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SN74LVC08A: Preventing Floating Inputs

Josh Corra
Prodigy 90 points
Other Parts Discussed in Thread: SN74LVC1G08, SN74LVC2G08

In the LVC family of logic gates, can I use the output of a spare gate to reliably hold the unused inputs at a known level, or will it power up in an unknown and possibly unreliable state due to the linear/transition region? In other words, on a spare gate, can I connect its output to its inputs with a feedback resistor of 1-2 KOhms to prevent floating inputs?  Will the output on power up be driven sufficiently low to hold the inputs at a steady low state, or will they potentially get stuck at an in between voltage level (not high or low)?

Thanks,

Josh

  • 0
    Guru 243190 points
    This is safe; the extremely high 'amplification' of LVC gates ensures that even the slightest amount of noise tilts the signal either way. (Of course, any kind of inverting gate would result in an oscillator.)

    A resistor is needed only in a bus-hold circuit where you'd want another device to drive the signal the opposite way. Logic gates are designed to be connected to each other directly; here, the resistor is superfluous.

    Wouldn't it be easier to connect the inputs to Vcc or GND?

    To avoid having spare gates, consider using little logic devices (SN74LVC1G08/SN74LVC2G08).
  • 0 in reply to Clemens Ladisch
    TI__Guru 55970 points
    Thanks Clemens for your suggested answer . I appreciate your contribution in engaging with the Logic community .
    I would agree that the spare channels can be avoided using the single, dual and triple gate versions of LVC family .
  • 0 in reply to Clemens Ladisch
    Prodigy 90 points

    Thank you Clemens.  The reason for the feedback resistor was to keep the spare gate usable in the future without having to cut any traces on the board (and the resistor pads make very convenient wire points).  I was just considering an alternate approach for the spare gates (rather than the traditional method of connecting each input to Vcc/GND) that minimized the number of extra components while maximizing access to the spare gates in the future.  In our application, due to low volume, the cost and risk of cutting traces to rework a board to add functionality is much greater than the cost of a couple resistors.

    Josh

  • 0 in reply to Josh Corra
    Guru 243190 points

    If you actually want a spare gate, connecting the inputs to some signal with a resistor would indeed be the simplest approach. But it does matter what the actual signal is (Vcc, GND, or output), nor what the resistor value is (anything between 0 Ω and lots of kΩ would work).