I am analyzing the V9990 datasheet.
It has some very valuable information, some is missing, some must be guessed.
At present I am trying to figure out how to map V9990 modes into the VGA timing. The most data intensive at RGB555 modes are B2 and B3:
B2: DC=7.2 MHz, HS=15.7 kHz, Size=384*480
B3: DC=10.7 MHz, HS=15.7 kHz, Size=512*424
The target VGA mode in mind is 640x480, 60Hz, DC=25.175 MHz, area=800*525
If I stretch picture of 512 pixels to 640 using time domain, I suspect picture quality will deteriorate.
I suspect I still can display 512*424 picture @ 25 MHz dot clock, with black level around this picture, but there're two important questions:
1. I recall VGA monitors may have "stretch picture" function, if it detects actual video is less than the screen size, monitor may be able to resize the picture to its full screen;
2. if (1) is true, then what to do with border color? If I set color between 512*424 picture to black, there will be no border color but monitors may be able to stretch the picture. But if I set this color to border color, will VGA monitor detect it as not a part of the picture and be able to resize, and if it will somehow be able to resize, what picture geometry (including border) we may see?
Edit: found answer to the second question. I can include 8 pixels of the border color into front and back porches in H and V signal. Then if (1) is true, monitor will be able to sense black level and resize picture *including border* properly on its screen.
