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SN74LV4052A: SN74LV4052A pull up resistors needed for I2C signaling?

chet skalka
Prodigy 30 points
Part Number: SN74LV4052A

Hello,

I would like to use this part as an I2C mux and had a question regarding the pull up resistors.  Figure 13 of the datasheet shows a typical I2C mux configuration, is it really necessary to have pull up resistors on every single multiplexed I2C signal?  I would have thought that a single set of pull up resistors could be placed on the common path, reducing the resistor count from 8 to 2 (assuming using all 8 outputs).  Is there some reason that TI recommends having dedicated pull ups for each I2C pair?  Thanks.

-Chris Skalka

  • 0
    TI__Mastermind 45370 points

    Chris,

    Placing pull up resistors on both sides of the switch are recommended to prevent glitching when the switch changes from low impedance to high impedance state.  If you have pull ups only on the COM side the slave devices on the other side of the switch could be floating when the signal switch is high impedance.  If there is no issue with I2C bus floating for your master or slave devices in your system you can decide to use less pull up resistors. 

    Thank you,

    Adam

  • 0 in reply to Adam Torma
    Prodigy 30 points

    Thanks Adam, one clarification, do you recommend pull ups on both sides or just the 'non-common' pins. The figure in the datasheet only shows resistors on the mux side ('non-common side).  Thanks.

    Chris Skalka

  • 0 in reply to chet skalka
    TI__Mastermind 45370 points

    Chris,

    The signal switch itself doesn't have any requirement to have pull up resistors at all.  The example in the datasheet is simply showing that you could use this device in an open drain system like I2C.  The pull up resistor requirements will be dictated by the I2C master and slave requirements and no the signal switch.

    The signal switch will create a Hi-Z path when the channel is not selected which can cause the I2C bus on either side of the switch to float if it doesn't have a pull-up.  I recommend placing pull up resistors on all I2C master and slave devices that would have an issue if their I2C bus was floating. 

    Thank you,

    Adam