# 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
1
0
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:
http://users.otenet.gr/~athsam/samgordon_clock_eng.htm

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.
And
Do I need to connect the end of my new 390 to another gate (74HC32), then to the 74HC08?

Francis.

2. ### rjenkins AAC Fanatic!

Nov 6, 2005
1,015
69
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
15,815
290
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 - http://www.allaboutcircuits.com/vol_4/chpt_3/5.html

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.