A20 + 7" LCD with flickering

[dudo]

Hi all.

As I also mentioned in one A13 topic, I have A20 OLinuXino board with 7" LCD and X-windows started on Debian wheezy. There are some sporadical flickerings like display is fast moving up and then down in short period of time. Without cursor movement it can happen a few times in single minute. script.bin is from original image downloaded from Olimex and adopted for LCD display. Kernel is 3.4.79+, custom precompiled.
The same thing happens with second A20 board, but with A13 is ok, without flickering.

When I connect 10.3" LCD on A20, flickering happens rarely, maybe few times in single hour and those aren't so expressed or intensive like ones with 7" display. I could say that it could be hardly seen.

[martinayotte]

Hi Dudo,

I've saw that you read my latest post on A13 forum.
I have also some LCD 10in, and don't have any flickering with those.
I've notice that if I put a finger on the 7in PCB, the flickering seems to be reduced.
I hope it is not an hardware design problem with an unconnected/floating wire getting into "antenna effect" ...
I wonder if Olimex folks can tell us what is the changes requires to "change_display.sh" script as it is mentionned in the other thread. Maybe they have done the fix on recent Debian image, but I want to stick with the current image I'm using, so I need to merge the fix into it.

[dudo]

As I mentioned above, the same 7" LCD works fine (without flickering) on A13 board, so I suppose connectors should be fine. But, I can make one more test. A20 board + second 7" display, to be 100% sure where the problem is.

[Ntronics]

I have the same issue on A20 + 7" LCD

[kylebassett]

Hi guys,

Try using a shorter cable or shielding the one you have.  I had some issues with this as well, and a shorter cable solved the problem.  Hope this helps!

[dave-at-axon]

This sounds more like a configuration issues than a hardware one and also the fact that Olimex mentioned this in another posting with the same issue when running Debian.

I have an A20 running Android and with the original Olimex A13-LCD7 it's rock solid, yet someone here posted that their A20 with the 7" and Debian was also having the same issue.

A small error in the timing would cause the display to be unstable. I've seen this with other displays when initially setting up timing etc.

[Alep]

Hi all

For this problem I solved by wrapping the CABLE-IDC40 with aluminum foil for food without connect it to the ground. To prevent short circuits insulated with tape or other system

[otyugh]

https://www.olimex.com/forum/index.php?topic=1942.msg10886#msg10886

Worked quite much for me. Still few flickering, but much lesser.
Good luck.

[BMK]

I have the same issue today with a newly purchased A20 and LCD7.

I am using the
'official' debian image from the link on the wiki.

I found this:

https://www.olimex.com/forum/index.php?topic=2933.msg12278#msg12278

LubOlimex mentioned that there was a typo in the display configuration script but we never got to hear what exactly this was...

If it was fixed, it looks like it never made it into the 'official' image.

I spent a while trying different settings in the change_display_a20_OlinuXino.sh. I noticed that reducing the freq value slightly improved the flickering. I kept reducing in steps to see what value worked best and eventually arrived at 23.

I have no idea what this means, or what I have actuall done, but it works!
Maybe something has drifted in the LCDs or other components since Olimex initially tested the settings?
I would appreciate the opinion of someone more experienced


  "7.0")
    x=800
    y=480
    freq=23
    hbp=46
    ht=1055
    vbp=23
    vt=1050
    vspw=1
    hspw=30

[martinayotte]

Is the someone who figured out how to get the real fix ?
BMK, could you share your modified "change_display_a20_OlinuXino.sh" script ?

I still get flickers depending of what is been displayed on LCD (some background images make the problem worst than others)

LubOlimex ? could you explain what was the fix you've mentionned back in March ?

[dudo]

Currently, I don't have flickering problems any more. But, there are some strange or invalid properties in script.bin which came with A20 image release7. When translated to .fex, 2 propeties have some different names:

lcd_vspw
lcd_hspw

than defined in fex guide or from script.bin from A13 image that I have:

lcd_hv_vspw
lcd_hv_hspw

Also, value of lcd_io_cfg0 is sometimes 0 and sometimes 268435456.

[Marc Coussement]

Hi,

I have a setup with A20 and 10inch Olimex LCD connected with the standard IDC40 cable.

I have the same issue vertical jumping of the image.

After done some measuring, i found some basic hardware design errors.

1/ The Dotclk is critical and running > 40 Mhz TTL signal over a IDC40 cable is simply not done unless one uses a signal/ground wire layout in the IDC cable, uses correct impedance matching and uses the correct driver.
2/ in case of the Olimex cable only a few Ground signals are used for up to 27 fast changing and time critical TTL signals. Equals bad design!
3/ The Dotclk signal is routed next to the vertical sync signal. Due to capacitive coupling in the flat cable the vertical sync signal is disturbed.

Solutions.
1/ I have a semi solution: at the LCD side of the cable. Put a 220R resistor on the vertical signal to the ground. And put a 470R resistor on the Dotclk to ground.

2/ Final solution don't use IDC40 cable, or use a very very short one. The best solution is connecting the LCD direct to the CPU board with a solid ground plane.

Marc,

[dave-at-axon]

Nice work Marc.

I've seem some issues with the Olimex IDC cable. It's too long you as noted and I switched to a shorter one of about 100mm. This seems to be stable but I would like to consider a replacement option seeing as I use a custom LCD anyway.

I am looking at making a small 4 layer PCB that plugs into the PCB header and routes the LCD signals via a 50 way FPC cable instead. The additional lines will allow me to place GND lines between those that need it. The more flexible FPC will also help with manufacture too.

When I have gotten to this stage, I will check carefully the DOTCLK and keep this away from the other lines. My LCD does not use V and H sync so I can drop these from the cable although if I plan to allow others to use the same board with the Olimex displays, I may leave them in.

[Marc Coussement]

Hi,

Be careful with the length of your FPC cable. depending on the length a 7inch with WVGA resolution may work as the dotclk is lower than the 10inch WVGA resolution.
Consider a LVDS transmitter-receiver solution for longer cables.
Running 27 TTL signals on a cable may give bad result on EMC compliance test.

In fact all signals are critical even RGB signals, all signals should respect the "5 ns" setup time to dotclk.
See your LCD timing specifications.

Another problem can be ground bounce (difference between Ground on LCD and ground on CPU) when all signals are changing form logic "0" to logic "1" at the same time, consider white characters on a black background. If signals are not within specifications one will have a less sharp image.

Marc,

[deskwizard]

Just a random thought about the need for more grounds in the cable, isn't that why ata/133 cables used 80 conductor cables on 40 pins IDC connectors? I don't have the required cable or hardware to test this theory but it sounds like the fix to a similar problem...

But then again, I'm no engineer ... Just a thought

DW