Disable H/VSync at LVDS output

Technical Support
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1

Hi,

I have a 1024x768 LVDS Display here that requires Hsync and Vsync to be held low during operation.
(Running Linux image 2.6 on Apalis iMX6)

Satement in Datasheet:

Since this assembly is operated in DE
on ly mode, Hsync and Vsync input
signals should be set to low logic
level. Otherwise, this assembly would
operate abnormally

I tried to simply set this in the device tree:

hsync-len = <0>; /*136*/
vsync-len = <0>; /*6*/
hsync-active = <1>;
vsync-active = <1>;

In my opinion this should do the trick, but I’m not getting any output as soon as i set one of the two sync-len parameters to 0. When both are 1 or greater i get the Image shortly before it vanishes and gets all colored vertical lines.

Is there any special Parameter to disable the sync signals to set?

Thank you very much in advance,
Moritz Wagner

2

Hello

We’ve faced this issue already.

The driver tests and refuses to work when hsync and/or vsync are set to 0.
However we have not seen any adverse effects when removing the test.

Remove the following lines around line 1064 in drivers/mxc/ipu3/ipu_disp.c

       if ((v_sync_width == 0) || (h_sync_width == 0))
                return -EINVAL;

Regards
Max

3

Thank you very much for the quick answer,
but when I comment out these two lines and recompile the kernel this makes no difference.

Only the following boot message error vanishes after making the changes:
mxc_sdc_fb fb.19: mxcfb: Error initializing panel.

But still no signals at the LVDS in my case until setting h/vsync at least to 1.

4

Hello,

I tried a whole set of configurations now but still won’t get any correct display output.
Is it possible, that i have to set a special option for using DE-only without sync?
I’m still confused that the whole LVDS output is quiet as soon as any sync length is zero. I investigated the driver further but could not find a problem yet. Neither do i get any errors/warnings for ldb during boot.

Thank you very much for your help!

Moritz

5

My problem was finally solved by leaving the driver untouched and simply setting bpp=32 at the vidargs uboot variable (although the former display worked fine without this argument). I still don’t really know what this parameter changes but it works now.

6

bpp sets the number of bits per pixel used in RAM.

I don’t see why that affects the ldb driver to work with HSYNC/VSYNC=0, however great news.

Max