Parts of a digital clock I don't understand

Discussion in 'The Projects Forum' started by Elashen, Apr 21, 2010.

  1. Elashen

    Thread Starter New Member

    Dec 30, 2009
    Hello Everyone

    First of all, I'm a french native, so I'm sorry for my mistakes.

    I'm not following any electronic class. I'm making a project in my Physic Electricity class.

    I chose this diagram:

    I finished the 'Time clock A'.

    Because I was too easy, my teacher gave me some problem to solve. First, he gave me a Crystal oscillator of 10 MHZ instead of 1MHZ, it was not a problem, I just put a 74HC390 in the diagram, using only the first part. I have the good signal, but the part with the gates, it's the part I don't understand. I know what's a gate, but not the effect. I asked my teacher, but he doesn't know. This is why I'm asking you.

    So the questions are:
    What is the use of the gate part.
    Do I need to connect the end of my new 390 to another gate (74HC32), then to the 74HC08?

  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    The gates together with the switches enable pulses from earlier in the divider chain, running the clock section fast forward.

    I'd put the extra 390 straight after your oscillator stage, so it feeds the start of the existing divider chain where the existing 1MHz osc was used.
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    If, by "what is the use of the gate part?", you are referring to those AND and OR gates (the schematic uses a really poor convention for the OR gates), the purpose is to allow signals to pass at defined times. You do not wish a counter for the hours to increase its count until the minutes counter has reached 59 and the seconds as well.

    To see how the gates work, try our Ebook -

    As to that LS390, tie the unused inputs high. TTL logic can have inputs go short, so always use a resistor between Vcc and the inputs. For LSTTL, about 10K is good.