how 74191 counter works?

Discussion in 'The Projects Forum' started by cupcake, Oct 8, 2010.

  1. cupcake

    Thread Starter Member

    Sep 20, 2010
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    Hi, I need some explanation about 74191 synchronous 4-bit up/down counter with mode control, works.. I have read the datasheet, but I still don't quite understand how it works.

    Suppose, I have an input value, and I want to make 74191 counts it, three times, then terminated. for instances, my first input 00, 01, and 11, I want the counter to read it one by one, then after it reaches three times, it should terminate.

    how to make the counter read it? in 74191 got 4 input value, 4 output and 1 enable chip. do I connect the input of the counter to my previos input (00,01 and 11) ? or just use the enable chip to counts? but then how do I know it's already reach three counts?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    The presets are just that, you can start it from a specific number instead of zero. CMOS has a very similar chip, they are very useful for clocks.
     
  3. cupcake

    Thread Starter Member

    Sep 20, 2010
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    I still don't get it, then what should the input value for the counter? and how should i make it stop, when it already counts three times?

    can you give me some examples...?
     
  4. marshallf3

    Well-Known Member

    Jul 26, 2010
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    It
    s generally done by having one of the output pins activate the /RESET
     
  5. cupcake

    Thread Starter Member

    Sep 20, 2010
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    hmm..there's no reset output I think, for 74191 then how do I know when its already counts three times? by clock signal?
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Odd, you're right. Try looking for a different chip? There's bound to be something out there that will do exactly what you're wanting.
     
    Last edited: Oct 8, 2010
  7. Wendy

    Moderator

    Mar 24, 2008
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    Here is the datasheet, which should always be your starting point.

    You can put any binary number on the load inputs. When the load input (pin 11) is taken low the number is entered into the registers, therefor the load input should normally be high.

    Enable (pin 4) allows it to count, while down/up (pin 5) picks which direction of counting you want. Ripple clock (pin 13) and max/min (pin 12) allow it to feed to another 74191 to make a larger count circuit.

    There is no reset pin, which means you need to load 0000 into the presets (pins 15, 1, 10, 9) and toggle load.

    A simple combination of inverters and a 4 input AND gate will detect any number, including a 3 count, you need.

    Personally I prefer the 74192, which is very similar in many ways. It is what I thought you were talking about originally.

    Perhaps you could show us the count sequence you are after?

    BTW, most counters are sequential, which means a 3 count would be something like...

    00
    01
    10
     
  8. cupcake

    Thread Starter Member

    Sep 20, 2010
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    Thanks a lot! your explanation is really helpful. sadly, I only have 74191, btw, could I just use two pins input? and put another input two ground? so, I load 00 to pin 15 and 1 for example, and start counting up.

    and the sequence will be..
    00
    01
    10
    *should i stop counting right here? or counting up again until it reached 11, then reset 00?
     
  9. Wendy

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    Mar 24, 2008
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    Is that your entire count? Any other conditions? You could use much simpler chips, such as dual flip flops. I suspect this is a sledgehammer approach to a simple problem.

    While I don't have the schematic handy, two JK flip flops will do the count directly wired appropriately, while the reset method will have a transient number that changes very fast. They are used for old style digital clocks (time pieces). Look at internal schematics of the 7490 and 7492 for example.
     
  10. cupcake

    Thread Starter Member

    Sep 20, 2010
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    actually this one related to my other thread http://forum.allaboutcircuits.com/showthread.php?t=43934

    I'm trying to understand the chips first before drawing my schematic diagram, but I have problem in understanding the counter, how to make it counts three cycles only? I still don't get the part to terminate the count after three clock cycles period.
     
  11. retched

    AAC Fanatic!

    Dec 5, 2009
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  12. Wendy

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    Mar 24, 2008
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    The problem I see with using a 74191 is simple, no reset pin. A 74192/93 has this feature, as does any dual flip flop in the world.

    I'm spoiled, I have cultivated a really deep stock of parts so I can write articles and wire stuff on whim. For example, see Project: Resistor Parts Storage.

    As it is, you going to have to use extra gates just to add a feature similar to reset. Unless this is a must do (as in school assignment) I suspect you would be better off stepping back and getting something like a 7474.

    I was taught TTL in college (30+) years ago. If you have a choice CMOS is much better, since it will operate off of any voltage you care to use, no precision power supplies needed. Having said that, I notice cell phones and other devices have some really small wall warts that are 5V @ 1A, which I would have killed for in my TTL days.

    I tend to use diode gates a lot, they are easy to lay out and do the job.

    Do you want me to sketch out something using the 74191 that is a simple counter? It will require extra gates.
     
  13. cupcake

    Thread Starter Member

    Sep 20, 2010
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    ahh...I think I have 74193 maybe I'll try take a look and understand the datasheet first... Thank you..
     
  14. Wendy

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    Mar 24, 2008
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    BTW, this is a standard question, one you don't have to answer. It does help us refer sources and parts. What part of the world are you in? You'll note most of the old hands have it in their profile, this is why.
     
  15. cupcake

    Thread Starter Member

    Sep 20, 2010
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    does 74193 have a reset pin? I'm looking to the datasheet right now, but didn't notice anything.. is something called 'borrow' and 'carry'??
     
  16. Wendy

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    Mar 24, 2008
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    That is to cascade counters, so it can count up and down. One part is BCD, the other is binary.

    Look at post #7, I posted a pinout I made as part of my PaintCAD package (used for quick and dirty drawing of schematics).

    Think pin 14 (clear).
     
  17. cupcake

    Thread Starter Member

    Sep 20, 2010
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    AH now I see... Thank you Sir!
    so, basically clear is an input,right? so, how am I gonna use it to count three cycle? once I enable the input and count up, how should I terminate and clear? I'm so sorry I'm a bit slow.. my lecturer has not covered counter yet..
     
  18. retched

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  19. cupcake

    Thread Starter Member

    Sep 20, 2010
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    thanks, I'm watching it right now..
     
  20. Wendy

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    Mar 24, 2008
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    Again, not the best way, there are better.

    When the count 11 the chip is cleared.

    [​IMG]
     
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