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VGA or TV with an I2C Bus Expander, is it possable? — Parallax Forums

VGA or TV with an I2C Bus Expander, is it possable?

Jorge PJorge P Posts: 385
edited 2011-04-22 10:57 in Propeller 1
Has anyone tried to connect VGA or TV using an I2C bus expander? I am recieving a 16bit, and 24bit I2C expanders thursday, Just want to know if it has already been tried and weather it was successfull.

If it has been done, also would like to know if I should instead use an 8bit expander to VGA for speed, quality, etc...

These are the chips I will recieve tomorrow:

16bit I2C expander - http://search.digikey.com/scripts/DkSearch/dksus.dll?detail&name=568-2027-5-ND
24bit expander - http://search.digikey.com/scripts/DkSearch/dksus.dll?detail&name=296-27844-1-ND

data sheets are on their respective pages.

Thanks in advance for any replies

Comments

  • kwinnkwinn Posts: 8,697
    edited 2011-04-20 20:44
    Sorry Jorge, no way those chips are going to do TV video or VGA. I2C chips run at 400Kbits per second while standard NTSC video IIRC needs about 6Mbits a second for a monochrome display (pixel lit or off). VGA needs about 25MBytes a second for 256 (8 bit) colors.
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2011-04-21 03:57
    Are you trying to build something like an I2C signal analyzer with video display?

    A prop might work for you in combination with the chips you listed.
    The prop could be made to watch the I2C data and create a display.
    NTSC or VGA.
  • Jorge PJorge P Posts: 385
    edited 2011-04-21 04:42
    I was getting the I2C bus expanders for a different purpose but was curious as to wether VGA or TV could be done since I never realy saw anyone talk about it here. The January 2000 I2C bus specifications from Philips state that High Speed Mode "Philips’ I2C" bus can communicate at 3.4 Mbit/s. The 400Kbits is the fast speed mode. So taking into consideration kwinn's comments, I would need at least 6Mbits.

    I suppose if you broke up the signal over 2, or more, "High Speed", not fast speed, I2C busses and combine the signals from the two through bus expanders then in you can probably get away with something. Thats just my guess. But I suppose that would also depend on the Expander Chips themselves too. It would most likely need some modifed cone and maybee an aditional cog so I would also guess that even if it could be done, it would eat up to much processing power and ram.

    @HollyMinkowski No, I just got them to play around with, I am still trying to play around with adding an IDE Drive interface even if it is slow, based off of some old 8bit micro schematics I have from the mid 80's like the 6809E and Zilog mcu's. I think after I get something actualy working It can be expanded. Since It is basicaly a parallel bus, I need the bus expanders to give me some more I/O pins to play around with buttons and leds and such, since the IDE interface itself would be directly handled by the prop.

    Most of the old 8bit micros that had IDE or SCSI interfaces ran at a whoping 2khz, lol, and that was fast at the time. If I muck around with it, I can figure something out for my enhoyment with the prop. Alone, I might be able to get something done within year or so, but If I get some help from a friend, localy, I might be able to get something much quicker. But I will keep that for a different thread.

    Back the the NTSC or VGA on the I2C bus, I will read over the Philips docs on the I2C bus the figure out how to get into the High Speed mode, I will try toying with a few ideas and reading up some more on the datasheets to see if the chips I orderd actualy support I2C in High Speed mode.

    This brings me to ask another question. Since the Propeller already supporst the Fast Speed I2C (400Kbits) Can the Propeller handle Philips I2C High Speed Mode(3.4Mbits)???
  • KyeKye Posts: 2,200
    edited 2011-04-21 06:34
    yes, its all written in ASM. So, the speed can be whatever. The prop will probably be abe to hit 1-2Mbits @ 80Mhz.
  • ericballericball Posts: 774
    edited 2011-04-21 09:54
    NTSC line rate is 15.734kHz, so at fast speed that's only 25 bits/line. Even at max high speed that's only 216 bits/line. At 2 bits per pixel (sync, black, grey, white) you'd get less than 85 active pixels per line. Pretty low res. Better to use the Propeller's video generator and one of the low pin-count video drivers.
  • HollyMinkowskiHollyMinkowski Posts: 1,398
    edited 2011-04-21 11:42
    Hey Kye!
    I like your new avatar image, very cute...nice look with the black jacket. :-)

    Jorge
    As ericball said.... better to use the video generators in the prop...
  • lonesocklonesock Posts: 917
    edited 2011-04-21 11:57
    Jorge,

    I'd love for this to work, but I have two concerns:

    1 - I didn't see anything in the datasheets about the faster i2c speed (I only see 400kHz mentioned)

    2 - Don't forget that i2c protocol will use some bits sending chip addresses which eats into your actual data rate.

    I'm betting you could get the expander to do sync for you, though getting the timing exactly right wouldn't be fun, and I think you would need to modify the VGA-out circuit (no idea on the TV portion).

    Jonathan
  • kwinnkwinn Posts: 8,697
    edited 2011-04-21 13:50
    Everyone seems to be overlooking the fact that an I2C device is essentially a serial to parallel converter. In the case of one of the chips specified you send out 36 bits of serial data at 400KHz and get 16 bits of output at 25KHz. The original bit stream even at 3.4Mb/s would barely be able to provide a low resolution two shade/color display. Far better to use a prop to produce video.
  • Jorge PJorge P Posts: 385
    edited 2011-04-22 04:34
    @lonesock me neither, I just read the datasheets on the chips, even though the one, PCA9555, is an NXP (formerly Philips) it don't seem to support High Speed mode. Who would have thought that the developers of I2C wouldnt support their own bus. I will contact NXP to see if it is supported but not documented with this IC.
    As for the TI Chip, The datasheet seems to leave out any info about the High Speed mode as welll. After doing some reasearch on I2C, it seams that only very few companies fully incorperate the High Speed mode (http://www.i2c-bus.org/). The link on i2c-bus.org's site to the I2C Specification (Linked to NXP) is updated to 2007. I am assuming that the bus speed all depends on the master, not the IC's, and wether or not it initializes the High Speed mode.

    @Kye thanks for the info.

    @kwinn & ericball: Thanks for your insight, I will be using the prop to display NTSC/VGA for now. I will still toy with the Idea and probably try doing it anyways just to see if I can split up the video signals.

    I will update this thread if I find anything out about the I2C High Speed, regarding these 2 IC's, mode or if I have some success with NTSC/VGA over the I2C Bus, but that is on the backburner for now.

    The attached schematic is my general Idea as to how I think this could work with VGA, the schematic is from PCB123 by Sunstone. the video signal would be split up amongst 4 data line going to 4-8Bit I2C bus expanders which would then combine their outputs to VGA.
    zip
    zip
    I2C VGA.zip
    724K
  • kwinnkwinn Posts: 8,697
    edited 2011-04-22 10:57
    Jorge P wrote: »
    @kwinn & ericball: Thanks for your insight, I will be using the prop to display NTSC/VGA for now. I will still toy with the Idea and probably try doing it anyways just to see if I can split up the video signals.

    I will update this thread if I find anything out about the I2C High Speed, regarding these 2 IC's, mode or if I have some success with NTSC/VGA over the I2C Bus, but that is on the backburner for now.

    The attached schematic is my general Idea as to how I think this could work with VGA, the schematic is from PCB123 by Sunstone. the video signal would be split up amongst 4 data line going to 4-8Bit I2C bus expanders which would then combine their outputs to VGA.

    It never hurts to pursue an idea. Even if you do not succeed you always learn from it. In many cases I find that I learn much more from ideas that do not bear fruit than from those that succeed. I would be interested in your progress regardless of the outcome, and would be most impressed if you made it work.

    I downloaded the Sunstone software to look at your schematic but had no luck with it. Perhaps you could download one of the PDF printer programs (I use PDF Creator) and post it as a PDF. Almost everyone can view PDF files, but schematic and PCB layout files usually require the software they were created with for viewing.
  • Allen1953Allen1953 Posts: 3
    Would this help?
    Hobbytronics serial VGA board http://www.hobbytronics.co.uk/serial-vga?keyword=serialvga
  • tritoniumtritonium Posts: 543
    Hi
    oooold thread but-
    Would this help?
    Hobbytronics serial VGA board http://www.hobbytronics.co.uk/serial-vga?keyword=serialvga

    -and wouldn't you know- it uses a propeller, with chips vga code- all open source...

    Dave
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