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Question About IC Chips

Discussion in 'Electronic Basics' started by Guest, Mar 28, 2005.

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  1. Guest

    Guest Guest

    Should the used pins be left unconnected or should they be taken to ground?
  2. Chris Dugan

    Chris Dugan Guest

    It depends entireley on the device in use.

  3. I suggest leaving _used_ pins alone...;)
  4. I read in alt.binaries.schematics.electronic that
    wrote (in
    You mean 'unused pins'. It depends on the device; you need to read the
    data sheet. Some devices require a resistor to ground or to supply. Some
    MUST NOT be connected to anything at all.

    Newsgroups reduced to FOUR.
  5. I read in alt.binaries.schematics.electronic that Vidar Løkken
    Once they have been used, they should be cut off and discarded.

    Newsgroups reduced to four.
  6. Guest

    Guest Guest

    Sorry for my typing skills. I ment to say:

    What do you do with unused pins?
  7. Mark Jones

    Mark Jones Guest

    That depends on the chip used. Generally, "inputs" can be grounded while
    outputs are left unconnected. Check the chip's datasheet for current consumption
    with inputs at Vss and Vcc, it might consume less power if the pins are pulled
    up to Vcc instead.
  8. Jim Thompson

    Jim Thompson Guest

    Try reading the data sheet? On some I/C's spare pins are just
    floating objects... on others they may be used as test points during
    manufacture and are designated NC (no connection).

    ...Jim Thompson
  9. Luhan Monat

    Luhan Monat Guest

    Sell them on Ebay?
  10. CJT

    CJT Guest

    If they're truly unused, it shouldn't matter. If they're connected
    internally, then logic should prevail.
  11. Generally, unused output pins should be left unconnected.

    What you do with unused input pins will depend on the chip - for CMOS
    logic parts, all input pins must be connected to something, and unused
    input pins must be connected in a way that will allow the chip to work
    as required. For example, and active low "reset" pin must be
    connected to the positive supply, otherwise the chip will be
    permanently held in its reset state.

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  12. Steve Rush

    Steve Rush Guest

    All the jokes aside, an unused input to a logic circuit should never be
    left floating. You never know what state it will take on powerup. Worse,
    you never know when it's going to change.

    Tie each unused input to whatever value won't change the output. For
    instance, if you use a two-input NAND gate as an inverter, you can tie one
    input high or to the other input. Both will work, although paralleling
    inputs increases the fan-in, which may be significant. With an OR or NOR
    gate, unused inputs need to be held low. Some logic families may require
    a resistor instead of a direct connection to either supply rail; check
    your data sheets.

    Unused outputs can float.
  13. And everyone who replied meant to say: "It's entirely dependent on
    the chip used, and the circuit it's used in." In general, unused
    outputs can be left alone, unused inputs should probably be tied high
    or low, sometimes through a resistor, but you can't tell without a
    detailed schematic and a datasheet and/or application note for the
    specific IC used. I can probably think of a dozen examples of where
    my "general" rules above are wrong, for instance...
  14. Along those same lines, what about unused microcontroller ports? Say a PIC
    16F8xx. Can you just set them as outputs and ignore them?

    Glenn Ashmore

    I'm building a 45' cutter in strip/composite. Watch my progress (or lack
    there of) at:
    Shameless Commercial Division:
  15. Robert

    Robert Guest

    The recommendation for logic gates is to connect unused AND gate inputs to
    the positive supply through a 1K resistor.

    OR gate inputs are to be connected to ground.

    Why this is recommended is obvious from the logic function the gates

    If the unused inputs are not logic gate inputs it gets more complicated, as
    Jim and others have mentioned.

  16. "Glenn Ashmore" wrote ...
    That would certainly seem like the logical thing to do.
    No pun intended.
  17. I read in alt.binaries.schematics.electronic that Richard Crowley
    Oh, go on, intend away! No-one was ever punished for a really good pun.

    You know what is paved with good intentions? (;-)
  18. Aidan Grey

    Aidan Grey Guest

    Unused inputs should be tied to either VCC or ground. If they are left
    open, it is possible the chip will start "oscillating", if the input is
    between on and off. This will probably not affect the chip, but may cause
    odd problems elsewhere in the circuit.

    At one time, it was the military practise to attach unused inputs through
    a 1K resister to VCC. Doing this is longer thought necessary, just attach it

    Aidan Grey
  19. Jim Thompson

    Jim Thompson Guest

    I know digital designers who do this as standard practice... it allows
    easily forcing a test signal when trying to track a fault.

    ...Jim Thompson

  20. It was only "necessary" for TTL, where the base-emitter
    junction following a strapped-high input could be stressed
    if forced into breakdown by excessive VCC. This is not
    a concern with CMOS, where, by the time an input gate
    is in jeapardy, so are many of the internal gates.

    Another reason I've seen for strapping thru a resistor is
    so, during board test development, canned routines can
    be applied to the part, testing used and unused gates
    alike. It takes a tiny extra effort to not test an unused
    gate or ignore one that fails because its input cannot be
    wiggled enough. (That practise always stuck in my craw,
    since unit cost was among my responsiblities, but I heard
    it from enough different test folks that I guess it could be
    part of their lore of "good" practise.)
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