Open Example Model

Add Port Value Labels

To understand how signal values change as the simulation progresses, you can add port value labels to signals of interest throughout your model before and during simulation. The port value label is displayed in the model canvas, near the port that produces the signal.

To add a port value label on the Sine Wave signal:

You can also add port value labels to multiple signals at once. To add port value labels for the Pulse and Iteration Number signals:

New port value labels display no data yet until the next time the block that produces the signal executes.

Set Breakpoint to Pause Within Time Step

By default, models are configured to pause within a time step as soon as the condition for a breakpoint is met. To check whether your model is configured to pause within a time step, open the Breakpoints List. In the Simulink^® Toolstrip, on the Debug tab, click Breakpoints List. Check that Pause within time step is selected.

To pause execution within a time step so you can analyze the model by progressing the simulation one block at a time, set a breakpoint. The simulation pauses within the time step as soon as the condition for the breakpoint is met. For example, to pause just before the while-iterator subsystem gets enabled, set a breakpoint on the output of the Pulse block.

Because the while-iterator subsystem executes only when the Pulse signal value is not 0, this signal breakpoint causes the simulation to pause before each time the while-iterator subsystem executes.

The red circle on the signal line indicates the active signal breakpoint.

Step Through Block Execution

To start analyzing the model, run a simulation. In the Simulink Toolstrip, on the Debug tab, click Run.

When the simulation reaches a time of 0.5 seconds, the Pulse signal value becomes 1. The simulation pauses on the breakpoint just after the Pulse block executes and updates the output value. To indicate why and where the simulation paused, the breakpoint is annotated with a green arrow, and the Pulse block is highlighted green.

To step the simulation execution to the next block, in the Debug tab, click Step Over. The green highlight moves to the while-iterator subsystem, and the green arrow on the breakpoint disappears.

To see the number of iterations the while-iterator subsystem executes in this time step, click Step Over again. Because the while-iterator subsystem is nonvirtual, the Step Over action steps over the entire subsystem as a single unit. The port value label updates to show that the while-iterator subsystem executed the algorithm 13 times for an input value of 86.6.

If you click Step Over one more time, the Outport block executes, and the Dashboard Scope block updates to show the new value for the Iteration Number signal.

If you continue clicking Step Over, you can step through the block-by-block execution in subsequent major time steps. The Pulse signal value stays 1 for the next several time steps.

Each time the simulation pauses on the Pulse block after you click Step Over, clicking Step Over again causes the simulation to pause on the breakpoint. The first time the simulation pauses on the Pulse block, the simulation pauses just before the Pulse block runs its output method. The second time the simulation pauses on the breakpoint is because the condition is met when the Pulse block runs its output method.

Step into While-Iterator Subsystem

To progress the simulation until the next time the while-iterator subsystem is enabled, in the Debug tab, click Continue. If you did not continue stepping block by block until the Pulse block output value changed to 0, continue clicking Continue until the simulation reaches a time of 1.5 seconds and pauses on the breakpoint.

To progress the simulation to the while-iterator subsystem, click Step Over. Then, to observe the algorithm, click Step In. The model canvas navigates inside the while-iterator subsystem. The simulation pauses on the Display block just before it executes to update the displayed value. The block displays 14 from the last time it executed.

To see the signal values as you step through the algorithm implemented in the while-iterator subsystem, add port value labels.

The port value label for the iteration number output from the While Iterator block shows inaccessible, so that signal value is displayed using the Display block. The other port value labels show no data yet.

Click Step Over. The Display block executes and the displayed value updates to 1 to indicate that this is the first iteration. The simulation pauses on the Memory block, which is next in the execution order.

As you click Step Over two more times, the simulation pauses on the Sum block then the Relational Operator block. When you click Step Over again, the canvas updates to show the top level block diagram because the while-iterator subsystem executes only once in this time step. When you click Step Over once more, the Dashboard Scope block plots the new value for the Iteration Number signal.

Pause on Specific Iteration

When you want to debug or analyze the algorithm inside an iterator subsystem, you can use a breakpoint inside the while-iterator subsystem to pause simulation on a particular iteration. Setting a breakpoint on the iteration number output from the While Iterator block is not supported. Where you place the breakpoint and the condition for the breakpoint depends on how your algorithm uses the iteration number.

In the example model, the algorithm computes the cumulative sum of the iteration number. To break on a particular iteration, you can add a breakpoint to the output of the Sum block or to the output of the Memory block. For example, suppose you want to analyze the algorithm for a time step where the while-iterator subsystem algorithm executes more than once. In the first iteration, the output of the Sum block becomes 1. In the second iteration, the Memory block output becomes 1. Add a signal breakpoint to the output of the Memory block to pause simulation when the value is greater than 0.

To progress simulation directly to the first time the while-iterator subsystem executes more than once, disable the breakpoint on the Pulse signal.

To progress the simulation until the next time step that the while-iterator subsystem executes more than once, in the Debug tab, click Continue. The simulation runs for 3.5 seconds and pauses on the breakpoint on the Memory block output signal. The port value label for the Memory block output shows a value of 1. The other port value labels display the value from the last time the while-iterator subsystem executed.

Continue clicking Step Over to step through the algorithm one block at a time. The while-iterator subsystem continues to execute so long as the cumulative sum is less than the input value from the Sine Wave block.

Because the simulation pauses on the breakpoint in each iteration, you can use the Continue button to step through the simulation one iteration at a time.

On the 13^th iteration, the cumulative sum becomes 91, which is greater than the input value of 86.6. The port value label for the output of the Relational Operator block shows a value of true because the block has not executed yet.

When you click Step Over once more, the Relational Operator block executes, and the simulation exits the while-iterator subsystem loop. The model canvas navigates back to the top-level view of the model. If you click Step Over once more, the Outport block executes, and the plot in the Dashboard Scope block updates to show the value from this time step.

View Execution Order

To understand how the software steps from one block to the next, you can view the execution order on the block diagram while a simulation is paused. The execution order indicates the order in which the software executes each block during each major time step. Viewing the execution order can help you understand the behavior of your model and the progression of the simulation as you click Step Over.

To progress the simulation to the next time step where the while-iterator subsystem executes more than once, click Continue. The model canvas navigates inside the while-iterator subsystem, and the simulation pauses on the Memory block.

To display the execution order, on the Debug tab, in the Diagnostics section, click Information Overlays. Then, under Blocks, click Execution Order. The Execution Order viewer opens on the right of the Simulink Editor with the current task selected. The Execution Order viewer highlights and annotates the model to indicate the blocks involved in the selected task and their order of execution.

The execution order highlighting overlays the green debugging highlight that indicates where the simulation is paused. You can continue to step through the simulation with the execution order highlighting in the model, or you can clear the execution order highlighting to see the debugging highlights. To clear the execution order highlighting, in the Execution Order viewer, click Clear Highlighting .

The Execution Order viewer does not show the execution order for any tasks, and the highlighting and annotations are cleared from the model. You can see the green debugging highlight on the Memory block again.

To view the execution order again, in the Execution Order viewer, select the task you want to view.

End Simulation Debugging Session

When you are done analyzing and debugging your model, you can end the simulation debugging session in one of several ways, depending on what you plan to do next.

To continue the simulation from the current time point and retain the set of breakpoints configured in the model, disable all breakpoints.

To continue the simulation from the current time point without retaining breakpoints, remove all breakpoints from the model.

To stop the simulation at the current time point, in the Simulink Toolstrip, on the Debug tab, click Stop.