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I2C over long bus

Discussion in 'Electronic Design' started by Jon Slaughter, Apr 9, 2008.

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  1. Anyone here familiar with using I2C over a long bus, say up to a few hundred
    feet? Basically either using charge pumps and/or slower speeds(even < 1k
    will work).

    I know there are some driver chips(but quite expensive for what they do) and
    I'm curious as to if there are any simple solutions. (basically connecting
    an I2C temp sensor(or several) over about 200 feet to a uC.

    That or I was thinking of using a single to differential converter and run
    it over twisted pair... but this is more complex and has some other issues.

    Thanks,
    Jon
     
  2. Joerg

    Joerg Guest

    Also, consider AC termination, using a very small C. I don't know I2C
    too well, I am mostly an SPI guy, but maybe that works.
     
  3. Guest

    I think it's "up to" 100KHz.
    What's wrong with using RS-485 or Ethernet, something designed for the
    long haul?
     
  4. Is there? the I2C low speed mode is 100kHz. Although I can run it at any
    speed I want for the most part(shouldn't be too hard). I will do some tests
    soon and see. Maybe just lowering the clock to 1khz will be fine for my app.
    (I don't need any real speed as I'm just monitoring temp and each conversion
    takes a second anyways)
    Yes, that was one recomendation I saw online as some others that use driver
    chips and audio cable. Basically trying to avoid any expensive cabling(want
    to use 4-wire telephone cable if possible but not sure about noise) and any
    expensive chips.
    I imagine this would be difficult cause the uC can only sink so much
    current. Although its about 20mA so it should be fine... not sure about the
    sensor though.

    I'll just try a lower speed, which will be the best option cause I want to
    run in low power mode anyways(see if I can get it battery operated) and if
    not I'll look into the charge pump method and see if that works.

    Thanks,
    Jon
     
  5. That would mean I need one uC per IC. Essentially quadrupling the cost(well,
    sorta)... and definitely ruling out battery power operation.
     
  6. You sure?

    I have the spec and from what I saw its only 100khz, 400khz and 3.4Mhz.
    These are low speed, fast speed, and high speed modes.

    But AFAIK there is no specific need to run at these modes because the master
    provides the clock... so these, I guess, are the max speeds for the
    different modes.

    Wiki does mention that its from 10khz to 100khz so I guess your quoting the
    min speed. But I've seen an app note for such things that say they've ran it
    down to 500hz for over 1 mile.
     
  7. Yeah. I might have too. I2C is basically like SPI cept only data and clock
    lines and lower speed with a specific protocol(sorta like RS232 almost) +
    has some features like multi-master and such.
     
  8. Joerg

    Joerg Guest

    .... plus it has some nasty hang-up features :-(
     
  9. Might be worth looking at the Maxim/Dallas 1-Wire devices, here is an app
    note that discusses distances etc:
    http://www.maxim-ic.com/appnotes.cfm/an_pk/148

    Dave.
     
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