A handy tool is the zoom to fit magnifying glass. After running commands, your simulation will often looks like just one solid like where nothing happened. Just click the zoom to fit magnifying glass to quickly fix your woes!

When you first open a simulation, it will look like nothing happened, most of the black screen will be blank. To make something happen, you have to type in commands yourself. Go to the TCL Console to type in commands. These commands use the Tool Command Language, or TCL. (It's not a very complicated language, and you won't need to learn very much to use it here.)

One command that you will use extensively is the add_force command. This command allows you to put values on input ports. You will have to type in the command for each input. You can use this command to set a long list of values to be added at future times. You can also use this command to set a repeating input pattern. The following example explains this command's syntax.

  add_force myInput {0 0} {1 5} -repeat_every 10

• add_force starts the command.
• myInput is to be replaced by the port name of your input.
• {0 0} {1 5} is what actually adds values to the input. The syntax for this is: {<value> <time>} where <value> is added to the input at time <time> where time is in nanoseconds. In the above example, myInput is made a 0 at time 0 and then is made a 1 at 5 nanoseconds later. The time is relative to the beginning of the simulation run, not the last value. You can have as many {<value> <time>} commands in a single line of add_force as you want! You can even have just one if you want.
• -repeat_every 10 causes the given input sequence to repeat every 10 nanoseconds. This is not required.

A simple example that doesn't repeat might be an enable signal or a reset signal.

  add_force enable {1 0}
  add_force reset {1 0} {0 5}

When adding values to your inputs, make sure the most significant input alternates the slowest, and the least significant alternates the fastest.

After executing a run 100 command, which is explained in the next section, the previous code example for myInput will create the following simulation waveform.

When adding values onto signals, you will be instructed as to which signal is the most significant to least significant bits. Alternate each signal between and 0 and a 1 as if you're going down the rows in a truth table with the given MSB and LSB. This method allows you to exhaustively test your circuit in an easy manner. Your simulation should end up looking like a truth table turned sideways. Below is an example screenshot for a simulation of a 4:1 multiplexer.

After using add_force to put values on all your inputs, you have to run the simulation before any simulation time will pass. Use the run command after all of your add_force commands are finished. The follow example will illustrate run's syntax.

  run 40

• run starts the command.
• 40 is the amount of time in nanoseconds that the simulation will run.

After you execute a run command, you'll most likely need to use the zoom to fit tool to view your schematic. Read about this tool from the earlier section about it.

You can press the up arrow key while in the TCL console to reuse commands that were previously entered. (This is a nearly universal command in any command line interface.)

Those are the most useful commands that you need for your simulations! If you'd like to read about how to make a TCL file read the Using TCL Files in Simulations tutorial. This will allow you to run a file containing your commands rather than typing in each command line-by-line every time you run a new simulation.

Usually your waveform colors are green. Most of the time, that is what you want. You may notice your waveforms turn other colors though. This will briefly explain what those colors mean. In the context of this class, if your waveform is not green something went wrong!

• Blue means that the signal holds no value. This might happen if you forgot to use add_force to give one of your inputs a value. You will see a Z for a blue signal's value.
• Red means that your signal has a conflict. This can happen if you have Verilog code that tries to output different values to a wire at the same time. This also can happen if you have a multiplexor with a select signal that holds no value, or is the color blue. The output of a flip flop will also appear red until you clear it or give it a value. (Flip flops are memory devices explained later in the class.) You will see an X for a red signal's value.

If you have made changes to your Verilog file, you must first save the Verilog file and then run the simulation again by clicking Run Simulation as is explained in the Setup to Run a Simulation section.

If a popup appears titled Simulation is Already Running, click Yes to close and relaunch the simulation with your updated Verilog module.

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