Project Overview
For this project, you will use Logisim to create a 4 bit adder. The reading material from Chapter
1 and Appendix B of the Brookshear text along with the Logisim tutorial you completed last
week should provide you with sufficient background to successfully complete the project.
Instructions
1. Open Logisim.
2. First, create a sub-circuit. To add a sub-circuit, right-click on the project name (default is
Untitled) and select "Add Circuit..." and name the circuit SR "Flip Flop". Build the
circuit (based on the redrawn NAND gate flip flop from the Chapter 1 Exercise
Solutions) as follows (Make sure to set the "Label" property of each pin to its given
name, rather than adding text comments directly to the diagram. This way, the labels will
still be visible from outside the sub-circuit):
Create two input pins named S and R.
Create one output pins named X.
Add two NAND gates.
Configure the wiring as required.
You will use this SR Flip Flop circuit to provide inputs to, and receive outputs from, your
Adder.
3. Save your project and use the poke tool (the finger button on the toolbar) to verify your
circuit is performing correctly. When both inputs to the flip flop are on, the output of the
flip flop should remain stable. Dropping the S input low temporarily should set the
output value to 1. Dropping the R input low temporarily should set the output value to 0.
4. Now, create a new sub-circuit called Full Adder. You can easily find images of Full
Adder circuits on the internet, but one such circuit can be found at
http://www.theorycircuit.com/full-adder-circuit-diagram/ . This circuit will require the
following aspects:
Create three input pins named A, B and Cin.
Create two output pins named Sum and Cout.
Add the necessary gates and wiring to match the Full Adder Logic Gate diagram
referenced above.
5. Save you work and use the poke tool to verify this sub-circuit is performing correctly. By
working through the input combinations listed in the truth table of the above referenced
website.
6. Using the SR Flip Flops and Full Adders circuits, build the 4-bit adder described in
Appendix B of the text. This will be your main circuit (double click "main" to design this
last circuit.)
a. You can use your sub-circuits while designing the main circuit by clicking on the
sub-circuit name and dragging it into the main circuit window.
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