ARM Technical Support Knowledge Articles

Why do I get a single black line on my display when i change the resolution?

Applies to: PL111 Colour LCD Controller

Scenario

I have a 480 x 272 colour TFT panel connected to PL110. When I program PL110 to drive it as a 480 x 180 panel, or other values less than 272 pixels in the y-direction, I get a black line part way down the panel then the picture repeats. Why?

Answer

This is due to the Vertical Front and Back Porch valus not being set up correctly in the LCD controller. To display an image that is not the full depth of the panel, you must program the VFP and VBP parameters correctly.

The formula to use for equal-sized Vertical Front and Back Porches is:

Porch width = (Size of display - size of image)/2

So, using a panel depth of 272 pixels and a desired image depth of 180 pixels:

Porch width = (272 - 180)/2
            = 92/2
            = 46

Therefore, VFP = VBP = 46

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