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A strange problem with CMOS Up/Down counter

Discussion in 'Electronic Design' started by csl, Apr 25, 2005.

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  1. csl

    csl Guest

    Hey all,

    Been searching for topics relating to what I have been experiencing and
    to no avail.

    Hopefully some of you might have a hint or tip. I am using a TI
    CD40110B CMOS Up/Down decade counter. It has a separate clock up and
    clock down. I connected to a Kingston 7-segment LED and here's what I
    have been observing:

    1. Counting up from 0 to 9 is ok.

    2. Counting from 0 to 1 to 2 to 3 back to 2, when clock up is activated
    nothing happens. When activated AGAIN, it jumps to 3.

    3. Counting down from 9 to 2 to 3 then back to 2. Then clock up is
    activated, nothing happens. When activated again, it jumps to 3.

    I can't seem to figure out what's the problem. It only happens when one
    returns from 3 to 2 and making an attempting back to 3.

    If you have any suggestion, please post. Thanks for reading.
     
  2. Luhan Monat

    Luhan Monat Guest

    I would check the clock polarity. Make sure that the unused one is not
    left in its active state when clocking.
     
  3. Something strikes me as odd about your description
    that may help sort this out, (assuming Mr. Monat's
    comments don't do it).

    What is the difference between the count 2 to count 3
    transition during your step 1 and what should be the
    same transition during your step 2 or step 3? If the
    problem is repeatable, there must be some difference
    in conditions, either in the intended logic inputs or
    in some unintended input arising from cabling, the
    way the circuit is wired, interference, or ... something.
    So, what is it? The answer will be telling.
     
  4. What are you using for the clock signal? If you have pull-ups on the
    clock line, how big are they? I've had strange results like this when
    the rise time was too slow on the clock.
     
  5. csl

    csl Guest

    Thanks so much for the replies.

    Luhan Monat: I checked the clock polarity. The "down" clock stays as it
    should.

    Larry Brasfield: Indeed, this observation is very perculiar. Yes,
    problem is definitely repeatable. Somehow it appears to me that the IC
    knows whether I am starting from 0 to 1 to 2 and so on. But whenever I
    count from 3 to 2 and try to count back to 3, it won't allow me to do
    so unless I double clock it. I tested everything. I checked everything.
    Just doesn't make sense.

    Anthony Fremont: I am using a tactile switch. I used a 10k resistor and
    1uF capacitor. Thing is regardless of the size of the resistor or
    capacitor, this problem is still there. But you brought up an
    interesting point. So, if rise time is slow, it will take two "clicks"
    to swich from 2 to 3 instead of one click? But why is it that it only
    happens from 2 to 3?
     
  6. csl

    csl Guest

    I want to add another observation...

    Counting from 1 to 3 back to 2. At this point, if I click down to 1,
    it's ok. Then count from 1 to 2 to 3. It's ok.
     
  7. My experience was with a serial shift register. Every thing worked good
    until I clocked in a 0 bit, then all the outputs would reset
    simultaneously. You could try using a Schmitt trigger inverter to
    square up your clock signal and see if that helps.
     
  8. csl

    csl Guest

    Anthony, thanks for the follow up.

    I solved the issue this morning by adding a 0.15uF to the clock up
    input and ground it. And now the "2 to 3" issue gone. Was there too
    much noise?
     
  9. Bob Monsen

    Bob Monsen Guest

    According to the TI datasheet, the maximum clock rise or fall time is
    15us. Thus, your clock is far too slow. A 1uF cap + 10k resistor has a
    tau of 1/100. That means it is about 1000 times too slow. There is some
    wacky arbitration circuitry to protect against races between the clocks
    which could be getting you.

    Regards,
    Bob Monsen
     
  10. Fred Bloggs

    Fred Bloggs Guest

    Unbelievable !!CRAP!! "What is it?"- that's what he's asking you,
    stupid? And as usual, all anyone gets is a bunch of verbose gibberish.
    If you don't know what you're doing then just stay the hell out of the
    thread, you are a worthless jackass pseudo-intellectual.
     
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