Interfacing to a TFT display controller

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Interfacing to a TFT display controller

Contributor IV

I am interfacing to a 320x240 pixel TFT with a Himax HX8238-A controller.
I have difficulties understanding the timing relationships between
One problem concerns the vertical and horizontal front/back porches.
What signals are needed here?
I have AN3493, which is helpful but not detailed enough when
it comes to timing issues.
All comments and pointers to litterature are most welcome.
Anders J
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NXP Employee
NXP Employee
Anders, I assume you are talking about configuring the TFT controller to correctly provide those signals to the TFT panel?
If so, that isn't directly relevant to the Freescale product but here are some pointers that may help.
There are usually up to 4 relevant control signals for the panel: VSYNC, HSYNC, enable & pixel clock. These vary from panel to panel and so controllers need to be configurable to adjust the timing relationship between them. Not all panels need all the signals
The root signal is normally the pixel clock which ticks for each pixel value sent to the panel. The frequency of this must be correct. For a QVGA it is normally about 5MHz.
Next the enable signal indicates when the pixel data is being sent to the panel. The reason this is needed is that TFT panels require time to synchronise each line of data and each frame of data therefore the pixel information is not fed in a continuous stream.
The HSYNC signal indicates the start of each line of data and controls the delay in feeding each new line
The VSYNC signal indicates each start of a completely new frame of data and controls delays for this.
The relationships of these signals are how you must configure the controller and these are defined in the TFT panel timing diagram/table. You normally read the information off the TFT panel datasheet and plug those values into the controller. Sometimes the information is not explicit in the panel spec so you need to do some math to calculate the value required.
You need to check the configuration of your controller for their definition but normally the horizontal (line) "back porch" is the delay from the de-assertion of the HSYNC signal to the start of pixel data being sent (in pixel clocks). The horizontal "front porch" is normally the delay from the end of the line's pixel data to the assertion of the HSYNC signal. Basically these allow the panel to get ready for the data and have time to begin displaying it before getting ready for the next line. Vertical back and front porches are basically the same but for vertical signals (i.e. the delays are normally expressed in numbers of horizontal lines rather than pixel clocks).
If you are still in doubt it would be best to consult the manufacturer of the controller for some advice.

Message Edited by Steve on 2008-09-26 02:13 PM
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Contributor IV
Thanks for valuable input.
I especially appreciate how you explained the differences
between horizontal and vertical porches,
and how one is expressed in pixel clocks and the other one in horizontal lines.
I was confused about this, and you cleared it up.
Sorry about posting a non Freescale question.
I had nowhere else to go for competent help.
Thanks again,
Anders J
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