[beagleboard] VGA display issue

Hi All,

I'm trying to bring up display on the VGA interface on my board. We
are using THS8200 interfaced to the OMAP3525. I've attached the
display schematics (VGA_Display.jpg in the files section). I've also
uploaded the DSS configuration and the THS8200 configuration values in
the file THS8200_and_DSS_Configs.txt (uploaded in the files section).
We are configuring for 1024x768@60Hz. we are using the following
kernel command line for the display interface: omapfb.mode="dvi:

We have measured the Pixel clock, VSYNC pulse width, VSYNC frequency,
HSYNC pulse width and the HSYNC frequency and these are matching the
expected standard values for the above resolution:
1. Pixel clock: 65.4MHz
2. HSYNC pulse width: 2.1us
3. HSYNC Frequency: 48.54KHz
4. VSYNC pulse width: 124.0us
5. VSYNC Frequency: 60.004Hz

The initial console (including the Beagle logo) is getting displayed
on the VGA monitor (Dell monitor). The issues we are facing are as

1. Only the Dell VGA LCD monitor is able to sync and display kernel
console messages and the logo. Other VGA LCD monitors and CRT monitors
are not displaying the image.

2. Once every few minutes, the display goes blank for about a second
and the time between the blanking does not seem to be periodic.

3. Once the Angstrom distro comes up, we see the Angstrom login
screen, but the colors are messed up.

What could be the issue?


Hi All,

Some more observations:

  1. There are subtle flicker in the console output messages (that are visible on close scrutiny of the screen).

  2. Also there is a phase shift between the VSYNC output from the OMAP and the VSYNC output from the THS8200. The relationship between the HSYNC and VSYNC at the output of OMAP and THS8200 are drawn in the attached file. Is this OK?


HSYNC_To_VSYNC_Waveforms.txt (662 Bytes)


The real problem is in the Framebuffer of DSS driver, i have been worked on the kernel and nothing.
The only way that i founded was this link:


it works on THS8200.

Best regards.


The pix data are in phase with DSS or THS output?

I am using the beagleboardtoys vga adapter, this have THS8200, and i don´t have any issues with the script.
And the flicker is normal like the wrong colors when a full screen image is displayed.


2010/4/26 Sajith P V <sajithpv@gmail.com>

Hi Jaime,

Thanks a lot for the suggestion of playing around with the display settings through sysfs. With these changes, now the flicker is some what reduced and also the whole display is now getting blanked out for about a second with long gap in between.

Can you please tell me what causes the whole display to be blanked out?



Do you remove de MR flags on the bootargs?

I don´t have that issue, here are the values on my timming file.




If you modified the DSS driver, try to restore on default parameters for display.


2010/4/27 Sajith P V <sajithpv@gmail.com>

Hi Jaime,

We are not using the MR flags now. With your settings we are getting a display that almost looks right on a CRT monitor, but it still does not sync in other VGA monitors.

I checked the voltages on the analog outputs from the THS and found them to be as follows:

Max voltage on the RGB signal lines: 1.3v.
HSYNC & VSYNC voltages 1.8v.

Going through some of the references on the web, it looks like the VGA standard specifies that the HSYNC and VSYNC voltages should be TTL (5V) and the max peak-to-peak swing on the RGB lines must be limited to 0.7v. Could this be the reason why some of the monitors are not syncing, or some monitors are showing the display, but with color issues? We are parallely trying to build a circuitry that will translate the voltages to TTL level and do the test.



Hi Sajith.

The THS can be powered with 3.3 V on the analog supply, so you can try change this.


2010/4/30 Sajith P V <sajithpv@gmail.com>

Hello Jaime,

Today we tried with the external circuitry to boost up the HSYNC and VSYNC voltages from 1.8v to 5v. It finally started syncing with all the monitors. Also the RGB signal voltages was brought down to 0.7 peak.

THS was already powered from 3.3V.

We are now fine tuning the timings using the sysfs hack you mentioned in the earlier mails.

Thanks for all the info you gave us.