Lesson 12: Run Simulation on ModelSim (Pre-Simulation)

Create a New Project

1. Create a new folder for this exercise at "<the folder you created for this lab>/L05_ModelSim"
2. Launch ModelSim-Intel Starter edition
   On the left end of the taskbar, select the Start icon, or press the Windows logo key  on the keyboard. and scroll to find the Intel FPGA xx.x.x.x Edition folder. Select on the folder and click on ModelSim-Intel FPGA Starter Edition icon to launch the software.
3. Create a new Project
   1. Select File ➤ New ➤ Project... from the menu bar.
      This opens the Create Project dialog where you can enter a Project Name, Project Location, and Default Library Name (Figure 1)
   2. Enter the project name, in this case, the project is called cmp4bit
   3. Click Browse... button for the Project Location field to select a folder where the project file will be stored. Change it to the folder you create in step 1.
   4. Leave the Default Library Name set to work.
   5. Click OK.
   
   Figure 1: Create Project Dialog
4. You might be asked if you want to create the project directory, click the Yes button.
   
   Figure 2: Create a new Directory
5. Once you click OK to accept the new project setting, a blank window and the Add items to the Project dialog will appear (Figure 3). While we can use this dialog to create new source files or add existing ones, we will not be using this option for the exercise. We will add source files later, so just click on the Close button.
   
   
   Figure 3: Adding New Items to a Project Dialog
6. You now have a project by name of cmp4bit.
   
   Figure 4: Main Window of ModelSim

Add New Verilog Files to the Project

Now we need to add a new file to the project.

7. Click on the menu File ➤ New ➤ Source ➤ Verilog, or click and hold on the button, then select the Verilog.
   
8. Now you can edit the Verilog code in the edit area.
   
9. Type the Verilog for a 4-bit comparator using behavioral modeling.
   Then be sure to save the file to the comp_4bit.v file by clicking on File ➤ Save As...
   The Verilog code for the 4-bit comparator.
   

   //Save file to comp_4bit.v
   module comp_4bit(a, b, lg, eq, ls);
   input [3:0] a;
   input [3:0] b;
   output lg, eq, ls;
   
   reg lg, eq, ls;
   
   always @(*) begin
       lg = 1'b0;
       eq = 1'b0;
       ls = 1'b0;
       if (a > b) begin
           lg = 1'b1;
       end else if (a == b) begin
           eq = 1'b1;
       end else begin
           ls = 1'b1;
       end
   end
   
   endmodule
   
   

10. Click on the Project Window. Right-click and select  Add to Project ➤ Existing File..., then select the file we just created for the project. In the project windows, you should see the file has been added to the project.
     
    Figure 8: Add file to the Project Dialog
11. We also need to add a testbench to the project. Follow steps 7 ~ 10 again to add comp_4bit_tb.v to the project.
    The Verilog code for the testbench is shown below.
    

    //Save file to comp_4bit_tb.v
    `timescale 1ns/1ns
    
    module test();
    reg  [3:0] a;
    reg  [3:0] b;
    wire lg, eq, ls;
    
    comp_4bit DUT (.a(a), .b(b), .lg(lg), .eq(eq), .ls(ls));
    
    initial begin
            a = 4'd10;  b = 4'd14;
        #5  a = 4'd3;   b = 4'd12;
        #5  a = 4'd6;   b = 4'd6;
    
        #5  $stop;
    end
    endmodule

12. You should now see two files listed in the Project window (Figure 9). Question mark icons in the Status column indicate that the file has not been compiled or that the source file has changed since the last successful compile. The other columns identify file type (e.g. Verilog or VHDL), compilation order, and modified data.
    
    Figure 9: Newly Added Project Files Display a  for Status

Compile the Design

13. Compile the files.
    1. Right-click in the Project window and select Compile ➤ Compile All from the pop-up menu.
       
    2. ModelSim compiles both files and changes the symbol in the Status column to a green checkmark . A checkmark means the compile succeeded. If the compile fails, the symbol will be a red , and you will see an error message in the Transcript window.
       
14. View the design units.
    1. Click the Library tab.
    2. Click the icon next to the work library.
       You should see two compiled design units, their types (modules in this case), and the path to the underlying source files.
       
       Figure 12: Library Window with Expanded Library

Load the Design for Simulation

After compiling the design, you can load the design into the simulation.

1. Load the test module.
   1. In the Library window, click the icon next to the work library to show the files contained there.
   2. Double-click test to load the design.
      
      Figure 13: Verilog Modules Compiled into work Library
2. You also can load the design by selecting Simulate ➤ Start Simulation in the menu bar. This opens the Start Simulation dialog. With the Design tab selected, click the  sign next to the work library to see the comp_4bit and test modules. Select the test module and click OK.
   
   Figure 14: Loading Design with Start Simulation Dialog

Prepare Simulation

After loading the design into the simulator, but before a simulation can be started, you need to set up the simulation/debug environment for the design. That is, you need to open the debug windows you consider important and select signals and variables, ... which you want to trace on those debug windows.

A good choice in the beginning for debug windows include:

• Wave — See waveforms
• Objects — See signals and their values
• Source — Trace your code line-by-line
• Locals — See variables and their values

These windows can be opened from the main menu View.

Next, you need to tell the simulator which signals you want to trace in the Wave window.

You can add signals into the Wave window using one of the following methods:

• Select signals in Objects pane ( Shift + Left Key on the mouse) and drag-n-drop the selection to Wave panel.
• Right-click in Object window and select Add to Wave.
• Click Add ➤ To Wave ➤ Selected Signals from the main menu.

Now, everything should look something like below:

Figure 15: ModelSim Main Window Showing Workspace, Object, Locals and Source Windows

It is very beneficial to save the current signal configuration of the wave window by selecting File ➤ Save.

Run Simulation

You can simulate the design.

1. Enter 5 ns as the length of time we would like to simulate for in the Run Length box and click the  Run icon as shown below:
   
2. You can click the Run icon again to continue the simulation for the next 5 ns, or click  Run-All icon to run all testbench code until $stop or $finish instruction in the testbench code.

Now run the simulation for a few hundred nanoseconds. You should see how each signal takes at startup their initial values and how inputs and outputs change after a few clock cycles. The Waveform window should look something like below:

Figure 17: Results for the Simulation