Breakpoints List

View, configure, and manage breakpoints for debugging models

Since R2022a

Description

The Breakpoints List provides information about all breakpoints configured in your model, including breakpoints in Stateflow^® charts and code for MATLAB Function blocks. The Breakpoints List displays breakpoints configured throughout the model hierarchy.

Using the Breakpoints List, you can:

• View all breakpoints in the model along with breakpoint information, such as the breakpoint source.

• Navigate to the location of each breakpoint in the model hierarchy.

• Enable and disable individual breakpoints or all breakpoints.

• Delete breakpoints.

• Enable and disable low-level debugging that provides options to:

  • Step through simulation block by block.

  • Pause within a time step as soon as a breakpoint condition is met.

  • Add model breakpoints to pause when a condition, such as an Inf or NaN value, occurs anywhere in the model.

• Highlight and trace signal paths to their sources by using Model Slicer when your simulation pauses on a breakpoint. A Simulink^® Check™ license is required to open Model Slicer from the Breakpoints List. For more information, see Trace Faulty Signal Paths Using Model Slicer at Signal Breakpoints (Simulink Check).

Breakpoints are not saved as part of the model.

Open the Breakpoints List

• Simulink Toolstrip: On the Debug tab, click Breakpoints List.

• Simulink Editor: Click the breakpoint icon on a signal that has a breakpoint.

  When you open the Breakpoints List by clicking a breakpoint, the row for the breakpoint you clicked is highlighted purple in the table.

Examples

Add Signal Breakpoints

A signal breakpoint causes the simulation to pause when the signal value satisfies the specified condition.

Add a signal breakpoint using the Simulink Toolstrip.

1. Select a signal in your model.

2. On the Debug tab, click Add Breakpoint.

3. In the Add Breakpoint dialog box, specify the condition for the breakpoint. From the drop-down list, select the relational operator for the condition. Then, in the text box, specify the value.

   For example, to pause the simulation when the signal value is equal to 0, select the equals (==) relational operator and enter 0 in the text box.

4. Click OK.

The red circle on the signal line indicates that a breakpoint is configured for that signal.

Add another breakpoint using the context menu for the signal.

1. Select a signal in the model.

2. Right-click the selected signal, and then select Add Breakpoint .

3. In the Add Breakpoint dialog box, specify the condition for the breakpoint.

   For example, to pause the simulation when the signal value is greater than 5, select the greater than (>) relational operator and enter 5 in the text box.

4. Click OK.

View and Manage Breakpoints

After adding breakpoints to your model, you can use the Breakpoints List to view and manage the breakpoints throughout your model.

To open the Breakpoints List, on the Debug tab, click Breakpoints List. The Breakpoints List opens docked at the bottom of the Simulink Editor. The table lists all the breakpoints configured in the model hierarchy, including breakpoints in Stateflow charts, along with information about each breakpoint.

Using the Breakpoints List, you can enable and disable individual breakpoints or all breakpoints. The simulation pauses when the condition for an enabled breakpoint is met and ignores disabled breakpoints. You can enable and disable breakpoints during and between debugging sessions. To enable or disable an individual breakpoint, use the check box in the Enabled column of the table. To enable or disable all breakpoints in the model hierarchy, click Enable or disable all breakpoints .

The table summarizes how a breakpoint appears in the block diagram based on its status.

Breakpoint Appearance	Breakpoint Status
	Enabled
	Hit
	Disabled
	Invalid

You can also delete breakpoints.

• To delete an individual breakpoint, pause on the row for the breakpoint, then click Delete breakpoint .

• To delete multiple breakpoints, select the breakpoints to delete in the table. Then, click Delete selected breakpoints .

• To delete all breakpoints, click the Delete selected breakpoints button arrow and select Delete all breakpoints.

Breakpoints are not saved as part of the model.

Navigate to Breakpoints in Model

Using the Breakpoints List, you can navigate easily to each breakpoint in the model and determine which row of the Breakpoints List contains information about a given breakpoint.

To navigate to a specific breakpoint from the Breakpoints List, click the hyperlink for the breakpoint in the table. A new tab opens in the Simulink Editor to show the component that contains the breakpoint. The source for the breakpoint is highlighted purple.

To get more information about a breakpoint you find as you navigate through your model, click the breakpoint . The row for that breakpoint in the Breakpoints List is highlighted purple.

Highlight and Trace Signal Paths from Breakpoints Using Model Slicer

When a simulation is paused, you can use Model Slicer to highlight and trace the signal paths from breakpoints back to their sources. The Debug using Slicer button changes from to in the Breakpoints List when the simulation is paused. Clicking Debug using Model Slicer sets a starting point to the signal associated with the current breakpoint. You can add multiple breakpoints, change between the breakpoints, add additional starting points, and switch the highlight direction to see the upstream or downstream impact of the values.

Pause the simulation and add a signal breakpoint.

Open the Breakpoints List. In the Simulink Toolstrip, on the Debug tab, click Breakpoints List.

To add a starting point from the breakpoint, click Debug using Slicer .

To add another starting point, select a block or signal and click Add as Starting Point in the Breakpoints List.

To view the upstream, downstream, and bidirectional flow of the signals in the model, click the upstream button , the downstream button , or the bidirectional button .

To close the Model Slicer, click Close Model Slicer in the Breakpoints List.

Tracing paths are not preserved when you exit the debugging session.

Related Examples

• Debug Simulation Using Signal Breakpoints
• Debug Simulation of Iterator Subsystem
• Step Through Simulations Using the Simulink Editor
• View Signal Values Using Port Value Labels
• How Fast Restart Improves Iterative Simulations

Parameters

Debugging Behavior

Pause within time step — Option to disable low-level debugging

on (default) | off

By default, models have Pause within time step selected to enable low-level debugging. This table summarizes debugging feature behavior and availability with and without low-level debugging enabled.

Debugging Feature	Low-Level Debugging Enabled	Low-Level Debugging Disabled
Block by block stepping	You can start stepping block by block to start a simulation and when a simulation is paused.	Not supported.
Signal breakpoints	The simulation pauses within a time step, as soon as the condition for the breakpoint is met.	The simulation pauses at the end of the time step in which the condition for a breakpoint is met.
Model breakpoints	You can add breakpoints to pause the simulation if these conditions occur anywhere in the model: Zero crossing detected Inf or NaN value	Not supported.

Note

Low-level debugging is supported only for normal mode simulations. This parameter has no effect on accelerator and rapid accelerator simulations.

Debug Stateflow charts and MATLAB Function blocks — Option to enable debugging capabilities for Stateflow charts and MATLAB Function blocks

off (default) | on

Since R2025a

Select this option to enable the Allow setting breakpoints during simulation parameter. By default, the parameter is disabled.

When you select this option to enable the parameter, you can add breakpoints during simulation to Stateflow charts, State Transition blocks, Truth Table blocks, and code for MATLAB Function blocks.

When Pause within time step is selected, selecting this option also enables stepping into and out of Stateflow charts, State Transition blocks, Truth Table blocks, and MATLAB Function blocks while stepping through the simulation block by block.

Active breakpoints — Option to enable and disable breakpoints

on (default) | off

Using the check box next to each breakpoint in the Breakpoints List, you can enable and disable individual breakpoints during and between debugging sessions. When the check box is selected, the breakpoint is active, and the simulation pauses when the condition for the breakpoint is met. When the check box is cleared, the breakpoint is inactive, and the simulation does not pause because of that breakpoint.

To enable or disable all breakpoints configured in the model, click Enable or disable all breakpoints .

Signal Breakpoints

Signal breakpoint — Breakpoints configured for one or more signals in model

Simulink signal breakpoint

The Breakpoints List contains a row for each signal breakpoint you configure in the model. Each signal breakpoint has a condition, and the simulation pauses when the signal value satisfies that condition. The condition consists of a relational operator, such as equals (==) or greater than (>), and a value. For example, for a signal breakpoint condition of > 1, the simulation pauses when the signal value is greater than one.

Model Breakpoints

Zero crossing — Option to pause simulation when zero crossing detected

off (default) | on

By default, no model breakpoints are configured for a model. To add the zero-crossing breakpoint to a model, on the Debug tab, click the button arrow next to Add Breakpoint and select Zero Crossing. After you add the zero-crossing breakpoint, you can enable and disable the zero-crossing breakpoint from the Breakpoints List during and between debugging sessions for the model.

When the simulation pauses due to an active zero-crossing breakpoint, the Simulink Editor opens a tab to display the subsystem or model that contains the block where the zero crossing occurred and highlights the block in green.

NaN or Inf Value — Option to pause simulation when nonfinite value computed

off (default) | on

By default, no model breakpoints are configured for a model. To add the NaN or Inf value breakpoint to a model, on the Debug tab, click the button arrow next to Add Breakpoint and select NaN or Inf Value. After you add the NaN or Inf value breakpoint, you can enable and disable the NaN or Inf value breakpoint from the Breakpoints List during and between debugging sessions for the model.

When the NaN or Inf value breakpoint is enabled for a model, the simulation pauses when a computation results in a NaN or Inf value. NaN or Inf values can occur when the computation result is infinite or outside of the range of values supported by the machine running the simulation.

When the simulation pauses due to an active NaN or Inf breakpoint, the Simulink Editor opens a tab to display the subsystem or model that contains the block where the NaN or Inf value occurred and highlights the block in green.

Use the NaN or Inf value breakpoint to help locate the site of computational errors in a model.

Limitations

These limitations apply to signal breakpoints when Pause within time step is either selected or cleared:

• Signal breakpoints do not support specifying the breakpoint condition using variables, MATLAB^® expressions, or symbols such as π.

• Signal breakpoints are supported only for real scalar signals with these data types:

  • double

  • single

  • int

  • Boolean

  • Fixed-point

    For fixed-point signals, the software converts the data type of the signal value to double and uses the converted value to check for the breakpoint condition.

• Signal breakpoints are supported for only Simulink signals and are not supported for other types of signals, such as Simscape™ signals.

• Breakpoints on buses are not supported.

• Setting a breakpoint on an input port created using an In Bus Element block is supported only when the port loads a leaf signal with scalar values.

• Breakpoints in model references are not supported in accelerator mode simulations.

• In normal mode, breakpoints in model references are supported only for the instance that has normal mode visibility. For more information, see Simulate Multiple Referenced Model Instances in Normal Mode.

• Breakpoints on the outputs of root-level input ports of referenced models inside variant subsystems are supported only for the active variant choice.

• The limitations for port value labels also apply for signal breakpoints. For details, see Additional Considerations for Using Port Value Labels.

These limitations apply to signal breakpoints only when Pause within time step is selected:

• Stepping into a model reference is supported only for the instance that has normal mode visibility. For more information, see Simulate Multiple Referenced Model Instances in Normal Mode.

• You can set breakpoints on input ports of subsystems, but the breakpoints do not pause the simulation and appear as invalid in the block diagram.

• Accelerator mode does not support stepping block by block or pausing within a time step.

• Signal breakpoints always pause accelerator mode simulations between time steps. (since R2026a)

• Before R2026a: Signal breakpoints are supported only in normal mode simulations.

• Before R2025a: You can set breakpoints on the outputs of root-level input ports, but the breakpoints do not pause the simulation and appear as invalid in the block diagram. To pause the simulation based on the signal value, add a breakpoint on the output of the block that produces the signal the port receives.

Model breakpoints are supported only in normal mode simulations and only when Pause within time step is selected.

Using Model Slicer to trace signals from breakpoints is supported when:

• The model simulates in normal mode.

• The simulation is paused on a signal breakpoint.

The stepping and breakpoint options in the Simulink Editor do not have a direct programmatic equivalent. The Simulink debugging programmatic interface includes options for setting breakpoints and stepping through simulations. However, the behavior of these functions is not identical to the behavior of the stepping options in the Simulink Editor. For more information about programmatic debugging, see Simulink Debugging Programmatic Interface.

Tips

• To view signal values in the model while debugging, use port value labels. To add a port value label for a signal, select the signal. Then, on the Debug tab, in the Tools section, next to Output Values, click Show Output Value of selected signal . For more information, see View Signal Values Using Port Value Labels.

• To save time when iteratively debugging, consider enabling fast restart. With fast restart enabled, the model compiles only the first time you simulate. Subsequent simulations run faster by skipping the compilation phase. While fast restart is enabled, you can modify the model only in ways that do not require compiling the model. For more information, see How Fast Restart Improves Iterative Simulations.

• To enable stepping into Stateflow charts during a simulation debugging session, enable the Allow setting breakpoints during simulation configuration parameter before starting the simulation debugging session.

• While paused within a time step, the MATLAB command prompt is replaced with the sldebug command prompt. You can use some of the programmatic debugging functions, including the disp function, to get more information about the system behavior. For more information, see Simulink Debugging Programmatic Interface.

• By default, the Breakpoints List opens as a pane at the bottom of the Simulink Editor. You can reposition the Breakpoints List to suit your workflow.

  • To dock the Breakpoints List to a different part of the Simulink Editor, drag the pane to another part of the window.

  • To minimize the Breakpoints List, click the arrow in the upper right and select Minimize.

  • To use the Breakpoints List as a separate window, drag the pane out of the Simulink Editor or click the arrow in the upper right and select Undock.

Version History

Introduced in R2022a

R2026a: Improved support for breakpoints in accelerator mode

The Pause within time step option now affects the behavior of breakpoints only in normal mode simulations. Supported breakpoints pause between time steps in accelerator mode simulations even if Pause within time step is selected. Previously, breakpoints were not supported in accelerator mode simulations if Pause within time step was selected.

Accelerator mode still does not support some breakpoints that normal mode supports. For example, breakpoints inside model references are not supported in accelerator mode.

The software now indicates which breakpoints are supported in accelerator mode by dimming unsupported breakpoints in the block diagram and the Breakpoints List. Previously, unsupported breakpoints appeared active during accelerator mode simulations but did not pause the simulation.

R2026a: Trace and highlight signal paths from breakpoints using Model Slicer

Open Model Slicer from the Breakpoints List when your simulation pauses on a breakpoint. A Simulink Check license is required to open Model Slicer from the Breakpoints List.

R2025a: Set breakpoints on root-level input ports

You can add breakpoints to outputs of root-level input ports while debugging a simulation block by block. When you add a breakpoint to the output of a root-level input port, the simulation pauses when the output value of the port satisfies the breakpoint condition. Set breakpoints on the outputs of root-level input ports when you want to investigate or debug model behavior in response to specific input values.

Root-level input ports include input ports created using Inport and In Bus Element blocks at the root level of either a top model or a referenced model.

• You can set breakpoints on the outputs of both unique and duplicate root-level input ports.

• You can set breakpoints on input ports of subsystems, but the breakpoints do not pause the simulation and appear as invalid in the block diagram.

• Breakpoints on buses are not supported. Setting breakpoints on an input port created using an In Bus Element block is supported only when the port loads a leaf signal with scalar values.

• Breakpoints on the outputs of root-level input ports of referenced models inside variant subsystems are supported only for the active variant choice.

In previous releases, you could add breakpoints on the outputs of root-level input ports, but the breakpoints did not pause the simulation and appeared as invalid in the block diagram. To pause the simulation based on the signal value, you had to add a breakpoint on the block that produces the signal the port receives.

R2025a: Enable Stateflow and MATLAB Function block debugging from the Breakpoints List

You can enable the Allow setting breakpoints during simulation model configuration parameter from the Breakpoints List by selecting Debug Stateflow charts and MATLAB Function blocks. With the Allow setting breakpoints during simulation parameter enabled, you can:

• Add breakpoints in Stateflow charts, State Transition Table blocks, Truth Table blocks, and MATLAB Function blocks during simulation.

• Step into and out of Stateflow charts, State Transition Table blocks, and MATLAB Function blocks while stepping block by block through simulation.

R2024b: Debug simulations on demand by stepping block by block

When the Pause within time step option is selected in the Breakpoints List, you can now start debugging a simulation on demand by stepping through time steps block by block. In previous releases, you could step block by block through a time step only when the simulation was paused and only if the model contained an active breakpoint before you started the simulation.

With on-demand simulation debugging, you can now debug a simulation by stepping block by block from the beginning of the simulation and without first adding a breakpoint. To start a simulation by stepping block by block, in the Simulink Toolstrip, on the Debug tab, click Step Over. The simulation starts and then pauses just before executing the first block in the execution order. To control the simulation execution while debugging, you can:

• Step block by block by clicking Step Over.

• Add breakpoints to conditionally pause the simulation.

• Step through time steps by clicking Step Forward and Step Back.

• Pause or resume on demand by clicking Pause or Continue.

For more information, see Step Through Simulations Using the Simulink Editor.

R2024b: Add signal breakpoints during simulation runtime

You can add signal breakpoints to a model while a simulation of the model is running. In prior releases, you could add signal breakpoints only when the simulation was paused.

R2024b: Add signal breakpoints and port value labels to iteration number outputs in iterator subsystems

You can now pause a simulation on a particular iteration of an iterator subsystem by adding signal breakpoints to signals produced by:

• Iteration number output ports of While Iterator blocks inside While Iterator Subsystem blocks

• Iterator value output ports of For Iterator blocks inside For Iterator Subsystem blocks

• Partition index output ports of For Each blocks inside For Each Subsystem blocks

In previous releases, you could add a breakpoint in these locations, but the breakpoints did not pause the simulation and appeared as invalid in the block diagram.

Port value labels at the same locations now display the iteration number. In previous releases, port value labels at these locations showed inaccessible.

For an example, see Debug Simulation of Iterator Subsystem.

R2024a: Debug simulation execution using the Execution Order viewer while stepping block by block

The Execution Order viewer has improved support for debugging simulations by stepping through simulation block by block.

• Execution order highlighting no longer conflicts with the block-by-block debugging highlighting that indicates the block on which the simulation is paused.

• The Execution Order viewer highlights the row in the execution list that corresponds to the block on which the simulation is paused, so you can track the position in the execution list alongside the location of the block in the model.

• The Breakpoints List now has a button to open the Execution Order viewer.

R2024a: Increased flexibility for stepping block by block in simulation debugging session

When the Pause within time step option is selected in the Breakpoints List and a model contains at least one breakpoint before you start a simulation, you can use the Step Over button to step through the simulation block by block. Starting in R2024a, the Step Over button is available to start stepping block by block anytime such a simulation is paused, including when the simulation is paused between major time steps.

In R2023a and R2023b, the Step Over button was available only while paused within a time step. After the simulation paused within a time step, the Step Over became unavailable if you advanced the simulation through major time steps by using the Step Forward and Step Back buttons. You could not step block by block again until the simulation paused within a time step on a breakpoint.

R2024a: Improved behavior for stepping block by block between Simulink block diagrams and Stateflow charts

The simulation debugging options in the Simulink Editor have enhanced support for stepping between Simulink block diagrams and Stateflow charts, including support for these situations:

• Stepping into and out of a function-call subsystem that is controlled by a Stateflow chart

• Stepping into and out of a Simulink function inside a Stateflow chart

• Stepping into and out of a Simulink-based state inside a Stateflow chart

R2024a: sldebug prompt in MATLAB Command Window indicates when simulation debugging session is paused within time step

When you start a simulation debugging session using the Simulink Editor, you can issue some programmatic simulation debugging commands, such as probe or stop, while the simulation is paused within a time step. The sldebug command prompt (sldebug>>) now replaces the MATLAB command prompt (>>) to indicate in the MATLAB Command Window when the simulation is paused within a time step. As in prior releases, you can issue both MATLAB commands and the supported simulation debugging commands at the sldebug command prompt.

For more information, see Simulink Debugging Programmatic Interface.

R2023b: Keyboard shortcuts for block-by-block debugging

You can use keyboard shortcuts to advance a simulation block by block while paused within a time step. The keyboard shortcuts for the buttons in the Signal Editor are the same as the keyboard shortcuts for these buttons in the Stateflow Editor, the MATLAB Function Block Editor, and the MATLAB Editor.

Task	Shortcut
Step over	F10 On macOS, press Shift+Command+O.
Step in	F11 On macOS, press Shift+Command+I.
Step out	Shift+F11 On macOS, press Shift+Command+U.