Lab 7
 Page 4

More Practice in Circuit Design

Now that you know how to use the Logic Design Tool, here are a few more exercises to help you become more comfortable building logic circuits. You do not need to submit these, but you are strongly encouraged to complete them since they will prepare you for the more advanced assignments in the next section.

Solutions are available for each exercise. Try not to look at them before you have finished unless you are really stuck.

Exercise 1

Build a circuit with three inputs A, B and C. The output should only be true when at least two of the inputs are true, as shown in the truthtable below. Note that this circuit computes the carry bit for the 1-bit full adder that we discussed in lecture: A and B are the bits to add and C is the carry from the previous bits added.

ABC-->Output
000-->0
001-->0
010-->0
011-->1
ABC-->Output
100-->0
101-->1
110-->1
111-->1

A solution to Exercise 1

Exercise 2

Build a circuit with three inputs such that the output is only true when at least one of the inputs is false. Try to find the smallest circuit possible with the gates available.

ABC-->Output
000-->1
001-->1
010-->1
011-->1
ABC-->Output
100-->1
101-->1
110-->1
111-->0

Hint: You should need no more than two gates if one is a 3-input gate
A solution to Exercise 2

Exercise 3

Using any logic gates, build a circuit with three inputs (A, B and C) and one output. When A is true, the output should be the same as B. When A is false, the output should be the same as C. The input A acts as a selector between B and C.

Here is the truthtable:

ABC-->Output
0X0-->0
0X1-->1
10X-->0
11X-->1

When you see an "X" in a truth table, it means that we don't care what that value is. It is not necessary to know that value in order to determine the output.


A solution to Exercise 3
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