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SN74CB3T16211: 74cb3t16211 i/o left floating

Dom Beesley
Prodigy 10 points
Part Number: SN74CB3T16211

Hello,

I'm designing a board that will be using 74cb3t16211's as level shifters and pin multiplexers to interface to plug-in modules. In some situations (when a board is not plugged in or where a plug-in board does not use all pins) the nOE of the part will be held high and the pins on one side not connected to anything. Will the weak current pull-up source be sufficient to pull these pins high - it doesn't really matter which state they are pulled into so long as excessive current isn't drawn.

Another 74cb3t16211 will be used to provide configuration pins, where certain pins are pulled low by either jumpers, links or links on the plug in modules. Is it necessary to proved pull up resistors for these inputs?

I'm trying to keep the part count as low as possible

Thanks

D

  • +1
    TI__Genius 10974 points

    Hello,

    Thanks for your question! It is recommended to leave unused pins floating or tied to GND. In this case, I do not think you need any pullup resistors to mitigate the leakage current when the part is in Hi-Z. This part has an IOZ parameter which describes the leakage current when the part is powered, but the switch is in Hi-Z/OFF. It is maximum +/-10uA per channel across temperature.

    Let me know if you have further questions!

    Thanks!
    Bryan

  • 0 in reply to Bryan Marshall
    Prodigy 10 points

    Thanks Bryan, sorry for the delay in replying.

    That's put my mind at rest about the floating inputs

    On the second question - do I still need pull ups for the configuration pins or will the weak pull ups on the fpga side be enough? At boot time the reset takes around 500uS during which time the nOE for the configuration pins will be enabled and those pins read near the end of the reset.

  • 0 in reply to Dom Beesley
    TI__Genius 10974 points

    Hi Dom,

    For configuration pins, definitely do not leave them floating. Pullup or pulldown is fine, they just need to be in a defined state to avoid any unwanted switching in your application.

    Thanks!

    Bryan