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Interface a digital RGB LCD controller to a VGA monitor or projector

Question asked by pmrshudson on Jan 22, 2013
Latest reply on Feb 12, 2013 by GuenterL

I've got a project where I want to convert RGB digital into a VGA output that could be used with a monitor or projector. I've created a circuit using the ADV7123 that works OK with an analog monitor, but gets bad results using a LCD monitor or projector input. I believe the issue is due to timing of the sync signals, as I can adjust the timings and it behaves better but not all the time. Here are more details on the design:

 

The CPU I am using has a RGB output (6 bits per channel), pixel clock, horizontal sync, and vertical sync. I have some control (via software) of the pixel clock period, vertical and horizontal sync pulse width, front porch, and back porch timings. I'm trying to get a 640x480 output at 60Hz. Here are the ideal timings I have seen listed on the internet:

 

Pixel clock freq = 25.175MHz

Horizontal

front porch = 635ns

sync pulse = 3.813us

back porch = 1.907us

Whole line = 31.77us

 

Vertical

front porch = 318us

sync pulse = 63.6us

back porch = 1.049ms

Whole frame = 16.683ms

 

The closest pixel clock I can get from my CPU is 26.096MHz. I've adjusted the other timings to be as close as possible to the ideal values. I also found that I needed to make a composite sync from the horizontal and vertical signals and fed this into the BLANK input of the 7123 to get a blanking signal on the RGB outputs from the 7123.

 

If I plug the circuit output into the VGA connector of a flat screen monitor or projector, it has a hard time getting in sync with my signal. Can anyone offer advice on what could be wrong? What signal(s) does the projector sync to?

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