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SN74LVC16245A: SN74LVC16245A with a floating input pin

amnon pomerants
Intellectual 780 points
Part Number: SN74LVC16245A
Other Parts Discussed in Thread: SN74LVCH16245A

Hello

I have read the SCBA004D application note and noticed the SN74LVC16245A XCVR as part of the WideBus family has the Bus-Hold feature.

I understood from your appnote that in case the input was once being driven by a "0" or "1" there is no over current/ heating problem when this driver

starts to float (last input level is being kept by the XCVR until next change).

However, what is the situation when the XCVR is being powered up where there is no driver available at one of the XCVR pins (such in case a JTAG connector is connected

to the Buffer pins , but during power up there is nothing connected over this JTAG connector...)

Will the XCVR pin  float until JTAG machine is connected to the connector and starts driving "0"/ "1" ?

If so, can it harm the XCVR?

Thanks

Amnon

  • 0
    TI__Guru 58020 points
    Hi Amnon,

    Glad that you have referred to the app note about the floating inputs and trying to mitigate the issue using the bus-hold circuit.
    As long as the system doesnt need a defined high or a low level at the startup, you can use the bushold device; note that the default state most likely is going to be low, however, there is certianly a possibility that it can be high.
  • 0 in reply to ShreyasRao
    Intellectual 780 points
    Hi Shreyas
    Thanks for the prompt and elaborated answer.
    I would like to further ask on the aspect that started my inquiry, and double verify that what you are saying is that leaving a floating input at the SN74LVC16245A XCVR doesn't pose a dangerous condition on this pin nor at the whole device?
    The HOLD feature will functionally serve as a pull down (or pull up) since power up and nothing can be damaged from this floating input status, true?
    Thanks again
    Amnon
  • 0 in reply to amnon pomerants
    TI__Guru 58020 points
    Hi Ammon,

    I am sorry for the confusion, the SN74LVCH16245A has the bus-hold functionality and not the SN74LVC16425A.
    You can use the SN74LVCH16245A in your system and have a floating input without damaging the device.
  • 0 in reply to ShreyasRao
    Intellectual 780 points
    Hi Shreyas
    I looked at both buffers and they look alike but I wanted to be sure, are the SN74LVCH16245ADGGR and the SN74LVC16245ADGGR are form fit replaceable, take out one and put on its pads the other?

    Thanks
    Amnon
  • 0 in reply to ShreyasRao
    Intellectual 780 points
    Hi Shreyas,
    In the SCB004D app note, at TABLE-1 , the last row states that all LVC WideBus chips have the BUS HOLD INCORPORATED.
    As the SN74LVCH16245A is part of the WideBus family why do I need the SN74LVCH16245A?
    The "Hold" feature doesn't exist at the SN74LVC16245A?
    To replace all the buffers on my card at this point will be very unpleasant...

    Thanks
    Amnon
  • 0 in reply to amnon pomerants
    TI__Guru 58020 points
    Amnon,

    I am sorry for the unpleasant experience but the datasheet of the SN74LVC16245A doesnt mention the bushold feature in the description or the specs. We have to go by the datasheet.
  • 0 in reply to ShreyasRao
    Intellectual 780 points

    Hi Shreyas

    I took a look at the SN74LVCH16245A datasheet (June 2014 revision) and I think I have noticed contradiction information about the Hold mechanism solving or not the floating input pins issue.

    There is comment (1) at the end of section 7.3, but mainly section 12- LAYOUT which even brings figures showing NO floating inputs are allowed !!! 

    Can you please explain the above?

    Can I leave my inputs floating with this chip?

    Thanks

    Amnon

  • 0 in reply to amnon pomerants
    TI__Guru 58020 points

    Amnon,

    The control inputs DIR/ OE doesnt have bus hold feature in them.

    The IOs A/B do have the bushold feature.