Logic Gates

Discussion in 'Homework Help' started by ibemopar, Nov 19, 2009.

  1. ibemopar

    Thread Starter New Member

    Nov 19, 2009
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    Hi I need to make circuit using only 74HCT family. I have 2 issues so far. I one chunk requires 7 2-input AND gates or'd together with an OR gate. But there arent any OR gates large enough. I also have a similar situation but it requires 5 3-input AND gates or'd with an OR gate. Any ideas? I am lost? Thanks
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    Because OR is commutative and associative you can build a multi-input OR gate from smaller OR gates. The only penalty for this approach is increased propagation delay.
     
  3. bertus

    Administrator

    Apr 5, 2008
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  4. SgtWookie

    Expert

    Jul 17, 2007
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    Adding to what Papabravo said about propagaton delay;

    It takes time for the output of a gate to react after receiving a change of logic level(s) on the input(s). When you are connecting even a few gates together, this can cause timing problems or "glitches" which can be hard to fix later.

    See the attached. The circuit on the right looks neat, but the propagation delays will vary anywhere from 2 gates' worth of delays to 7 gates' worth. This could result in severe problems.

    The circuit on the left will have a minimum of 3 cumulative propagation delays, and a maximum of 4 delays. The chance of having logic "glitches" will be far less.

    To make the propagation delays all equal, one OR gate with both inputs wired together could be added between the output of the lower left AND gate and it's subsequent OR gate input.
     
  5. zgozvrm

    Member

    Oct 24, 2009
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    ... or, since you have extra gates left over in both U2 and U4, you could add a gate, resulting in a strictly 4-gate propagation delay (as stated in the last paragraph of SgtWookie's last post). Remember that A OR A = A. See attachment.
     
    Last edited: Dec 3, 2009
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