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TCA9539: Leave floating on Pin 22, 23

Oliver Lin
Expert 3725 points
Part Number: TCA9539

Hi Expert,

Customer has met some I2C address conflict.

Their workaround is disconnecting the pin 22 and 23 to prevent master to read back from the device.

Customer would like to know is there any concern or side effect to leave pin 22, 23 floating? We are not able to find this information from the datasheet.

Please kindly share some comment.

  • 0
    TI__Guru 50835 points

    Hi Oliver,

    Disconnecting the I2C lines on same-address devices is a common method of allowing more devices to share an I2C bus. However, the disconnected pins should not be left entirely floating. Doing so may cause the signal lines to fall to a low-level input and disrupt the state machine of the device, requiring a reset before communication can be restored. To avoid this, the disconnected lines should be pulled-up to Vcc, similar to the normal pull-up requirements for these lines. If you are using an I2C switch to do this, a pull-up resistor should be included on both sides of the switch. Be sure to value the resistors appropriately so that the equivalent pull-up strength is not too large when both lines are connected. 

    Let me know if you have any more questions.

    Regards,
    Eric Schott

  • 0 in reply to Eric Schott1
    TI__Expert 3725 points

    HI Eric,

    Thank you for your response for customer question, that is nice and clear, customer got a following question,

    Pin A0 and A1, these two pins, do they have any internal pull up or pull down?

    Regards,

    Oliver

  • 0 in reply to Oliver Lin
    TI__Guru 50835 points

    Hi Oliver,

    Thank you for the feedback!

    The address pins have no significant internal bias and should not be left floating. It's recommended to connect these pins directly either to Vcc or to GND to assign the IC's address. If it's desired that the address be changeable post-fabrication, I'd recommend including 0-ohm resistor footprints to both voltage potentials for easy address reassignment. 

    Regards,
    Eric Schott