HW 0b001: Verilog Tools and DeMorgan's Law

Due: Friday, September 15

Install tools

For this assignment you will need the Icarus Verilog simulator (iverilog). You can manually install it (fairly easy), or wait for the forthcoming course virtual machine that includes it (along with several other tools you'll need later in the class).

Hello, Verilog!

In your favorite text editor, create a file named hello.v with the following contents:

// Simple Verilog test
module hello_test ();
initial begin
    $display("Hello, CompArch!");
end
endmodule

We can run the test through iverilog with the following commands:

iverilog -o hello hello.v
./hello

iverilog actually creates an executable program based on your Verilog (here, hello), which you then run to see the simulation results. If you do not specify -o <foo>, the executable will be called a.out just like for a C compiler.

Your first Verilog

Prove DeMorgan’s Law using the exhaustive proof method. To do so, you will use Verilog to create truth tables for the following 4 equations:

Schematic capture

First, draw a schematic of each of the four equations (on paper is fine). To help you get started, here is one for (~A)*(~B):

Then, give each gate and each wire a name. A and B are already named.

Translate to Verilog

Each element becomes a line of Verilog. Create a new Verilog file called hw1.v to contain your module. Here is the schematic above, captured:

module demorgan
(
  input  A,          // Single bit inputs
  input  B,
  output nA,         // Output intermediate complemented inputs
  output nB,
  output nAandnB     // Single bit output, (~A)*(~B)
);

  wire nA;
  wire nB;
  not Ainv(nA, A);  	// Top inverter is named Ainv, takes signal A as input and produces signal nA
  not Binv(nB, B);
  and andgate(nAandnB, nA, nB); 	// AND gate produces nAandnB from nA and nB

endmodule

Create a testbench

The statements above show the structure of the module. We still need to add some stimuli to drive the module for testing. We will write this test code within an Verilog initial block. You could put the test code within the module we wrote previously, but it is better practice to separate circuits and their tests.

Create a new file called hw1.t.v with the following contents:

`include "hw1.v"

module demorgan_test ();

  // Instantiate device/module under test
  reg A, B;                // Primary test inputs
  wire nA, nB, nAandnB;    // Test outputs

  demorgan dut(A, B, nA, nB, nAandnB);  // Module to be tested


  // Run sequence of test stimuli
  initial begin
    $display("A B | ~A ~B | ~A~B ");            // Prints header for truth table
    A=0;B=0; #1                                 // Set A and B, wait for update (#1)
    $display("%b %b |  %b  %b |    %b  ", A,B, nA, nB, nAandnB);
    A=0;B=1; #1                                 // Set A and B, wait for new update
    $display("%b %b |  %b  %b |    %b  ", A,B, nA, nB, nAandnB);
    A=1;B=0; #1
    $display("%b %b |  %b  %b |    %b  ", A,B, nA, nB, nAandnB);
    A=1;B=1; #1
    $display("%b %b |  %b  %b |    %b  ", A,B, nA, nB, nAandnB);
  end
endmodule    // End demorgan_test

When you use iverilog to run hw1.t.v, you should see the following output:

A B | ~A ~B | ~A~B 
0 0 |  1  1 |    1  
0 1 |  1  0 |    0  
1 0 |  0  1 |    0  
1 1 |  0  0 |    0  

Add the other 3 equations

Continue in the manner described above to create the remaining three equations. Your final result should exhaustively prove DeMorgan's Laws.

Submission

Create a plain text results.txt file containing your completed truth table. Push results.txt, hw1.v, and hw1.t.v to your Github in the HW1 folder. There is no write-up or other deliverable. Note: you should get in the habit of using the exact filenames and module names specified, as we will occasionally use automated grading tools.

Tips and Tricks

• If a signal shows up as “z”, it means it is not being driven. Are you sure you wired it?
• The $display syntax is similar to what you might find in a printf style args function. The first input is a format string. %b means "interpret the next argument as a bit and display it here".
• begin and end are the equivalent of { and } in C.