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74LS76 and 74F112PC JK flip flops weird behavior

Discussion in 'Electronic Basics' started by Mohammed Hamed, Feb 27, 2004.

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  1. I tried to hook a simple circuit using a 74F112PC or 74LS76 (both get
    the same behavior) dual negative edge triggered flip flop, but I get
    weird behavior when both JK inputs are set to high. Whenever I made
    the toggle manually by toggling the clock input, the flipping
    (complementing) action doesn't always occur. This could be explained
    easily by switch bouncing/debouncing. Although I don't know how much
    bouncing exists in regular DIP switches. But the most puzzling thing
    happens when I tie the Clock input to a regular square wave clock
    (generated off a logic training board. The clock doesn't show any
    glitches on oscilloscope output. However, sometimes the output doesn't
    change, sometimes it follows the clock faithfully when it's supposed
    to change on the negative edge, sometimes it doesn't work unless the
    clock input pin is tied to an onboard LED, and sometimes it works well
    when i touch the plastic coating of the wire between the clock and the
    onboard LED and usually not all the LEDs. This weird behavior happened
    with all the chips that i have for these 2 families, and it's
    interesting that it's happening across families. I don't know what it
    could be. The fact that touching the wire made me think it needs some
    path to ground to lose extra charge (although i'm touching the
    plastic). I tried to tie a large resistor across the clock input with
    no much success. Do you have any ideas?

  2. chris.welsh

    chris.welsh Guest

    How is the circuit grounded? It sounds like there might be a floating
    ground somewhere...
  3. CFoley1064

    CFoley1064 Guest

    Subject: 74LS76 and 74F112PC JK flip flops weird behavior
    Youdefinitely have a problem with the clock. For the LS76, you need a single
    transition with a fast rise time -- if you don't get it, you're toast. You
    can't do that with a switch alone. All a resistor between the switch and the
    input will do is slow the rise and fall times (an RC network with the
    capacitance of the input).

    You'll want a logic change on the NGT, so here's a trick that might work if
    you've got a couple of CMOS NOR gates handy (view in fixed font or M$ Notepad):

    Debounced Clock Switch Using Hardware NOR
    .-. 47K
    | | ___
    | | 22K .-----|___|--------.
    T '-' 10K | |
    --- ___ | ___ | __ |
    .----o o--|___|-o----|___|---o--|>=| __ |
    | 10 Ohm | |1 |o-----|>=| | CLK'
    | --- .--|__| |1 |o-o----o
    === --- 1uF | .--|__|
    GND | | |
    === === ===

    If you've got CMOS inverters, just replace the NORs with inverters -- it will
    work just as well. The circuit uses hysteresis to overcome the slow
    transitions at the switch.

    Good luck
  4. (CFoley1064) wrote in message
    Thanks for the response. 2 things here. First, The clock has rise and
    fall time on the orders of nanoseconds as checked on the oscilloscope.
    Second, does that explain the behavior when the output follows the
    clock for long a time (many seconds sometimes). My guess that if these
    were random glitches, I would get this behavior every once in a while.
    It doesn't explain either when touching the wire makes it work, as i
    don't believe this will make the rise time any faster.
  5. Roy McCammon

    Roy McCammon Guest

    I suspect ringing of the clock signal at the clock input of the flip
    flop. It there is only one load on the clock, try a small (30-75) ohm
    resister in series with the clock at the clock driver.
  6. I think the problem is with clock ringing, I drove the circuit from a
    clean clock source and it works fine. I tried the small series
    resistor but with no much success. Also I find that the instances in
    which the flip flop is working have always the clock with no ringing
    or very less. This happens when I connect an LED across the clock. The
    LED is in series with 5k resistance and a darlington pair from a
    ULN2803A chip.

    My questions here are: If the ringing is not too severe to alter the
    voltage levels, why does it cause the chip to malfunction? What I
    understand is that the logic gates inside the flip flop have predefine
    voltage levels with some noise margins that are most likely than the
    ringing noise.

    A 2nd question is :
    Why the series combination of the 5k resistor and Darlington pair
    across the clock input pin (i think the LED has no effect), why they
    almost eliminate the ringing?

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