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SN74AVC2T245: Messages not being received, OE line to GND

Go Bravos
Expert 6090 points
Part Number: SN74AVC2T245

Team,

This part is used on the

UART Tx and Rx lines between two boards, called B1 and B2.  We have a scenario where, on power up

the Tx messages from B1 are not being received on B2.  

 

The board with the SN74AVC2T245 does not handle the OE line of the SN74AVC2T245 in the manner the

datasheet describes on page 16. 

 

Instead of following the datasheet, the OE line is tied directly to ground.  Can you say what the consequences

are if connected this way?  Will it produce unexpected output until after the 2 power supplies come up, or

is it possible the chip will be “hung up” in such a way that it does not cleanly translate from one level to

another?

Thank you.

  • 0
    Guru 243220 points

    If OE is low before power-up has completed, then the state of the I/Os is undefined, i.e., the direction might be wrong, and/or the outputs might drive the wrong value.

    As long as you do not have exceeded the absolute maximum ratings, the device will work correctly after power up. (All inputs are overvoltage tolerant, so the only problem could be an external voltage applied to an output. What exactly happens on the Tx/Rx lines during power up?)

  • 0 in reply to Clemens Ladisch
    TI__Expert 6090 points

    Clemens,

    As I understand it, adding the capacitance of a scope probe onto the signal, causes it to work! So we aren’t exactly sure what the signal

    looks like during power up. There are some boards that are failing, which start working with no changes, and vice versa.  So it’s tough

    to get a clear picture of what’s happening.

  • 0 in reply to Go Bravos
    Guru 243220 points

    Blind guess: a short pulse at power up causes ringing, and thus might exceed the normal signal voltage.

    Try adding any kind of LPF (series resistor, capacitor, ferrite etc.) to slow down the edges.