Electronic – VGA timing difference in pixel frequency

vga

I'm trying to determine what is the correct timing for 640×480 VGA. Using Tinyvga.com for 640×480@60Hz I get a pixel frequency of 25.2 Mhz and not 25.175 Mhz.

800*525*60 = 25200000

I have wrote assembly for a AVR outputing Hsync and Vsync and my logic analyzer shows the freq per frame is 60.06hz which is correct for a oscillator of 25.175Mhz.

If I use a 25.2Mhz oscillator the freq per frame is 59.94hz.

Which is right?

Best Answer

25.175 MHz is the official frequency, used back in the day in original VGA card and today in digital video interfaces. The horizontal and vertical timings are designed to be very close to NTSC frequencies, so the vertical refresh rate is not 60 Hz but approximately 59.94 Hz