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LCD Timings

Hi, I have a board with an ARM9 (AT91SAM9G45) and I have connected this LCD to the LCD controller of the microcontroller. I read the HX8262A datasheet but I only found the Pixel clock, Horizontal Pulse Width and Vertical Pulse Width parameters, but I need the Horizontal Front Porch, Horizontal Back Portch, Vertical Front Porch, Vertical Back Porch too. I made a test with the following configuration and I couldn't get the LCD working.

Pixel Clock = 33.33Mhz
Horizontal Front Porch (HFP) = 96
Horizontal Back Porch (HBP) = 96
Vertical Front Porch (VFP) = 21
Vertical Back Porch (VBP) = 22
Vertical Pulse Width (VPW) = 2
Horizontal Pulse Width (HPW) = 64 (I can't use 128 because AT91SAM9G45 internal LCD controller only support 1 to 64)

Does anyone know the correct values to run the lcd? Best Regards.


Guest's picture
June 2, 2011

Can you see anything on display? If there is no single dot or line, there can be more troubles than just porches.
Front and back porch just mean blank video signal time. For digital video inerfaces pixel data validity is ruled by DataEnable (DEN for HX2862).
If your board has digital video output, DEN should be there, ready for using. If you are digtizing analog video, you have to form DEN yourself. DEN should be active only in active video lines and only during active portion of line.
Take a look on chapter "11.1.2 Data input format" in datasheet, there are proper video signal waveforms.
BTW, in my project (on other but similar controller), which contained digitizing, I got DEN strictly from HSync. But analog video porches and sync waveforms were been altered from standard VGA timings, so active part of line was transmitted at the same time, as HSync was high.

Guest's picture
June 3, 2011

I can see something in the display, but the picture is not stable. I have tried to draw a third of screen red, but i can not exactly a third and the picture is not stable.
My board have digital video output, this have vsync, hsync, enable (DEN), clock and 8 bits for Red, 8 bits for green and 8 bits for blue. All lines are wired to the LCD interface.
I have also tried to wire only the clock lines, enable and color, but the result is the same.
Any idea?

Guest's picture
June 4, 2011

Okay, I digged datasheet. Looks like default timings of HX2862 for 800x480 mode are:
active pixels=800
DEN should be active during active pixels, e.g. from 216 to 1016. Check this with oscilloscope.
active lines=480
DEN should be active only during active lines, check this too.
To get things working you can tune either video source timings to match controller defaults or set up controller registers to ARM board defaults. The first seems easier.
I can even do X modlines for you, if your board runs linux :-)
Hope this will help you. Would like to know the result.

Guest's picture
June 5, 2011

I have tested your config, but not success. I have also checked DEN and is active only during active pixels and active lines, but i saw that the clock line is not square (looks more like a sine wave), so i suspect that the problem is the clock line.
I have planned put a fast smith trigger on the clock line signal to fit. Could it work?

Guest's picture
June 6, 2011

That's sad.
By the way I had been observing almost sine wave on DotClock in my design too, and screen controller (D51E5TA7601) got it right.
So probably trigger will not help you, but anyway you should try it.

Guest's picture
June 7, 2011

Finally a week ago and I have time to work on the LCD. The problem was the configuration of the DMA by default brust length is 4 words, but in my case had to be 8 words. Thanks for your time and help.