74LS90 chip

Discussion in 'General Electronics Chat' started by nismospecv, May 5, 2007.

  1. nismospecv

    Thread Starter Member

    Mar 14, 2007
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    Does anybody have any information on how to make a divide-by-2 counter and a divide-by-4 counter using 7490 chips?

    I'm not really sure how to even go about making a table for this to get me started.

    If you already know the pin layout for creating it, that would be helpful also.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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  3. nismospecv

    Thread Starter Member

    Mar 14, 2007
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    This doesn't make sense to me. What do you mean by use pin 12 for divided by 2?

    Also, what would a divide-by-3 look like?
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    The signal on pin 12 will be 1/2 the frequency of the signal on pin 14.

    Look at one of the datasheets in the link I provided.

    Divide-by-three will require more than just a 7490. You'll have to reset the chip every third clock cycle with external logic.
     
  5. Salgat

    Active Member

    Dec 23, 2006
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    Couldn't you do an approximate divide-by-3 by tying QA and QB to the ANDing MR0? That way when it hits 3 it resets to 0 and then has to count 3 times again to get back to 0(a pulse every 3 pulses for QB)?
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    I think that would work! It should be quite precise at lower frequencies, too - only a few dozen nanoseconds too long of a pulse at pin 9. Similarly, one could get approximate divide-by-five or divide-by-nine from using pin 8 or pin 11, respectively.

    One could also divide-by-seven by ANDing pins 12 and 8 in bi-quinary mode.

    Congratulations, Salgat! Its a divide-by-n counter!
     
  7. Iommi

    New Member

    Jun 3, 2007
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    I apologize, but I don't understand Salgat's answer to the "divide by 3" question. What exactly must I do in order to use 7490 IC's in a divide by 3 configuration. Thank you so much for your patience.
     
  8. Salgat

    Active Member

    Dec 23, 2006
    215
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    Tie the pins QA(Q1) and QB(Q2) to the pins MR0, which has 2 inputs. By tying each output to the different MR0 inputs, the counter will reset when the counter reaches 3. A schematic might be needed if you don't understand :\
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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