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combining two pulse streams for counting

Discussion in 'Electronic Basics' started by Paul Ciszek, Sep 20, 2005.

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  1. Paul Ciszek

    Paul Ciszek Guest

    Suppose I have two sensors that give off a pulse every time an
    event occurs; I have a choice of negative-going or positive-going
    pulse. I also have a counter that is triggered by a falling edge
    followed by a rising edge. I am looking for a simple way to
    combine these two streams of pulses to get an accurate combined
    count. The problem with using an AND or OR gate is that it wouldn't
    correctly handle the case where one pulse arrives while another is
    still in progress. So, I was thinking of doing the following:
    Have one of the streams be normally high and negative-going; have
    the other be normally low and positive-going. Feed the two signals
    to the inputs of an XOR gate. That way, a lone pulse on either
    line would cause the output of the gate to fall and rise; a
    pair of overlapping pulses should make the output fall and rise

    So, what's wrong with this? Usually something this simple is

  2. Alas, it will not be reliable. As apparently both streams are not related,
    what will happen when two pulses arrive at the same time? Or, maybe even
    worse, when they arrive almost at the same time? The first thing may be very
    rare and you will have -theoretically - no output at all. The second will
    give pulses that are too small or too low to be seen by the counter.

    A simple, straight forward solution will be counting both streams seprately
    and then use a full adder to obtain the result.

    Otherwise you will have to build a circuit that samples both streams and
    give exactly one outputpuls for every inputpuls, separated when both occur
    at the same time. For not too high frequencies - let's say some hundreds of
    kHz - a small micro is an ideal component. If you can't use one, you will
    need some flipflops and glue logic to build a state machine plus a clock to
    drive it.

    petrus bitbyter
  3. mike

    mike Guest

    This won't work when the edges are coincident.
    Simplest general solution might be to use two counters and sum
    the outputs digitally.
    If you can restrict the relationships between the input signals,
    things could get very much easier.

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  4. ehsjr

    ehsjr Guest

    Regarding the XOR, simultaneous same duration pulses will fail
    (3rd example below), as will simultaneous rise or fall with
    different durations (examples 4 & 5)

    Can you control the duration of the pulses from the sensors?
    I'm envisioning the possibility of "pulses" from sensors that
    are of random duration. For example, a power failure sensor
    or low water level sensor could be low (or high) anywhere from
    seconds to days.


    Sensor 1: ________| |_________
    Sensor 2: __________| |__________
    - --
    XOR : ________| |___| |_________ GOOD!

    Sensor 1: ________| |_________
    Sensor 2: __________| |__________
    - ---
    XOR : ________| |______| |_________ GOOD!

    Sensor 1: ________| |_________
    Sensor 2: ________| |__________

    XOR : __________________________ BAD!

    Sensor 1: ________| |_________
    Sensor 2: ________| |__________
    XOR : ____________| |_________ BAD!

  5. Jasen Betts

    Jasen Betts Guest

    If you get two events at exactly the same time they won't get counted.

    or if ome pulse ends as the other begins it produces one pulse on the output.

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