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What can cause logic glitches

Discussion in 'General Electronics Discussion' started by TommyF, May 18, 2015.

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  1. TommyF

    TommyF

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    May 18, 2015
    What can cause logic glitches and where can they come from in a logic circuit?

    Propagation delays, power supply spikes, different logic families
     
  2. davenn

    davenn Moderator

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    all your ones and improper tying of unused inputs to GND or +V via an appropriate resistor

    do you have a specific problem with a particular circuit ?
    if so, show us a circuit diagram


    other members may add to the list ;)
     
  3. Tha fios agaibh

    Tha fios agaibh

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    Bad programming. It's possible to have a line of logic that is scanned by a cpu later than the other portions of logic (example: subroutines). By the time one portion of logic executes, the other conditions have not been updated.
     
    ramussons likes this.
  4. poor mystic

    poor mystic

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    power supply problems. (I believe in testing the supply before anything else.)
     
  5. hevans1944

    hevans1944 Hop - AC8NS

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    Biggest source of glitches is inadequately by-passed power supply rails. Each time a high-speed logic device changes its output state, a current pulse occurs at the power supply input to the device. This pulse can then propagate as a "glitch" over the power supply rails to other devices. The solution is to always bypass the power pin on EACH device with a capacitor, typically 0.1 μF stacked ceramic type, connected as close to the device as physically possible between the power supply rail and the common rail. Back when logic ICs were sold in DIP packages, you could purchase sockets with this capacitor built into the socket. Good PCB layout is also important, especially when using the faster logic families with clocks approaching one gigahertz. Having power and signals on different layers of the PCB is virtually mandatory.

    Glitches in a production circuit design are an indication of either poor engineering or defective components. I would suspect both, given the current explosion of products originating from the Pacific Rim nations, and the fact that a new generation of engineers needs to re-learn techniques from the previous century while discovering new techniques apropos to the technology of this century. As things get smaller and faster the problem of "glitches" and how to eliminate them becomes much more difficult to solve. And the test equipment you need to even "see" a pico-second "glitch" is very expensive. <sigh> Sorta glad I am retired and will avoid such trials and tribulations.
     
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  6. TommyF

    TommyF

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    May 18, 2015
    Why do they use ceramic instead of electrolytics or tantalum?

    I have seen decoupling caps as electrolytics and sometimes i see them as tantalum and other times i see the molded ceramic caps. I'm not sure when to use electrolytic or tantalum decoupling caps.

    But why Stacked ceramic? do you mean the multi-layer ceramic
     
  7. davenn

    davenn Moderator

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    They tend to be more stable and they also have a smaller footprint on the circuit board for a given value

    tend to be used for the larger values and the initial brute force smoothing. Then after that, smaller value ceramics will be used

    yes

    Dave
     
  8. TommyF

    TommyF

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    May 18, 2015
    So it has nothing to do with ESL or ESR? I can interchange the capacitor material types using the same values, voltage rating

    I have noticed the electrolytics have a temperature rating, the higher temp rating handles high frequencies better?
     
  9. davenn

    davenn Moderator

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    do I really have to answer that ?

    what do you really think a higher temp rating means ?
     
  10. TommyF

    TommyF

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    May 18, 2015
    Heat dries up electrolytic capacitors. When a electrolytic capacitor is drying up it loses it's capacitance causes the ESR in ohms to go up to meg ohms. This changes the frequency response.
     
  11. davenn

    davenn Moderator

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    that didn't answer my question

    try again
     
  12. TommyF

    TommyF

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    May 18, 2015
    Handles more heat and higher temperature before it dries out the electrolytic liquid
     
  13. davenn

    davenn Moderator

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    Sep 5, 2009
    Bingo :)

    and obviously, that has other good effects
     
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