Discussion in 'Homework Help' started by fiendslyr, Mar 22, 2011.

1. ### fiendslyr Thread Starter New Member

Mar 22, 2011
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I am building a 2 bit ripple carry adder (one from logic gates and the other from 2 4:1 MUXes). I've built the first stage using logic gates with two outputs (the sum "S" and the carry out "Cout"). I am now supposed to take that Cout and build the second stage using dual 4:1 mux. I've made the truth table for the second stage but am not sure how to implement the two tables using MUXes. I would greatly appreciate it if someone could get me started on the second stage of the ripple carry adder. Thanks.

2. ### fiendslyr Thread Starter New Member

Mar 22, 2011
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So, this is what I got after reading my notes for the class... Could someone please verify it?

For SUM Mux controlled by A, B
mux input 0 connected to Cin
mux input 1 connected to Cin inverted
mux input 2 connected to Cin inverted
mux input 3 connected to Cin

For CARRY Mux controlled by A, B
mux input 0 connected to '0'
mux input 1 connected to Cin
mux input 2 connected to Cin
mux input 3 connected to '1'

3. ### Georacer Moderator

Nov 25, 2009
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1,266
Your connection seem correct. Do you understand the method for implementing Boolean functions with a MUX or do you need some crarifications?

4. ### fiendslyr Thread Starter New Member

Mar 22, 2011
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I would really appreciate some clarifications as my professor has not been too clear about multiplexers in class (and neither has his presentations).

5. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Let's first examine the S function. Take a look at the 3-variable truth table you made. It's columns are A, B and Cin.

Leave Cin aside for now and look at the first two columns. They contain 4 double entries of the A-B combinations. These will control a MUX of equal control pins (2 in our case). They are the inputs that point which MUX input will be selected each time.

In the first case, AB=00, you notice that the S result is the same as the Cin (the last input in general) input, 0 in the first row, and 1 in the second. That means that you must connect the 00-pin of the MUX directly to the Cin input.

If AB=01, you see that S is opposite to the Cin, and you must connect the 01-pin of the MUX with NOT(Cin).

Other cases are that the result function will be 0 in both rows or 1 in both rows. That means you have to either ground the MUX pin or attach it to the power supply.

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6. ### fiendslyr Thread Starter New Member

Mar 22, 2011
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Oh ok, I think I understand that. That was much better explained by you than my professor. Thanks for your help.