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Discussion in 'Electronic Design' started by D from BC, Nov 1, 2007.

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  1. D from BC

    D from BC Guest

    This has got to be a classic signal clean up problem....

    I need a circuit that triggers on edge A, then ignores about 0.1uS of
    jitter then triggers on edge B and then ignores a following 0.1uS of

    +-+ +-+ +----------------+ +-+ +-+
    In | | | | | | | | | |
    A | | | | B | | | |
    -------+ +-+ +-+ +-+ +-+ +-----------

    |<0.1uS>| |<0.1uS >|
    |< 0.5uS >|

    Out +------------------------+
    | |
    A' B'
    -------+ +-------------

    Edge A to A' is ~ less than 10nS
    Edge B to B' is ~ less than 10nS

    All values are approximates.
    "In" and "Out" are repeating waveforms.

    I think I can do it with:

    1 flip flop
    1 >0.1us delay circuit
    Sprinkled with gates..

    Or maybe I need 2 flip for edge A and one for edge B..

    I'm not even sure yet which type of FF to get.

    If anybody has done this problem before and doesn't mind sharing..let
    me know a topology...

    In the meantime, I'll be doodling until I get a solution...

    D from BC
  2. Winfield

    Winfield Guest

    Are you sure the word you want is "jitter"? Once an edge
    is detected, there's nothing until the corresponding next
    opposite edge, right? The word jitter describes a periodic
    time uncertainty for when an edge will arrive, not what
    happens immediately after the edge arrives. If in fact
    you're worried about "bounce" or additional transitions
    in the 100ns after an edge, that's not called jitter.
    It's a type of digital noise.

    Anyway, if you add an XOR gate to your mix of available
    pieces, you'll be able to find a simple solution allowing
    the use of only a single oneshot delay element. But I
    might point out that most oneshot ICs come as dual parts.
    You may get a lower chip count by using two timers. BTW,
    there's a simple way to adjust the delay of two oneshots
    with one trimpot or external panel pot. It could even be
    a 10-turn precision pot with a turns-counting dial.
  3. Fred Bloggs

    Fred Bloggs Guest

    It would be simpler to learn how to terminate your signals properly.
  4. John Larkin

    John Larkin Guest

    Looks like you can do it with a dflop, a quad xor, and an RC.

    Run the input through a delay-line edge detector (three gates of
    delay, then xor) and clock the dflop. Then rc lowpass the input and
    apply it to D. Q is the output.

  5. D from BC

    D from BC Guest

    Yeah...jitter is probably the wrong word..
    I thought of jitter as some interval of spurious digital behavior.
    I'll read up on:

    One shots ICs and xors...neato!
    I've haven't thought about that yet. :)
    I'll give it a shot (<<pun!).

    I'll post a link to schematic variations once I settle on a design.

    D from BC
  6. D from BC

    D from BC Guest

    Are you thinking reflection?

    The source signal is from pcb power section with high di/dt and dv/dt.

    D from BC
  7. Seems to me that you could do it with a D flop, and two RC nets from Q to R
    and /Q to S to hard-hold it high or low for a tenth of a usec.

  8. D from BC

    D from BC Guest

    Ahhhhh........... :)

    I'll try drawing it while stuck in traffic...


    D from BC
  9. John Fields

    John Fields Guest

  10. John Fields

    John Fields Guest

    I think this'll work, conceptually, but to get that <10ns in-out
    delay (especially considering the propagation delay through the
    one-shot)you'll probably need to go ECL since even FAST is iffy.

    View in Courier:

    IN>-+-----------------A /
    | NAND Y------A
    +-[250ns]--+------B NAND Y--+-->OUT
    | \ / \ +--B |
    | \ / DLY1 | |
    | / | A--+
    | / \ DLY2 +--Y NAND
    | / \ / B--+
    +-[250ns]--+------A |
    | OR Y------------+
    +-----------------B \___

    The blocks labeled '250ns are cross-coupled one-shots, the top one
    high-going edge triggered and the bottom one low-going edge
    triggered. When either one is hot it keeps the other one from
    triggering on the bounce transitions after the first edge.

    Here's the timing:

    _ _ _ _ ___________ _ _____ _ _ _ _
    IN___|_|_|_|_| |_|_|_|_|___________

    DLY1__| |___ _ _________________________

    ___ ___ __ _ _________________________
    SET |______________|

    ____ ____________________ _ ______ ___
    DLY2 |______________|

    ___ _____________________ _ _______ __
    RST |______________|

    ________________ _ ________
    OUT ____| |________________

    I'm working out a simulation in LTSPICE, and it should be ready
    sometime tomorrow. :)
  11. John Larkin

    John Larkin Guest

    What's all that ??? about?

  12. John Fields

    John Fields Guest

  13. Tom Bruhns

    Tom Bruhns Guest

    Right, so the xor plus the dflop clock to output needs to have less
    delay than that... should be easy with fast parts. The xor is just to
    get the same polarity clock pulse from each (leading) clock edge. The
    d input is RC delayed, so you capture the "old" level; thus you take Q-
    not as the output. RC must be long enough to get past the multiple
    transitions at each edge. In fact, you could do it, I think, with an
    SR f/f (cross-coupled NANDs) driven from a similar xor nanded with RC-
    filtered clock...

  14. John Fields

    John Fields Guest

    It's about equivalent to "Huh???", basically questioning whether
    your scheme could get a clean, debounced output less than 10
    nanoseconds after the input edge, which was one of the OP's

    Especially since, with that quad EXOR, you seem to be talking
  15. John Fields

    John Fields Guest

  16. John Larkin

    John Larkin Guest

  17. John Fields

    John Fields Guest

    No, the clock input is nominally high, so when a new edge comes
    along it'll drive the clock input low. Then, after _three_ EXOR
    gate delays it'll clock whatever's on D to Q, so the input-to-output
    delay will be three EXORs plus one dflop.

    BTW, what CMOS parts did you have in mind?
  18. John Larkin

    John Larkin Guest

    It is not.

    0 xor 0 = 0

    1 xor 1 = 0

  19. D from BC

    D from BC Guest

    oh nooo.. I rushed another post and again left out details...

    Ton can vary up to 0.5uS..
    (Ton is the 0.5uS time in my ascii drawing.)
    The circuit should at least function between 100khz and 1Mhz.
    The only thing that is constant is the bounce time of about 0.1uS.

    Was that one shot idea based on constant Ton? Or variable Ton?

    D from BC
  20. D from BC

    D from BC Guest

    Ideally..edge A is transparent through the circuit.
    Ideally. edge B is transparent through the circuit.
    But no no no...everything takes time and I have to allow a max delay.

    I have a timing budget of around 10nS for A to A' and B to B'.
    This circuit is being inserted in a chain . All the delays are adding
    up.. :(
    10nS is tolerable and at first guess seems attainable.

    By the way.. I just made up the A and A' nomenclature...
    I do try to express problems in some way to be clear and simple.

    D from BC
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