Problem with digital electronic

Discussion in 'Homework Help' started by Beginner0001, Sep 29, 2017.

  1. Beginner0001

    Thread Starter New Member

    Sep 29, 2017
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    I need to make mod-12 asynchronous down counter using T flip flop. It counts from 15 to 4. I don't know really how to do it and it's really important, so if you could help me i would be very grateful!
    Thank you
     
  2. dl324

    AAC Fanatic!

    Mar 30, 2015
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    Welcome to AAC!

    For homework, you need to show your best effort before members can try to guide you to a solution.
    Are you certain you're supposed to design an asynchronous counter?
     
  3. Beginner0001

    Thread Starter New Member

    Sep 29, 2017
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    yes, i'm sure :)
     
  4. dl324

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    Normally synchronous counter designs are preferred over asynchronous.

    Post your best effort so members can provide guidance in the area you're having difficulty.
     
  5. Beginner0001

    Thread Starter New Member

    Sep 29, 2017
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    Don't know, but right now i need asynchronous.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Do you know what a T flip-flop is?
    Do you know what a mod-12 counter does?
    We need to know your basic understanding level of what you need to do.
     
  7. dl324

    AAC Fanatic!

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    Post your best effort so we can see where you're having difficulty.
     
  8. Beginner0001

    Thread Starter New Member

    Sep 29, 2017
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    I know what T-ff is and i i kinda understand what mod-12 counter does, but not completely. Like i understood that means that i need to reset counter when it gets to 12, so it can start from begining. I hope you understand what i'm trying to say.
     
  9. dl324

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    That isn't what you said in your first post. You said mod-12, but it starts at 15 and counts down.
     
  10. WBahn

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    Mar 31, 2012
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    Your initial post says that your mod-12 counter is supposed to count from 15 to 4. The "mod-12" merely indicates that it has 12 states that it cycles through. The "15 to 4" tells you what those 12 states are, namely

    15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 15, 14, 13, ....

    There are many ways to approach this. Perhaps start with what you likely already know.

    If you were asked to design an asynchronous up-counter that went from 0 to 14, could you do that?
     
  11. Beginner0001

    Thread Starter New Member

    Sep 29, 2017
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    Oh, that was confusing to me, i think i understand it little better now.

    So, counter has to start at 15, then count down to 4. When it gets to 4 then needs to be reset. Am i right?

    Can i draw a truth table and send it so you can tell me is it right?
     
  12. dl324

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    You don't use truth tables for asynchronous counter design. That's why I asked if you were certain it wasn't supposed to be a synchronous counter.
     
  13. WBahn

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    Before you start doing truth tables, you need a state diagram and, from that, a state transition table.

    Your state diagram needs to reflect the basic approach you have chosen to take. Like I said, there are several viable approaches.

    Don't send it to me, post it here.
     
  14. xxxyyyba

    Active Member

    Aug 7, 2012
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    I think that this is explained in every Digital electronics book. There are some steps which you need to follow and that's it. It is explained very well in Digital Fundamentals by Thomas Floyd for example. Roughly speaking, you draw state diagram first, then you write transition table for T flip flop (every flip flop has its own transition table) and after that you have enough informations to fill k-maps, which are necessary to find logical connections between flip flops which make counter. Design procedure is always same, regardless of flip flop type used. So once when you are familiar with design using T flip flops for example, you can design using other types of flip flops also.

    I don't think that someone will write step by step tutorial in detail about complete process of counter design. That is because homework section is not meant to function like that. Consult some Digital electronics book or web, there is probably a lot of informations about this online. Then, when you are completely familiar with counter design process, if you stuck somewhere you come here :)
    Best regards
     
  15. Beginner0001

    Thread Starter New Member

    Sep 29, 2017
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    upload_2017-10-3_16-56-19.png
    Here is the table
     
  16. WBahn

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    You have two columns labeled "Q3". Why? How is a reader supposed to know what Q3 is when you have different values for it? Don't make people guess.
     
  17. Beginner0001

    Thread Starter New Member

    Sep 29, 2017
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    The second one represents down counting, the first one is there just to make the other clearer. So important part of table begins with second Q3.
     
  18. dl324

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    Your table has several errors:
    upload_2017-10-3_8-54-26.png
    Sure looks like you're doing a synchronous design to me...
     
  19. WBahn

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    If I understand you correctly, the first group is for the current state and the second group is the next state?

    If so, then you are saying that from State 15 you are going to go to State 4. Is that what you want?
     
  20. WBahn

    Moderator

    Mar 31, 2012
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    The state transition table doesn't really indicate whether it is synchronous or asynchronous. You could draw up a table for a ripple binary counter and it would look the same as the table for a synchronous binary counter. Where the different come in is what comes next -- the excitation logic. For a synchronous design you craft the logic for the control inputs of the FF, for an asynchronous design you craft the logic for the clock (and/or asynch set/reset inputs). Therein is where the demons lay in wait.

    If he REALLY needs to do this asynchronously, I would recommend making a simple mod-12 ripple up counter and then decode the outputs to get the desired sequence.
     
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