Wonky Digital Clock?

Discussion in 'General Electronics Chat' started by teejusb, Mar 30, 2012.

  1. teejusb

    Thread Starter New Member

    Feb 25, 2012
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    So for one of my course projects, I am designing a Digital Clock which using JK flip flops, and while the simulation in Multisim seems to work without any hiccups, when I physically build the circuit it goes out of whack.

    I just finished building the first two levels (the ones and tens of the seconds display) and while the ones works fine (counts from 0-9), the tens (0-5) digit displays odd values. I know both the tens and the ones works fine because if I use an independent clock for each level, both count correctly, but when I implement the 4 input AND gate the tens digit no longer displays correct values and for some reason, even changes when the AND gate isn't high.

    Below are part of my circuit. I also realized instead of a 4/3 input ANDs for the faux-clock, I could use a two input and and make my work easier.

    Part Circuit
    Full Circuit
    (Image is pretty small :/ )
     
    Last edited: Mar 30, 2012
  2. Wendy

    Moderator

    Mar 24, 2008
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    Welcome to AAC! You are that most rare of people, a first time poster who has included schematics! Way to go.

    The way to handle large images is to post them as attachments, and not post them as full screen images. That way the post looks OK, but you have all the information for those who are really interested.
     
  3. teejusb

    Thread Starter New Member

    Feb 25, 2012
    4
    0
    I changed the images to hyperlinks instead since I couldn't make them attachments through the edit feature.

    Does anyone have any advice? Any help at all?
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    How to Upload an Attachment To attach a document or picture to a post

    Click "Go Advanced" at the bottom of your post,

    One of several options will pop up,

    Click "Manage Attachments" much further at the bottom of your post,

    To upload a file from your computer click "Browse", then select the file.

    Schematics should be .gif or .png format, pictures should be .jpg. .jpg formats will fuzz out schematics, and should not be used for that purpose.

    Since you are a new user you will not be able to modify your posts after a set duration. That feature clicks in after 10 posts.

    I would repost the picture as you had had them, they were much more readable.

    One of several things I would question is bypass capacitors. They are not a luxury, but a requirement.

    Decoupling or Bypass Capacitors, Why?

    I have designed several digital clocks over time. It is a common theme here at AAC, and probably will be forever.

    Digital clocks
     
  5. teejusb

    Thread Starter New Member

    Feb 25, 2012
    4
    0
    Ah, power spikes could be an issue. So would I need to be connecting capacitors across the power and ground of EACH of my ICs (I have like 30). Where else would I be needing capacitors, and of what value? I'm not too sure where I need capacitors. In my circuit, I have two capacitors which I use with my 9V to 5V converter and a few with my clock, but no where else.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    It is considered good design practice. You might be able to get by with less though.
     
  7. billnow

    New Member

    Aug 4, 2010
    9
    1
    You may have a race condition which is causing decoding spikes out of the 4 input AND gate that is the clock for the ten's stage of the counter. These glitches could cause erratic clocking of the counter.

    As a quick fix try putting something like a 1000 pf capacitor from the output of the 4 input and gate to ground.

    In general it is a bad idea to create clocks with combinatorial logic
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    You do realise every school assigns exactly that project, right?
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    I have a partial project pretty similar...

    Digital clocks

    It will be for the AAC book when it is finished.

    We are always getting clock projects here. If you can't design a clock, there is a lot of other stuff you will no be able to design.

    I have seen people who could work marvels with PICs that didn't have a clue with gates and flip flops.
     
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