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rising-falling edges

Discussion in 'Electronic Basics' started by Bart, Jan 1, 2006.

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  1. Bart

    Bart Guest

    In my projects with flip flops I notice my led's have tiny off's or on's
    that are out of rythm with the function or counting of my circuit. I set my
    555 astable to about 1 herz so I can visually watch the sequencing of my
    led's and my O'scope shows a very nice clean rythmic square wave. My
    circuits are simple with two chips, both dual flip-flops and some led's. Now
    I'm trying circuits with counters, for example a 74LS90 and some led's and
    I'm noticing the same tiny "flickers" out of rythm. I read about
    rising-falling edges as triggers, might this be a factor on what I'm seeing?

    Any help is appreciated,
  2. Bart

    Bart Guest

    I found this:
    "These are ripple counters so beware that glitches may occur in any logic
    gate systems connected to their outputs due to the slight delay before the
    later counter outputs respond to a clock pulse. "

    Should I stay away from "ripple" IC's..... Is synchronous a better choice?
    Is there "glitches" in asynchronous?

    What would a "Rule of Thumb" be on when to use "ripple", "synchronous", and

    Still learning,
  3. Bob Monsen

    Bob Monsen Guest

    It is hard to say what you are seeing, because you haven't described
    the circuit, or what parts you are looking at. However, any change should
    be on either the rising or falling edge of the oscillator, no matter how
    you have things hooked up. If there is additional flickering at other
    times, something is wrong.

    Bob Monsen

    Strange as it may seem, the power of mathematics rests on its evasion of all
    unnecessary thought, and on its wonderful saving of mental operations.
    - Ernst Mach
  4. Andrew Holme

    Andrew Holme Guest

    The glitches produced by LS TTL ripple counters are not visible on an LED -
    they're only nano-seconds wide.

    Do you have resistors in series with your LEDs?

    If you turn-up the clocking speed, and look at it on your 'scope, you'll see
    that the output high voltage of a 74LS90 is not quite the same for all
    states in the counting sequence. This is allowed, as long as it always
    exceeds the minimum stated on the datasheet, and could be visible as a small
    change in brightness.
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