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diy vga signal from ram

Discussion in 'Electronic Design' started by mike, Dec 24, 2004.

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  1. mike

    mike Guest

    i'm looking for info re. producing vga signal based on contents of ram.
    don't want to use pic or fpga, from what i've found so far it looks do-able
    with the right clock freq. , some sram and a fistfull of 74XXXXX ic's.
    can anyone point me in the direction of some web based gen.?

    thanks for any info received..

  2. Rich Grise

    Rich Grise Guest

    More like a bushel basket full. )-;

    Good Luck!
  3. Leon Heller

    Leon Heller Guest

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

  4. Dave Garnett

    Dave Garnett Guest

    see The Cheap Video Cookbook, by Don Lancaster

  5. Don Taylor

    Don Taylor Guest

    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.
  6. Ian Stirling

    Ian Stirling Guest

    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

    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)
  7. Mike Monett

    Mike Monett Guest

    Ian Stirling wrote:

    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?


    Mike Monett
  8. Ian Stirling

    Ian Stirling Guest

    Somewhere I've got a schematic in pencil, and a big timing diagram.
    2 bit, black, grey, white and sync.
    It was designed to do PAL TV output.
    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.
  9. Mike Monett

    Mike Monett Guest

    Ian Stirling wrote:

    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:)


    Mike Monett
  10. mike

    mike Guest

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

  11. mike

    mike Guest

    thanks for the lead and the encouraging attitude.

  12. mike

    mike Guest

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

  13. mike

    mike Guest

    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

  14. Rich Grise

    Rich Grise Guest

    Wouldn't it almost be "easier", in this day and age, to just build
    a(an?) 8x8x8x2048x1600 video thing and not screw around with VGA specs?
    As long as you get the sync freqs right, of course. :)

    If you already have something making VGA and just want to display
    it, that's just an RGB monitor, and if you're trying to build one
    of those, you _are_ insane. ;-)

    And then you have the issue of getting the data in and out. Well,

    Good Luck!
  15. Rich Grise

    Rich Grise Guest

    A TV Typewriter, you can build with a few chips. Even color! ;-)

    A VGA, on the other hand, is not a hobbyist project.

    Have Fun!
  16. Ian Stirling

    Ian Stirling Guest

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

    Ian Guest

    Someone already has a head start on you...

  18. mike

    mike Guest

  19. mike

    mike Guest

    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 you say, for the "fun". just
    don't tell me i can't.

    thanks for reading, if not contributing.

  20. Rich Grise

    Rich Grise Guest

    No, it's not very helpful at all. But do you want to delve into the
    morass of VGA and SVGA specs to dope it out? They have very large
    logic array things on those things.
    Neither do I.
    Sure! Go track down the VGA spec! No big deal at all! Then, tell me
    if you want to build one from scratch with TTL.
    I never said any such thing.

    You can do anything your want to do.

    By "hobbyist," I meant the kind of guy who's still learning to put
    current-limit resistors on LEDs, and that sort of thing. I've done a TV
    typewriter at a "Hobbyist" level. Calling oneself a "Hobbyist," but
    claiming to take on a full-on VGA from scratch is a stretch.
    Well, you are certainly more then welcome to look into "How to make a
    VGA from scratch using TTL", but I can't guarantee that you'll have
    much success, unless you bite off little chunks, like a 640 X 480
    graphics TV Typewriter with VGA sync speeds.

    THEN you start on the graphics acceleration and all that crap. OK?

    Remember, you'll need three DACs that can spit out analog video at
    better than 6 MHz, probably more like 15 MHz.

    If you feel like you're ready to take on that challenge, then by all
    means, more power to you!

    I just wouldn't necessarily call it a "Hobbyist" project.

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