Digital clock (24hr format)

Discussion in 'The Projects Forum' started by ramy11, Apr 16, 2013.

  1. ramy11

    Thread Starter New Member

    Apr 16, 2013
    4
    0
    Hello I'm making a clock with the chips 74190,7447,7408 and 7404 so far I got it all working except one problem that my hours are starting with 1 right at the start and I can't manage to figure out why. I know that pin 3 in U20 is always at high causing the 1 to show but I dunno what's doing that...

    Hopefully someone can help me out I will attack both multisim file and a picture of it

    Thanks for the help
     
  2. ramy11

    Thread Starter New Member

    Apr 16, 2013
    4
    0
    anyone able to help me please? I'm stuck at the part why it's starting at 01 hours 00 min 00 sec
     
  3. KnRele

    New Member

    Jan 7, 2013
    20
    8
    Is this happening just after power-on, or is it that the hours don't reset from 23 to 00 but go from 23 to 01 instead?

    The count input (on U20) goes L-H when the hours digits advance, then when the count has reached 24, your gates U23A-U24A will make the LOAD(L) inputs go L, and re-set the counters a brief moment later. That all seems right. (LOAD(L) means that the LOAD command line is active Low)

    But remember that the initial state of sequential devices such as the 74190 are not generally predictable, so you may just be lucky that most of your counters start out at 0 or 1 -- they could start at any of the 16 different states (yes, even the decade counter could initially deliver combinations beyond 9 (1001) ), and require a lengthy process of cycling them through and into the sets of legal values.

    I'd recommend adding a reset line, RESET(H); in your diagram this would be functionally ORed with the existing LOAD(H) signals going into your inverters U12A U15A and U24A, replacing these with NOR gates (7402) and also feed the RESET(L) (that would be the RESET(H) inverted) into the LOAD(L) inputs of your U5 and U7, to reset these.
     
  4. ramy11

    Thread Starter New Member

    Apr 16, 2013
    4
    0
    What my problem is that it's starting at 01 at the power on it resets fine and everything else just on the power on the hours starts at 01 and the problem is that these are the only chips we're allowed to use for my class.
     
  5. ramy11

    Thread Starter New Member

    Apr 16, 2013
    4
    0
    was anyone able to figure out my problem or maybe a different way to connect them but with the same chips thanks
     
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