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Altera_Forum's avatar
Altera_Forum
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16 years ago

VGA 25MHz, Clock 100MHz, Help!

Hi, I would like to use a 100MHz clock to drive my logic, but the problem is that VGA needs a clock of 25MHz. Right now, I am using 2 PLLs to create a 100MHz clock and 25MHz clock by us...
Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
16 years ago

eject is right, I should be using 25.175 MHz, not 25 MHz. This is now corrected.

I am now using CLOCK_27 to generate the 25.175MHz clock, and CLOCK_50 to generate the 100MHz clock.

I have included a small test verilog module called vgatest in the vgatest.v attachment. It shows exactly what I am trying to achieve: Generating a counter at the normal clock, and feeding this to vga output.

However, when I program this test, my vga screen output is shaking.

When I move all the counter logic into the vga_clk domain, the image is correct and clean. This is ofcourse not an option, since the color needs to be generated from the normal clock.

MSchmitt, can you explain in more detail the DFF method?

I don't quite understand what you mean.

Regards