“Vectron VGA Plus v2 generates a 640x480@60Hz VGA signal and has an interface that works with retro computers or Arduino.
How It Works
Vectron VGA Plus v2 is an improvement on the earlier design.
The key to the simplified design of the Vectron VGA Plus boards is to use more RAM and less logic. So, for example, instead of having counters and flip flops, etc. to time each horizontal sync, that logic is embedded in the data in memory. Memory addresses are sequentially incremented by an osciallator, and the contents of each memory address specify what signals to output (R/G/B/H sync/V sync). Each of these memory addresses correspond with a specific pixel location (both visible and invisible) on the display. When the end of a frame is reached, the address counters are reset, and the process starts all over to draw the next frame.
This second version offers a number of enhancements over the original:
First, a second RAM chip was added to the design. This allows for an active/standby scheme, such that the standby chip can receive updates without interupting the active chip that is displaying the video output. When the host is done writing, the chips automatically switch roles in an instant, which updates the display without interruption.The original version used a single RAM chip, which led to momentary blips in the display during writes. This new type of design would have been simpler to achieve by using dual-port RAM, but in the world of DIP chips suitable for retro projects, I couldn’t source anything above about 4K, and I need a 256K chip for my selected resolution, so that wasn’t a reasonable choice.
The memory address at which the counter resets is no longer hard-wired. It can now be tweaked by setting jumpers at the top of the board, which allows the signal to be fine-tuned, or even an entirely different resolution to be produced. Since the storage of output signals in RAM offers total flexibility, it makes sense to allow for a frame of any number of pixels (up to the maximum allowed by RAM size).
The size of the PCB was reduced significantly, and lots more capacitors were added to ensure perfect stability of signal under all conditions. Additional power ports were also added to allow for grounding between additional peripherals.”