digital clock hours problem

Discussion in 'Homework Help' started by btroush, Feb 17, 2012.

  1. btroush

    Thread Starter New Member

    Oct 28, 2011
    7
    1
    I am building a digital clock using 7490s for the minutes and seconds, but I have to use a 74192, 2 jk flip flops, and 2 3-input NAND gates for the hours and am/pm indicator. Can't figure out how to hook it up. I have the minutes and seconds circuits done and working, just stuck on the hours. All I have are multisim files for schematics, will post as soon as I figure out how.
     
  2. btroush

    Thread Starter New Member

    Oct 28, 2011
    7
    1
    here it is
     
  3. btroush

    Thread Starter New Member

    Oct 28, 2011
    7
    1
    i have the NAND gate hooked up so that when the Q output of the FF is high, and the counter hits 3 (0011) it will clear the FF and load a 1 into the counter, so it jumps from 12 back to 1. not sure what to do with the other NAND gate though, or what to do with the clock and preset inputs on the FF. Not looking for someone to finish it for me, just push me in the right direction...
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Couple of points.

    The big one is your image format. No one wants to touch a .doc file because it can be a computer virus vector. You are much better off using a .gif or .png type file, which are image files already. These kinds of files can be displayed directly on the post.

    Your JK flip flop is not wired correctly. I'm not sure what you are trying to do with it, but you have several open inputs. This is always an illegal condition for digital circuits. Inputs must be a 1 or a 0. What are you trying to do with this flip flop anyhow?

    I've taken the liberty of converting this image over for you.

    [​IMG]
     
    Last edited: Feb 20, 2012
  5. btroush

    Thread Starter New Member

    Oct 28, 2011
    7
    1
    First off, thank you for the tips, and f
    or converting the file for me.

    The flip flop is to be used to control the tens of hours. I have had it connected in several different ways, but none of them worked properly. I was trying to start over, that's why there are open inputs. The only part I was confident was hooked up right was the Q output to the NAND gate, which will go low when Q=1 and a 2 (0011) appears on the counter to clear the flip flop back to 0 and load a 1 into the counter so it will jump from 12 back to 01 instead of 12 to 00.

    Once I get the hours functioning properly, I need to use a second jk flip flop to control a LED for an am/pm indicator.
     
    Last edited: Feb 20, 2012
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Start fixing known problems, tying the inputs down.

    U13 carry out should go to the U16A clock for example.
     
  7. Felo

    Member

    Feb 20, 2012
    91
    13
    Hi,

    May I ask if it is an absolute MUST to do this with TTL's or maybe you could use a MCU?? I have a project that only uses one 16F628 for a 4 digit clock that I would be happy to share if there is any interest. I believe that microchip has an APP note on building such clock with a F84...

    Good Luck
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Guy, this is homework forum, not the projects forum. What do you think?

    µC are good for many jobs, but they don't teach basic digital. Can you design a 1 to 12 counter using flip flops?
     
  9. btroush

    Thread Starter New Member

    Oct 28, 2011
    7
    1
    Got it.

    I tried clocking the flip flop with the carry out, but the carry goes low halfway through the "9" cycle and goes high again when it resets. That was giving me a half cycle on 9, then half cycle on 19, then 10.

    To fix it I needed to tie the preset from the FF high, and clock it with Qd from the 74192.

    Then, for the am/pm indicator, I had to use a second jk flip flop, and a second 3 input NAND gate. To do that I used Qb from the 74192, and Q from the first FF for two of the NAND gate inputs and tied the third input to Vcc. Then I used the output from that to clock the second FF which was also in toggle mode, and used an LED off of Q.

    Ill post the completed schematic when I get home.

    Thanks for the help
     
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