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Digital Monostable?

Discussion in 'Electronic Repair' started by cooldude, Aug 15, 2006.

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  1. cooldude

    cooldude Guest

    Hi,

    1) Is there any equivalent to monostable in fully digital electronics
    (logic gates)? I am trying to trigger a pulse (a set width) with a very
    narrow recovery time (in the range of 100ns or less) before the next.
    74LS221 seems to work although getting weird pulses (very short
    retrigger pulses) occasionally.

    2) And also whats the difference between the normal monostable and the
    precision ones? As the name suggested they'd be more precise
    triggering?

    3) Would 74HCT version will be more suitable although I read somewhere
    they are slower than the ls version.


    Thanks,
    John
     
  2. This is not a problem with the device but rather power supply decoupling,
    glitches on the clock signal, a race condition, or noise on the wiring.
    Monostables work reliably when properly fed.

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  3. Sam's right. Any noise on the power buses is going to be bad news for
    the monostable. That's because a monostable works by comparing a
    rising ramp voltage with some logic threshold. There's no problem at
    first, because the ramp is down near zero volts and the threshold is
    like 1/2 to 2/3 Vcc, But as the ramp rises, the monostable keeps
    trying to compare 1, 2, 2,.5, 2.6, 2.7 volts with the current
    threshold. As the time interval draws to a close, it's trying to
    compare smaller and smaller differences. Now the other fully-digital
    chips won't be bothered by a volt of noise, but this comparator will
    jitter all over the place, in proportion to the jitter on Vcc.

    Try putting a 100 ohm resistor in series with the +5 volts going to the
    monostable and bypass that to ground with a 10uF capacitor PLUS a 0.1uF
    ceramic. Make sure the timing resistor and capacitor are fed off this
    decoupled Vcc source too!..
     
  4. And if you need somewhat more stable delays, there are "delay line"
    IC's, basically 74HC14 inverters with little resistors or inductors
    between the inverters. Kinda kludgey but the kludges are hidden under
    an epoxy coating.
     
  5. All monostables in that region I'm aware of, use RC for the timing.
    (Although that R is not necessarily an external one.)

    Sam mentioned the necessity of powerline decoupling already. Another cause
    of spourious pulses are distorted trigger signals. Long lines (wires) or
    component mismatch are known for that.

    Generally speaking you should expect more precise timing. That's to say the
    pulse duration should be less depended on the IC and more or only on the
    external components. But the slogan "precision" can be interpreted in more
    ways. So the real properties should be extracted from the datasheets.
    Once more the datasheet(s) should tell you.
    petrus bitbyter
     
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