MCS-170 Project 7: Logic Circuits - Part II

Due: In Class

Project Task

In this lab, you will use a logic circuit simulator to investigate the design and analysis of logic circuits that carry out addition of binary digits.  During the lab you will be asked a series of questions. Record your answers and turn them in to the instructor at the end of the lab period.

In Lab

In the first part of this lab we investigated simple gates such as AND, OR, NOT, and XOR gates. We saw how these gates could be combined to form more complex logical circuits. One of the exercises from Part I was to investigate a half-adder. This is a circuit that adds two binary digits, A and B, and has two outputs, which represent the sum digit of A+B and the carry digit. Review the diagram on page 294 of the text to refresh your memory on the design of a half-adder.

We will again use the logic simulator located at
Open a new browser window and start the simulator befor proceeding on with this lab.

EXERCISE 1:   At the bottom of the list of gates on the left side of the simulator you will see a gate labeled "Half Adder". Drag this gate into the window, hook up two switches labeled "A"and "B", hook up LED's to these switches and to the outputs, and test out the half-adder. Write down the logic table for the half-adder as follows, and verify that it works just the same as the half-adder circuit you built from scratch in Part I.

0  0 |
0  1 |
1  0 |
1  1 |

The term "half-adder" is used to refer to this circuit because it only partially solves the problem of adding two multiple-digit numbers. When you add two numbers, you not only sum up the digits in a particular column, but you also have to keep track of whether there is a carry digit coming from adding up a previous column.

For example, consider the following addition of A=1011 and B=111:

c c c c
  1 0 1 1
+   1 1 1
1 0 0 1 0

When adding the digits in the second, third, and fourth columns, we have a carry of 1 coming in from the column to the right.  A better adder circuit would have a third input, labeled "cin" that would keep track of this carry from a previous column.  To construct such a "full" adder circuit, we'll need three inputs and two outputs. The inputs will be the two numbers A and B, and also the carry digit coming in from a previous column. The outputs will be the digit for the sum and a carry digit. Since we'll have both an input carry and an output carry, we'll designate them as "Cin" and "Cout". We'll use "Sum" to designate the digit of the sum.  How can we build a full-adder?

We can implement the full adder by using two half-adder circuits. The first will add A and B to produce a partial sum and a partial carry, while the second will add Cin to the partial sum to produce the final Sum output. If either half-adder produces a carry, there will be an output carry. Thus, Cout will be an OR function of the half-adder carry outputs. The resulting full adder circuit is shown below.

EXERCISE 2:   Construct this circuit in the simulator using two half-adders and an OR gate. Fill in the truth table for this circuit.

A  B Cin |  Cout  Sum
0  0  0  |
0  0  1  |
0  1  0  |
0  1  1  |
1  0  0  |
1  0  1  |
1  1  0  |
1  1  1  |

EXERCISE 3:  We could build multiple copies of the previous circuit and use them to build more complex circuits. This would be tedious. Fortunately, the Circuit Simulator has built-in Full-Adder gates (the last gate in the list of gates in the left-hand window).  Read the discussion on page 295 on how to connect full-adders together to add 4-digit numbers. Then, use 4 full-adder circuits as shown on page 295 to construct a 4-bit adder circuit in your simulator. Test it out on different numbers to verify that it is working. Then, hand in a printout of your circuit (or have the instructor okay your circuit).

So far we have focused solely on arithmetic and logical operations of the computer (activities of the ALU). How does memory get implemented with logical gates? The simplest circuit for storing a 1-0 value is called a flip-flop.  This circuit is illustrated in Figures 16.17 and 16.18 of the text (page 296).
EXERCISE 4:  To build this circuit in the simulator, use two "PushOn" switches for the two inputs. These switches are normally off (0) and turn on (1) only when pressed. Construct the flip-flop circuit in the simulator and then test what happens when you press the "Set" and "Reset" buttons.  Note the "feed-back" wire that runs from the output of the circuit back to the input of the lower AND gate. What purpose does this wire serve in the circuit?  Why is it reasonable to call this circuit a flip-flop circuit? What feature makes it useful for storing values? 

Project Report:

Hand in your answers to the exercises above.

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