help with d-type bistable

Discussion in 'Homework Help' started by killer6008, Feb 4, 2010.

  1. killer6008

    Thread Starter New Member

    Jan 26, 2010
    20
    0
    I have a truth table with sw1 as input and two outputs LED 1 and LED2.

    The outputs i am getting are

    SW1
    H 1 0
    L 0 0
    H 1 1
    L 0 1

    After this the sequence starts repeating itself.

    Can someone explain what is happening and what will this type of circuit be used for??


    Thanks
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Looks like a 2-bit binary down counter:-
    SW1 decimal
    H 1 0 1
    L 0 0 0
    H 1 1 3
    L 0 1 2

    And then it rolls back around as you say.

    You don't say but I assume that you have 2 D-types.

    The only thing I am not sure of is that your counter decrements on both transitions of the switch. Some extra logic is normally required for this.
     
  3. killer6008

    Thread Starter New Member

    Jan 26, 2010
    20
    0
    Sw1 is connect to Clk
    LED 1 is through SW1 and LED2 is connected to Q.
    D is connected to Q-

    Its in a logic box 74HCT74.
     
  4. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    Ah! I see. The D-type changes state (toggles) on the low to high (0 to 1) transition of the switch.

    This is a divide-by-two circuit. Used in frequency dividers and binary counters.

    Bit more explanation:-
    On the low to high transition of the clock input, the state of the D input gets transferred to the Q output.
    Because the /Q output (the opposite of Q) is connected to the D input, on every clock the Q output changes state.
     
    Last edited: Feb 5, 2010
  5. killer6008

    Thread Starter New Member

    Jan 26, 2010
    20
    0
    thanks for that :D
     
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