VGA Timings

[DevSter]

Hey hey hey i'm making a video card and i'm going to be using the VGA interface for a 65816 computer. Question though

Standard 640*480 resolution at 60hz requires a 25.175 mhz crystal. Around my area, these crystals are highly uncomon. I could go scrap some oldskool vga isa cards for these crystals, or something relative, but there might also be other alternatives i was thinking about. Would it hurt to use a 25.000 mhz crystal? The total scan width for a line would fall longer than standard timing, 200 ns longer, and verticle sync would be 59.524 hz (standard timing is 59.940 hz).

I really don't think it would hurt, i mean vga cards can switch resolutions dynamically changing the clock speed, and there are tons of clock speeds and they all can't get as close to 25.175 mhz =)

here is where i got most of my timing info. from http://www.epanorama.net/documents/pc/vga_timing.html if you don't wanna check google =P

[GARTHWILSON]

> ...a 25.175 mhz crystal. Around my area, these crystals are highly uncomon.

Digi-Key www.digi-key.com has them. Even if you're outside the USA it's easy to buy from these companies if you have a credit card. Shipping may be a killer if all you order is one crystal; but with a 1300-page fine-print catalog, they're bound to have something else you need.

[greener]

DevSter wrote:

Would it hurt to use a 25.000 mhz crystal? The total scan width for a line would fall longer than standard timing, 200 ns longer, and verticle sync would be 59.524 hz (standard timing is 59.940 hz).

I really don't think it would hurt, i mean vga cards can switch resolutions dynamically changing the clock speed, and there are tons of clock speeds and they all can't get as close to 25.175 mhz =)

I've been lurking again - still haven't got space to get back into things. Per your question though - it shouldn't hurt. With my old Gateway 15" (1571?) CrystalScan monitor, it just didn't show a picture or couldn't sync. When I was measuring the timing, I never hit the exact refresh rate.

If you're curious, visit my page.

Whatever you do, let us know. I'm watching to see what people are doing... it's really amazing what's done "just for fun" around here!!

[Rob Finch]

crystal frequency isn't critical. sync and blanking timings are. As long as you keep the right sync width and frequency, and right blanking intervals there should not be a problem.

I've seen some VGA adapters that use a 25.000 MHz crystal and other frequencies as well.

I'm using 28.636MHz in my own system, which potentially gives me 728 displayable pixels horizontally, but I just have a pretty red and white border of 84 pixels with a 560x384 display area in the centre.