Debug Simulation of Iterator Subsystem
Iterator subsystems repeat the execution of the algorithm modeled inside the subsystem one or more times within a simulation time step. To analyze or debug the algorithm inside an iterator subsystem, you need to pause within a major time step when the iterator subsystem executes and step the model execution by a more granular interval than major time steps.
This example shows how to analyze a simple summing algorithm implemented using a while-iterator subsystem.
Open Example Model
IterateSum uses a while-iterator subsystem to implement an iterative summing algorithm. The while-iterator subsystem has two inputs: a sine wave signal and a pulse signal. The pulse provides the initial logical condition that determines whether the while-iterator subsystem is enabled in each time step. A Dashboard Scope block is connected to the
Iteration Number signal to display the output of the while-iterator subsystem.
Simulate the model. You can use debugging utilities in the Simulink® Editor to analyze the algorithm implemented in the while-iterator subsystem and understand the signal displayed in the Dashboard Scope block.
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:
On the Debug tab, under Tools, in the Port Values button group, click Show port value label on selected signal .
You can also add port value labels to multiple signals at once. To add port value labels
Iteration Number signals:
Press Ctrl and select the
On the Debug tab, in the Port Values button group, click Show port value label on selected signal .
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.
On the Debug tab, under Breakpoints, click Add Breakpoint.
In the Add Breakpoint dialog box, configure the signal breakpoint condition so the simulation pauses when the Pulse block output value becomes greater than zero. From the drop-down list, select
>and then enter
0in the box.
Because the while-iterator subsystem executes only when the
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
If you click Step Over one more time, the Outport
block executes, and the Dashboard Scope block updates to show the new value
Iteration Number signal.
If you continue clicking Step Over, you can step through the
block-by-block execution in subsequent major time steps. The
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
0, continue clicking Continue until the
simulation reaches a time of
1.5 seconds and pauses on the
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
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.
Select everything in the block diagram by clicking in the upper left of the canvas and holding while you drag the pointer to the lower right of the canvas or pressing Ctrl+A.
In the Debug tab, under Tools, in the Port Values button group, click Show port value label on selected signal .
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
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
To navigate inside the while-iterator subsystem, double-click the block in the model.
Right-click the output signal of the Memory block.
From the context menu, select Add Breakpoint.
By default, clicking a signal line during simulation toggles whether the signal has a port value label. If you click the signal line to add a breakpoint and the port value label disappears, click the signal again to add the port value label back. The port value label shows
no data yetuntil the block that produces the signal executes again. The software does not retain data for port value labels when they are removed.
In the Add Breakpoint dialog box, configure the signal breakpoint condition so the simulation pauses when the signal value becomes greater than
0. From the drop-down list, select
>and then enter
0in the box.
To progress simulation directly to the first time the while-iterator subsystem executes
more than once, disable the breakpoint on the
In the Debug tab, click Breakpoints List.
In the Breakpoints List, in the Enabled column, clear the check box for the row that corresponds to the breakpoint on the
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
other port value labels display the value from the last time the while-iterator subsystem
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 13th iteration, the cumulative sum becomes
91, which is greater than the input value of
The port value label for the output of the Relational Operator block shows a
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 while retaining the set of breakpoints configured in the model:
To open the Breakpoints List, on the Debug tab, click Breakpoints List.
In the Breakpoints List, use the check boxes in the Enable column or the Enable or disable all breakpoints button to disable all the breakpoints.
To resume the simulation, click Continue.
To continue the simulation from the current time point but stop debugging:
To clear all breakpoints in the model, on the Debug tab, click the Add Breakpoint button arrow and select Clear All Breakpoints.
To resume simulation, click Continue.
To stop the simulation at the current time point, in the Simulink Toolstrip, on the Debug tab, click Stop.