WF70YTIAGDNG0# display from it’s distributor - there is an error in official specification (revision F) for this display. It works with only 18-bit RGB interface. So it looks like my issue here is closed.

12

Thanks for your Input. Thus you should not see any colour banding if you configure the module to 18-bit.

Best regards,
Jaski

13

Thx. I would also need to change hardware connection to the display to handle differences in 18-/24-bit interface.

14

yeah, that’s true. All the best.