1. Seminar - Hardware description language - Introduction to Verilog

In the first part of seminars we will use Icarus Verilog.
The website is: http://iverilog.icarus.com/page (download, dokumentation, FAQ,..).

Alternatives:

Icarus Verilog for Windows: http://bleyer.org/icarus/ (tested on Win 7)
If you are using Eclipse, consider this link: http://sourceforge.net/apps/mediawiki/veditor/index.php?title=Main_Page
ISE WebPack from Xilinx (http://www.xilinx.com/support/download). After the instalation, you have to run Licence Manager to obtain a valid licence. xilinx_vytvoreni_projektu_a_simulace.pdf
Active-HDL Student Edition firmy Aldec (http://www.aldec.com/Products) - student licence for one year free of charge
ModelSim Student Edition (http://model.com/content/modelsim-pe-student-edition-hdl-simulation)

Short introduction to Verilog (in czech): verilog_2012.pdf

Git repository with verilog templates: https://gitlab.fel.cvut.cz/b4m35pap/stud-support

Exercise 1.

Consider the combinatorial circuit shown in figure below. Describe it in Verilog.

Solution. We will create a new file my_circuit.v with the following content:

module my_circuit(input a, b, c,
                 output d, e);
 
  assign d = ~(a | b) | (b & c);
  assign e = (b & c) ^ c;
    
endmodule

Exercise 2.

Create the TestBench for the simulation of the combinatorial circuit from Exercise 1; and perform the simulation.

Solution. We will create a new file, e.g. my_circuit_tb.v, which defines inputs to the circuit (stimulus list). Then we are able to simulate the circuit and observe its outputs.

module test();
  reg a, b, c;
  wire x, y;
  
  my_circuit my_circuit_XY(a, b, c, x, y);
  
  initial begin
    $dumpfile("test");
    $dumpvars;
    a=0;
    b=0;
    c=0;
    #160 $finish;
  end
  
  always #20 a = ~a;
  always #40 b = ~b;
  always #80 c = ~c;
  
  always @(x) $display( "The value of x was changed. Time=%d, x=%b. Inputs: a=%b, b=%b, c=%b.",$time, x,a,b,c);
  
endmodule

In the command line we will compile both the files, run the simulation, and visualize the results by following commands:

iverilog my_circuit.v my_circuit_tb.v
./a.out
gtkwave test

Alternatively, we can use vvp instead of gtkwave:

iverilog -otest.vvp my_circuit.v my_circuit_tb.v
vvp test.vvp

Exercise 3.

Sketch (by the hand) a schematic of the circuit according the following description:

  if A is equal to 1,
  then Y is equal to (B and C)
  else Y is equal to (B xor D) or C

The variables A, B, C and D are logical variables.

Exercise 4.

Describe the logic circuit from Exercise 3 in Verilog and perform the simulation. What is the value of Y in the case when A==0, B==1, C==0, D==0? Support your statement by the simulation results.

Exercise 5.

Describe in Verilog following circuit. Keep in mind that it is composed from three identical circuits according the Exercise 1. Use structural description. What is the output when A==0, B==1 and C==1?