handling unused xor gate pins

Discussion in 'Digital Circuit Design' started by testuserabcdef, Feb 23, 2017.

  1. testuserabcdef

    Thread Starter Member

    Jul 12, 2016
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    I learned that to reduce interference (EMI), one must not leave any input floating (disconnected). I've been used to tying inputs to VCC or GND but in the case of XOR gate, the output is low if both inputs are the same logic value (both inputs 1 or both inputs 0).

    I was thinking about tying only the inputs together for each unused xor gate individually but not connecting those inputs to anything else.

    Would I still get interference in my circuit if I terminated xor gates this way?
     
  2. WBahn

    Moderator

    Mar 31, 2012
    23,106
    6,951
    Yes, you would. You could also get shoot-through currents that could damage the gates (assuming we are talking about CMOS).

    But if you have unused gates, what do you care whether the outputs of those gates are high or low?

    If you are using one input and not the other, then why are you using an XOR gate at all?

    If you have a multi-input XOR gate (pretty uncommon in discrete logic) you tie all of the unused pins LO. This makes the gate behave like an XOR with fewer inputs.
     
  3. ian field

    AAC Fanatic!

    Oct 27, 2012
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    For unused gates, CMOS probably doesn't care whether you tie the inputs for logic high or logic low, TTL can sink more current than it can source, so setting the inputs to arrange logic high output might be just very slightly better.

    You can use surplus XOR gates as either inverters or buffers. With straight XOR you only get logic high outputs if the inputs are different, so tie one input high and the other is inverting.
     
    absf likes this.
  4. hp1729

    Well-Known Member

    Nov 23, 2015
    2,306
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    Why do you imagine it a problem? Tie them both low.
     
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