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Maximum number of devices on I2C bus — Parallax Forums

Maximum number of devices on I2C bus

4Alex4Alex Posts: 119
edited 2008-10-03 19:25 in Propeller 1
Hi all,

While having some problems on a MAX11041 project, I suspected at one point that I may have put too many devices on the I2C bus. Does anyone know if there is such a limit, and if so, what is it or how can we calculate it. I like using the I2C bus and I tend to select my components accordingly but perhaps too much of a good thing...

Cheers,

Alex

Comments

  • LeonLeon Posts: 7,620
    edited 2008-10-02 15:55
    You can probably work it out from the NXP spec:

    www.nxp.com/acrobat_download/literature/9398/39340011.pdf

    The max. capacitance on the lines is 400 pF. That is probably the limiting factor. Using lower value pull-ups will help.

    Leon

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  • TimmooreTimmoore Posts: 1,031
    edited 2008-10-02 16:09
    As Leon said the capacitance of the lines is the important thing. Though I have had ~16 devices with problem. It also depends on how far part the devices are, in the above case the total cable length was short < ~2.5 feet. Dont forgot with the prop you can easily move 1/2 the devices to another pair of pins and tell the I2C driver what pins the 2nd I2C bus is on.
  • Nick McClickNick McClick Posts: 1,003
    edited 2008-10-02 17:41
    The logical limit is 112, but yeah, capacitance is the limiting item.

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  • 4Alex4Alex Posts: 119
    edited 2008-10-02 17:54
    @Leon:

    Thank you for the link.

    @Timmoore:

    Thank you for your reply. Distance is not really an issue in my case as devices are populated on one pcb or a few stacked pcbs. So far, I'm using 9 devices, including many digipots, hence my question. You mentionned using ~16 devices with problem. Is it a typo or you really had problems with 16 units? I'm not that far from 16 and I plan on replacing a few SPIs (that I can't stand) with I2C...

    Cheers,

    Alex
  • TimmooreTimmoore Posts: 1,031
    edited 2008-10-02 20:47
    Sorry ~16 without problem. The only issue I had wasn't a no/distance problem, some of the devices required clock stretching so I had problems I blamed on too many devices until I found I needed clock stretching. I split the devices into 2 I2S busses and had the same problem, once I fixed it on 2 busses I put them back together and it worked fine.
  • TurboManTurboMan Posts: 19
    edited 2008-10-03 19:25
    I've used the phillips chip PCA9515 in past projects.· It's a I2C repeater that adds another 400pf string.· It will also interface a different voltage device(level shifter).·
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