# design

Discussion in 'Homework Help' started by unreal_khan, Jun 3, 2012.

1. ### unreal_khan Thread Starter New Member

Jun 3, 2012
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design a combinational circuit has three inputs x y z and three outputs bits A B and C . when the input is from 0(000)to 3 (011), the output is one greater than the input.For the rest of input values the output is one less than the input Assume that for your circuit have available only 2 input NAND gates , 2 input ex OR gates.
use the minimum number of gates for yur design ( i.e. your circuit may have allof these gates but the total number of gates must be samllest possible

2. ### crutschow Expert

Mar 14, 2008
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3,362
Interesting homework problem. So how much of the circuit have you designed so far?

3. ### absf Senior Member

Dec 29, 2010
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Have you created a truth table using EXCEL something like this?

• ###### home_work logic_cropped.PNG
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4. ### WBahn Moderator

Mar 31, 2012
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So, if I understand you correctly, you are allowed to use just two different types of gates, 2-input NAND and 2-input XOR. Correct?

If so, then I was able to do it with 5 NAND gates and 2 XOR gates. There may be a solution with fewer, but this at least gives you a target.

HINT: Tackle B and C first.

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5. ### Austin Clark Active Member

Dec 28, 2011
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Notice that the outputs are mirrored and inverted through the middle... Could a helpful hint in designing the circuit.

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6. ### WBahn Moderator

Mar 31, 2012
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That's a very good observation, Austin.

It permits the entire thing to be implemented using 3 NAND gates and 4 XOR gates. That hasn't reduced the gate count any, so using the metrics of this problem a solution leveraging this and my original solution are equivalent.

However, if building this using SSI logic, this would would require only two packages (a quad NAND gate and a quad XOR). Actually, my original would would also have required the same two packages because two of the NAND gates are used as inverters and hence could be implemented using XOR instead.

In practice, an XOR requires three times the transistors and is half the speed of a NAND (or worse, depending on how it is implemented). So mine ends up requiring fewer transistors. But I have a pretty long critical path that results in seven gate delays. Using your observation, the circuit is clean, simple, and only four gate delays. Even better, my worst case gate delays for the three outputs are {7,4,1} while yours are {4,4,3}, so very nearly equal delays for each of them.

If I were the grader, I would have to give equal points for both (namely full credit, assuming that there isn't a solution with fewer gates). But I would be sorely tempted to give you a bit of extra credit (or a bit of a pass on a minor mistake on a different problem).

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7. ### unreal_khan Thread Starter New Member

Jun 3, 2012
6
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plzz guys help me with this

8. ### unreal_khan Thread Starter New Member

Jun 3, 2012
6
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plz solve this i will be hghly thankful to u

9. ### WBahn Moderator

Mar 31, 2012
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What have you done so far?

Hell, absf GAVE you the truth table. Most of us would expect you to at least make SOME attempt at coming up with your own first.

Are you not capable of implementing the logic for ANY of the outputs given that table? More to the point, are you not capable of even attempting to figure out the logic for even one of them?

I bet you are and, once you do and post it, we will be more than willing to see where you are going right and where you are going wrong and HELP you make progress from there. But this is not the "Do My Homework For Me" forum.

10. ### WBahn Moderator

Mar 31, 2012
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I have solved it! Now, if you want me to just give you the solution, I will insist that you turn it in with my name on it so that I can get the credit for it. That seems only fair, doesn't it? Why should YOU get the credit for someone else's work, especially when you don't seem to feel you should even make any attempt at it yourself.

If this is the attitude that you have, then you have no business trying to become an engineer. Engineers are professionals who solve other people's problems for a living. If you can't be bothered to even attempt solving your own, then change majors and do it soon.

11. ### unreal_khan Thread Starter New Member

Jun 3, 2012
6
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wbahn
i knw that u have knowledge of electroics but for yourkind infrmation engnineers must be helpful in nature if they dont the they must be shameful of their behaviour and arrogrance as their knowledge will not help the society .
at last you want me to turn it in your name i will turn it bcoz i want to be part in sharig the knowledge and you can tell me how to turn in yur name i will surely turn it iin your name

12. ### WBahn Moderator

Mar 31, 2012
18,085
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Just giving you the answer will not help you at all. You may think that getting credit for work that you made no effort to do is somehow helpful, but it isn't. Part of helping society involved NOT "helping" to turn out incompetent engineers that are going to get others killed.

Feel free to post your instructor's name and institution/department as well as your name and/or student ID (or PM me with it). I will then contact your instructor and explain to them that you feel it is too much trouble to even attempt solving your homework problems and that you would like to have my solution submitted instead. I will then give them my solution (acknowledging absf's truth table and Austin Clark's very useful observation) and let them decide how to apportion the credit.

Or, if you want, I will post a circuit that has four NAND and four XOR gates and you can just take it and turn it in as your own, and get whatever credit your instructor thinks it's worth. How does that sound?

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13. ### Georacer Moderator

Nov 25, 2009
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WBahn, there is no need to mock around a subject, just for the sake of it.

Things are simple: either the OP shows some effort on his part, or this thread is considered closed.

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14. ### unreal_khan Thread Starter New Member

Jun 3, 2012
6
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just post it here it sounds good wbahn

15. ### unreal_khan Thread Starter New Member

Jun 3, 2012
6
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hey buddy i have solved it but there is oone thing clear u never solved it wbahn yu just playing the trick toget the solved results and i not gonna post it here may be u can suceeded with these tricks many times but not this time bro

16. ### vpoko Member

Jan 5, 2012
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Uhh, what? Is WBahn in your class or something? Why would he want your results?

17. ### WBahn Moderator

Mar 31, 2012
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He wouldn't.

It was a feeble attempt to get me to post one of my seven-gate solutions in order to prove I actually had one. But, in doing so, he could then snag it and turn it in without having to do any work himself. But, as you indicate, I have no need to see his and, if he actually solved it like he said, then he has no need to see mine.

I was going to post an eight-gate circuit, but am bowing to Georacer's admonition.

Unless and until he posts something that indicates some effort on his part, I don't plan to make any further responses in this thread since it would amount to feeding a troll.