Question - Treatment of unused inputs & outputs on ttl & cmos ic's

Discussion in 'Homework Help' started by hivagi, Mar 18, 2009.

  1. hivagi

    Thread Starter New Member

    Mar 18, 2009
    7
    0
    Any ideas please let me know
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Depending on what is the function of the pin and how it affects the overall function of the chip you can tie it to ground or Vcc via a resistor.
    It is not good to leave floating inputs because they acts like small antennas and they pick up noise.
     
  3. hivagi

    Thread Starter New Member

    Mar 18, 2009
    7
    0
    thanks for the help. Also is there any websites that will show me this sort or information
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    There should be some. Learn to use google, type the correct words and make the search. You will find many. For example, try to search for "floating digital inputs".
     
  5. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    There are a couple of exceptions to mik3's advice. Standard TTL and any of the older TTL families (74Fxx, 74Sxx, 74LSxx) all need pullup resistors to safely tie to Vcc. If the input shorts, the conduction with a direct tie may be haevy enough to destroy the power supply.

    Each and every unsed input must be connected so as to be in a known state. If not, the input floats up to the activation level and will cause an oscillation that can destroy the IC, or place so much garbage on the power bus that the entire circuit becomes unstable.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    I didn't say directly to ground but via resistors.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Going straight to ground is no problem for any and all logic families. It's just the old TTL that needs pullup resistors to Vcc.
     
Loading...