While I was hacking around with my ATtiny85 VGA / Sound generator, I found something interesting about the VGA signal, something I had never heard of or seen before.
You can send both Horizontal and Vertical sync on the Horizontal sync line in mode 720x400!
By mixing the syncs much like NTSC composite sync, all 5 monitors I tested work perfectly. I have never seen this documented, but it seems to work. In 720x400 or 640x400 mode, the vertical sync is positive and horizontal sync is negative, so I just bring the sync line high for one line during the vsync period, and then let hsync operate normally.
Perhaps the 2 pins are simply mixed internally anyhow, so this is why it works?
Anyhow, this is great, as an ATtiny85 only has 4 usable pins once you supply an external clock, so now I am getting 4 colors and sound into my monitor (2 color bit pins, 1 combined sync pin, and 1 sound output pin).
If this combined sync hack is actually documented someplace, I would sure like to see it.
I will be updating this project soon now that I have gained another IO pin to work with...