Over a year ago I wanted a graphics card for my Z80 computer. I made a prototype with the uPD7220, but I never got it working. Until...
As it turned out, I made TONS on mistakes in the original design. There are issues with the new design too, but it works well enough (after a hack, more on that later). Anyway, after playing with the first prototype and trying various hacks, I ended up with a valid VGA signal!
I don't think the uPD7220 was ever supposed to output VGA, considering VGA was introduced about four years after the uPD was, so it's awesome that it can output a VGA signal.
After studying the uPD datasheet and my schematic in depth, I eventually found most of the problems and ended up with my second prototype (which I am using now). I've managed to get it to do the basic things I need it to do. Getting from basic initialization to drawing pixels was very difficult, especially with such a complex device and no example code/libraries, but it was definately worth it. I now want to share what I've learned about it so that more people can use this GDC (GDC = Graphic Display Controller).
I plan on adding more and more information to this project so others can use my work as a reference. I also am going to create a Z80 driver, which I will post here as well.