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I2C communication between self powered multiple end devices

arvind kumar
Prodigy 30 points


hello all

for one of my design , i have to implement I2C communication(interface) between master and multiple self powered devices. so please suggest the hardware implementation. is isolation required between each end devices interface with master? 

  • 0
    Mastermind 7395 points

    The I2C lines need pull-ups. If the master is the last one to be powered, you can put the pull-ups on the master. If there are slaves that can get unpowered later, you have to check their datasheets to see if they can receive voltage at their IOs while they are unpowered. If they can't, they could disturb the communications and worst, they could get damaged.

  • 0 in reply to Avi Chami1
    Prodigy 30 points

    hello albert

    to better understand the system scenario, i am providing elaborate description.

    master is self powered with clock and data line pulled up to Vcc. then other slave devices, powered by their own power supply , are interfaced with the master I2C bus. such slave devices can be 4 or more depending on the bus capacitance.

    in this condition, do we need to isolate the I2c bus from master to slave. both master and slave are self powered before connection through I2C bus.

    in this scenario, do we need to isolate each slave from master, or we can directly connect scl and clk from each device to scl and clk bus from master.

     

  • 0 in reply to arvind kumar
    Mastermind 7395 points

    There isn't a generic answer to this. From my experience, I would say that isolation is NOT needed. That means that most I2C devices I know, are "friendly" enough to cope without problems with voltage in their clock and data pins while unpowered.

    But to give a 100% accurate answer you have to check the datasheets of the devices. Specifically, what are the admitted Vin levels. Some devices limit Vin level as a function of Vcc. If your I2C devices limit Vin as a function of Vcc, you will need isolation. But I would be surprised if that was the case for I2C devices. BTW, I checked the I2C standard but didn't find an answer to your question.

  • 0 in reply to Avi Chami1
    Prodigy 30 points

    hello

    if master and all the devices (slaves) are powered by same voltage amplitude( e.g 3.3v) , though derived from independent power supplies, does it require isolation between master and slaves on i2c bus.

  • 0 in reply to arvind kumar
    Mastermind 7395 points

    It is not dependent only on the voltages.

    1) The pullups, I guess, are connected to only one of the power supplies

    2) What happens to any device that is unpowered, while the pull-ups already receive voltage since they are powered by another source? That is the issue that you have to check. I would say there is no problem, but I can't be 100% sure if you don't check the datasheet of each of the devices.