VGA: 6pin VGA (save 2 pin); 2pin VGA; 256 color, etc

Cluso99

Thanks to this thread "Monochrome VGA" http://forums.parallax.com/showthread.php?128898-Monochrome-VGA...... and Phil Pilgrim's ideas, I have come up with this circuit.

I will check it out soon, including coding it. However, for anyone willing to try it and report back, here is the circuit. The circuit utilises a 74LVC86 and a 74LVC07. Separate buffers are required for each color to stop bleed between colors.

Presuming it works, I will add it (as an option) to a VGA pcb that I have in the works.

In fact, by adding some of the other pins, we should be able to increase the colors. So, with 6 pins we should get all the normal colors (i.e. we have just saved 2 pins from the normal VGAs 8 pins.

With this in mind, we should be able to increase the colors from 64 colors (6 bits) to 256 colors (8 bits). I am just not sure about how we would provide this in software at the moment - would no doubt require another cog for the extra 2 bits.

Cluso99

By using the 4 resistors (thanks to Phil) on a VGA pin, Sync Time can be detected with an XOR (74LVC1G86) from a VGA pin. Software can tristate the pin during HS & VS times.

This detection can then gate a mux to switch two pins from color out to become HS & VS out during the HS & VS times.

SO, with this type of circuit, what can we achieve?

1. A 2-6 pin VGA (saves 2+ pins)
2. An 8 pin color VGA (i.e. 256 colors instead of 64 colors) - i.e. say 2 bits for R, 3 each for G & B

So, an XOR 74LVC1G86, a 74LVC2G04 and a 74LVC139 would do the trick with 2 x 10K & 2 x 5K resistors. There may yet be a simpler logic solution.

Cluso99

Here is my 6pin VGA. There are a few ways to do the logic, but all take quite a few gates

I have not tested this yet.

Lawson

How acceptable would 254 color VGA be? I.e. use two 74HC688 chips to select two colors for use as H-sync and V-sync.

Lawson

Cluso99

Lawson: During the HS & VS blanking, the RGB have to be clamped to 0, so a pair of 74hc688 will not work without more gates. I can get the circuit in 3 quad 2 I/P Nands or the quad and, quad or and a 2G xor.