Hi. I'd like to build an 80s-style display controller, but I want to output the image to a VGA monitor.
I've seen references to a 320x200 "doublescan" mode, and an XFree86 config file that specifies the timings as:15.75MHz horizontal, 37.9kHz vertical. 85.3Hz. H: 320 336 368 416 / V: 200 200 202 222
I believe the numbers mean: 320 pixel Video. (336 - 320) = 16 pixel R.Border + F.Porch. (368 - 336) = 32 pixel H.Sync. (416 - 368) = 48 pixel B.Porch + L.Border.200 line Video. (200 - 200) = 0 line B.Border + F.Porch. (202 - 200) = 2 line V.Sync. (222 - 202) = 20 line B.Porch + T.Border
I have a couple of questions:
- 416 x 202 x 85.3 = 1/2 15.75MHz specified, so I'd like to know how the doublescan mode works.
Is it an interlace mode? If so how do you wiggle the VGA connector pins to signal odd/even frames?
Or is it really a 320x400 mode, with every line output twice?
- The vertical timing suggets a zero Bottom Border and Front Porch. Is that right? Does the V.Sync come immediately after the last video line?
- Ideally what I would like is 320x200 @ 60Hz (or 320x400 @ 60Hz if that's how doublescan works). Can anyone suggest the video timings for that?