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Looking for help desiginig (simple?) circuit

Discussion in 'Electronic Basics' started by Jack B. Pollack, Oct 11, 2003.

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  1. I am looking for some help designing a circuit, or finding a component that
    does the following:

    1. I need to be able to set the state of a TTL output H or L to follow its
    input (call this output X)

    2. If another input momentarily goes H, I need output X to toggle its
    state.


    I thought a D flipflop would be perfect for this.

    I was feeding the output of my circuit into the PRESET on the flipflop to
    set its state.

    I also wired the flipflop to toggle (-Q to D).
    My momentary H from step 2 generates the clock input for toggling.


    This was almost exactly what I wanted except the PRESET overrides the toggle
    input. I need a circuit that can be set or reset, but the toggle input will
    take priority over the set/reset input.


    Hope this is clear enough. Any help would be appreciated.

    Thanks
     
  2. Don Taylor

    Don Taylor Guest

    Maybe I'm off base here but it sounds like feeding your input to
    one side of an XOR and using your pulse alone to drive the D you
    have built and the output of the D to drive the other side of the
    XOR does what you want. The toggle will have priority then.

    This ignores timing glitches, which you haven't mentioned yet.
     
  3. Rich Webb

    Rich Webb Guest

    Errr... it's late but you might want to look at burning this into a GAL
    chip. The GALs are reasonably priced and the design software is free
    (WinCUPL -- see www.atmel.com) but you'll need a burner.
     
  4. John Fields

    John Fields Guest

    ---
    You may have conflicting requirements in that if your toggle input goes
    high momentarily and toggles the output then the sense of the S/R input
    will be reversed. If that's not a problem, then this ought to work for
    you:


    S/R >------------------A
    +-----+ EXOR Y--->X
    +--|D Q|------B
    T>-----|--|> _|
    | | Q|O-+
    | +-----+ |
    +-----------+
    __ ___
    Assuming an HC74, you can set the initial state with the FF's PR or CLR
    inputs, which I haven't shown.
     
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