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74LVCH245A implications of pull ups

Timothy Starr
Expert 2445 points


Hello-

We have an existing design (in the feild) which uses 74LVCH245A to buffer a JTAG chain (unidirectional). The parts are powered by 2.5V and there are pull ups of 4.7KOhm to 2.5V at both the inputs and the outputs - although some outputs are pull up to 2.5V with a 200 Ohm resistor. OE/ is tied to GND and DIR is tied difrectly to VCC. We have seen one device failure and we somtetimes see glitches on the signals.

1) Are there reliability issue when using pull ups with a bus shold part?

2) Could the glitches be the result of using the bus hold parts?

Thanks and Best Regards, -Tim Starr on behalf of ZM@BA

  • 0
    TI__Mastermind 46480 points

    The 4.7K resistors are too large. What could happen is if the bus hold is pulling the opposite direction than the pullups then it could hold the input in the transition region.

    This is not good for Cmos parts. You can use pullups or pull downs but they must have sufficient current to overdrive bus hold. See attached appnote.

    I dont think it would have anything to do with the glitches but I am not sure on that one.

     

    Bus hold appnote.pdf