# Digital logic, 0-59 using 2 7 segment LED

Discussion in 'Homework Help' started by Vaonix, Feb 25, 2015.

1. ### Vaonix Thread Starter New Member

Feb 25, 2015
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0
Good morning,

Working on a project and I really dont know where to begin. We have dabbled once with IC 7447 and the 7 segment in lab but that was just a strait shot from some BCD switches, have not done anything more complicated than what is drawn out in the book. I have something built on paper that when its over 59 it would knock both displays above 9, but i now realize that does not display as 0, it is just jibberish.

I do need to build a truth table, but in this instance would I need to build a truth table for all inputs into the 7447 decoders as it reads below?

o List of all components that are required for your design with a short description of each one.

o Pin assignment of each component.

o Truth table of the outputs

o Output functions

o A drawing of your full circuit.

o Short description of your circuit inputs and outputs.

o Short manual of how to use the circuit.

In this project you will implement a circuit that display and only display the decimal numbers that are less than 60 (0-59). The number should be observable using two 7-segmant displays. The number over 60 should be displayed as 00.

Note that the 7-segment display shows decimal digit correctly only when the input of the 4 binary digits is a legal BCD from 0-9, the input with value larger than 9 (from A to F) needs to be processed.

Hint: In the two-digit decimal number (< 60), each decimal digit can be represented as BCD so that each one can be displayed using one 7-segment display separately. The digit in ones can be coded as 4 binary digits, but the value larger than 9 (from A to F, illegal BCD) should be displayed as 0 by the 7-segmant display; the digit in tens can be coded as 3 binary digits, but the value larger than 5 (from 6 to 7) should be displayed as 0 by the other 7-segmant display.

We have not done anything complicated so I am doubtful this is as complicated as I am making it. We have use of all the IC's in our lab (NAND, AND, OR, NOR, XOR etc etc)

I realize I need to build is so that whenever the 10's digit is above 5, send a signal back to the 1's digits(all of them?) and have them go to 0. Any help would be beneficial, especially with the truth table, I should be able to work it from that point.

2. ### MrChips Moderator

Oct 2, 2009
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3,461
This is a classic homework exercise. However, the way it is presented is unusual as I will explain later.

You need two counters. One counter for the units digit and a second counter for the tens digit.

Can you implement these two counters separately?

3. ### djsfantasi AAC Fanatic!

Apr 11, 2010
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Why would the value be greater than 9 or 5? Don't do that! If it happens, then you need to do something to your counters. Keep this in mind as you try to implement the two counters, as suggested by MrChips.

What chips are you going to use for your counters? Have you researched this and reviewed the datasheets? What inputs and outputs are there?

4. ### Vaonix Thread Starter New Member

Feb 25, 2015
11
0
I seen a few counter tutorials while looking for the info, I dont know offhand but I can look it up. Actually, looking back to the 1st lab I see we worked with a IC7493 to make a BCD counter using the clock

Our trainer has 8 bits for toggling high and low, I was under the impression she wanted us to set a set of 4 bits to each digit so if we wanted to have it read say 32, we would set the tens to 0011 and set the 1's to 0010.

5. ### djsfantasi AAC Fanatic!

Apr 11, 2010
2,914
881
Ok, I reread your requirements and must admit I'm a little baffled. Your explanation is consistent with the problem statement. Perhaps that is what MrChips meant by "unusual".

I'm used to seeing similar requirements in a stopwatch or clock circuit...

6. ### Vaonix Thread Starter New Member

Feb 25, 2015
11
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I sent an email to the TA for clarification. She is very thick on the Chinese and I swear she took numbers as a second language instead of English, sometimes some explanation details fall through the cracks.

7. ### MrChips Moderator

Oct 2, 2009
12,652
3,461
The units counter has to count from 0 to 9. This is also known as a divide-by-10 counter.
Do you know how to implement this?

The tens counter has to count from 0 to 5. This is also known as a divided-by-6 counter.
Do you know how to implement this?

What is "unusual" about this exercise is that it asks for truth tables to be drawn. This would imply that the design would be restricted to the use of flip-flops but it does not specify such restriction.

Have you covered the design of finite state machines in class?

Last edited: Feb 25, 2015

Apr 5, 2008
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9. ### Vaonix Thread Starter New Member

Feb 25, 2015
11
0
According to the TA I have the option of doing it either way, but according to the TA "The switch is easier"(i asked about the toggle or counter, so I assume she meant toggles) but I've found much documentation on the counter/stopwatch method so I might go with that.

10. ### Vaonix Thread Starter New Member

Feb 25, 2015
11
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Negative and not even almost, pretty much everything you have said is gibberish. In class itself we have just started flip flops, but lab is entirely separate. In lab itself we have done basic boolean and combinational circuits, basic gray code to binary and most basic 7 segment(just 7447 accepting 4 inputs to display them) as well as half/full adders-subtractors. Looking back to the 1st lab we implemented a divide by 10 counter using IC 7493(uses 4 flip flops) though it never used that terminology, and through further reading I see how you can manipulate its R inputs to reset it upon any count you wish.

I am also confused by the request for a truth table. Of what? Where?

11. ### MrChips Moderator

Oct 2, 2009
12,652
3,461
If you have not done finite state machines then it is unlikely you would be asked to do the design from scratch using flip-flops.

If there are no restrictions on what ICs you are allowed to use then the solution is straight forward and dead easy.

7490 is a divide-by-10 counter.
7493 is a divide-by-6 counter.

Problem solved.

12. ### Vaonix Thread Starter New Member

Feb 25, 2015
11
0
I am not sure if we have 7490, but we certainly have 7493 and its not hard to change to output to be whatever. Yes it is easy and I think I figured it out, i'll generically draw it up and post, thanks.

Still no clue what she wants a truth table drawn up on if i am using these though.

13. ### Vaonix Thread Starter New Member

Feb 25, 2015
11
0
Oh actually I know that I will have a few AND and 2 OR gates, when it comes to incrementing the 10's and resetting the 1's, I guess she wants it drawn up on them

14. ### MrChips Moderator

Oct 2, 2009
12,652
3,461
Sorry, I meant 7490 and 7492.