CSE370 Laboratory Assignment 3

Using Decoders and Multiplexors to Implement Boolean Functions

Distributed: April 8
Due: By end of lab session (4/11,12)


In this laboratory assignment you will continue to learn how to use the Aldec Active-HDL tool.  This time the focus will be on schematic and Verilog modules.  You will start to use Verilog modules as test fixtures to help you verify your circuit.  By the end of this lab you should feel comfortable creating a module based on a schematic, creating a Verilog module directly, and setting up a test fixture.

In the hardware portion of this lab, you will use decoders and multiplexors in your kit to implement a full-adder circuit.


  1. Complete the first section of  ActiveHDL Tutorial #2 that covers test fixtures. It would be best if you do this before the lab since you will have to complete the entire tutorial as part of the homework. You'll be asked to use a test fixture to verify the schematic for a full adder circuit.  This first section is quite short, but it is very important. You'll be creating and testing many of your own modules over the rest of the quarter, and now is the time to get comfortable with the process.

  2. We have learned how to implement a truth table directly using a 2-level circuit with AND gates followed by an OR gate. The ‘138 chip is a 3-8 decoder chip that implements all 8 AND gates needed to implement any 3-input truth table. Using an ActiveHDL schematic, draw a ‘138 decoder and use simulation with Stimulators as you did for Lab #2 to explore what it does. Try out all input combinations.

  3. Now add to this decoder the gates that are needed to implement a full-adder, that is, the sum and carry functions. Create a top-level test circuit schematic using the test fixture given in the tutorial to test your design. As you design your circuit, be sure to use the chip maps as a guide, and pay close attention to the "enable" semantics of the decoder chip. That is, you must connect the enable input in order for the decoder to work properly.

  4. Implement this circuit using your prototyping board.  Connect the inputs to switches (use the same switches for the two circuits) and the outputs to lights. HINT: If your circuit is not working as expected use the logic probe to trace through the circuit to help you identify any wiring errors.

  5. Show the simulation results and the working circuit to a TA and have them check you off as having completed this laboratory assignment. They will ask you to show them your work and explain your design files and circuit.  Hand in your schematics for parts 2 and 3. Do not hand in waveforms this time.


NOTE: The gates in your Active-HDL solution must exactly match the gates used in your prototyping board solution.