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PCA9306: Connecting multiple pullp resistors in I2C bus with Jumper

Hari T O0
Genius 3820 points
Part Number: PCA9306

Hi Team,

I was told to connect multiple pull up resistors in an I2C  bus with an option to select the pull-up values using Jumpers.

Please find the attached figure. May I know connecting like this will make any communication issues on the I2C bus.

Regards

Hari

  • 0
    TI__Guru 62920 points
    Hello Hari,

    It is OK to give the option to connect a number of different pull-up resistances. If multiple resistances will be connected in parallel, though, you should be careful that the total equivalent pull-up resistance is not too low. (This will depend on the pull-down strength of the I2C devices connected to the bus. Most will specify a maximum VOL level based on an expected pull-down current, and this pull-down current is dependent on pull-up resistance and voltage.) During PCB layout, you may want to also give some thought to how these additional resistor options may introduce parasitic capacitance. This would generally be a concern for larger PCBs with a larger number of devices sharing the I2C bus - the maximum limit per the I2C standard is 400 pF, and since the PCA9306 does not implement buffering the total capacitance on both sides would need to be below this limit.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Genius 3820 points
    Hi Max,
    Thank you very much.
    If you don't mind could you please explain this statement"since the PCA9306 does not implement buffering the total capacitance on both sides would need to be below this limit."May I know what you mean by buffering the total capacitance.

    May I know whether TI have any IC which will implement buffering the total capacitance on bot sides.
    Regards
    Hari
  • 0 in reply to Hari T O0
    TI__Mastermind 23156 points

    Hello Hari,

    We have an app not that explains how a buffer works and why you need it.  Basically, most translators/level shifters for I2C are pass fet architectures which don't isolate the bus capacitance on either side of the device.   If you don't have much capacitance it isn't an issue, but if the total bus capacitance exceeds the I2C standard then you might have issues because of the pullup resistor range needed to meet rise time requirements from the standard.  That would be when you use a buffer to subdivide the bus capacitance. 

    Why, When, and How to use I2C Buffers

    You can use the following app note for part selection:

    Choosing the Correct I2C Device for New Designs

    Let me know if you still have questions or need clarification. 

    Francis