diy vga signal from ram

Why bother when you do it with a small FPGA or CPLD, an oscillator module, three resistors and three diodes.

Leon

I'm groping for the info and I can't put my hands on it but... I think I remember something in Circuit Cellar in the last year where someone did this with a surprisingly small number of parts and fitting into your constraints. I seem to remember that he saw he could do this in one mode with a couple of counters and a little glue logic, substantially less than a handful of 74XXXXX.

see The Cheap Video Cookbook, by Don Lancaster

Dave

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This sounds interesting. Can give some more info such as links, schematics, and code? Any articles?

Isn't there three dacs, one for each color? Does this mean you can have three independant AWG's? What resolution can you get? What is the sample rate? How do you do low frequencies?

Someone mentioned the blanking interval might mess up the waveform - how do you get around that?

Best,

Mike Monett

OK Ian, thanks for the info. I was thinking this might be useful if you were stuck on a desert island one Saturday night and needed a quick hack. Sounds like it might take a while to get working, and may have some limitations that would be difficult to work aound.

I guess I'd better stock up on Analog Devices DACS:)

Best,

Mike Monett

It's not hard. Especially if you can cheat and use a bit or two in the RAM as state controls. I did a simple design for a Z80 based computer with a graphics output.

Basically, a couple of gates to make a counter read a byte from an address in SRAM when the Z80 wasn't using it, and a shift register to read it out a couple of bits at a time and present it to a DAC.

The Z80 fills in the sync pulses (DAC has 3 levels and sync) and does all drawing.

IIRC, there were about 8 chips, and a few resistor arrays, counters, SRAM, shift register, ... (No eprom, it loaded in the program using the shift register and counter, used the wrong way round)

and code?

Nope. Somewhere I've got a schematic in pencil, and a big timing diagram.

do you do

you get

Simply set areas of the display to 'sync' level. The counter doesn't know about lines, interlacing, ... It's all done in software. In the above case, I was able to not even bother about overflow on the counter, as the clock rate was set so that it read out the entire frame memory in one frame time.

Nowadays, you'd almost have to be insane not to do it with programmable logic of some sort. Especially for VGA and colour. The bitrate and required frequencies are high for TTL.

thanks for the lead, i'll look into it.

mike

do-able

thanks for the lead and the encouraging attitude.

mike

that's all right then, i am almost insane...

mike

do-able

i found the article, by enoch hwang just this november, just paid for the pdf on the site (only $1.50) but can't get the password (page unavailable !!). have emailed the webmaster, not holding breath due to time of year...

thanks again for the tip

mike

Well, I have occasionally wondered about getting LEGO to sponsor me to make a mechanical CPU...

Someone already has a head start on you...

:-)

well, there's no "almost" about that insanity!!

with all due respect;

your implied suggestion that certain types of project are unsuitable for non proffessionals (is that what is meant by 'hobbyist'?) isn't really very helpfull (or friendly). i really don't want to start one of those pointless flame-war slanging matches with a negative comment, but i do feel strongly that all interested efforts, however flawed or over ambitious, should be encouraged at the very least. if i have a go and fail miserably, don't worry, i'm a big boy. i'll probably have a go at making a "space elevator" out of fishing line or something similar.like you say, for the "fun". just don't tell me i can't.

thanks for reading, if not contributing.

mike