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Set Breakpoints in Simulink Debugger

The Simulink® debugger allows you to define stopping points called breakpoints in a simulation. You can then run a simulation from breakpoint to breakpoint, using the continue function. You can define unconditional and conditional breakpoints. An unconditional breakpoint pauses simulation when the simulation reaches a specified method. A conditional breakpoint pauses simulation when the specified condition arises in the simulation. For example, you can specify a conditional breakpoint to pause the simulation when a signal has a certain value.

Breakpoints are useful when you know that a problem occurs at a certain point in your model or when a certain condition occurs. By defining an appropriate breakpoint and running the simulation using the continue function, you can skip immediately to the point in the simulation where the problem occurs.


When you stop a simulation at a breakpoint of a MATLAB® S-function, to exit MATLAB, you must first quit the debugger.

Setting Unconditional Breakpoints

You can set unconditional breakpoints from these locations:

  • Debugger toolbar

  • Simulation Loop pane

  • MATLAB Command Window

Setting Breakpoints from the Simulink Toolstrip

To add a breakpoint, enable the Breakpoint button.

  1. Simulate the model.

  2. Click the Step over current method button until simulationPhase is highlighted.

  3. Click the Step into current method button.

The debugger displays the name of the selected block in the Break/Display points panel of the Breakpoints pane.


Clicking Breakpoint on the toolbar sets breakpoints on the invocations of the methods of the block in major time steps.

You can temporarily disable the breakpoints on a block by deselecting the check box in the breakpoints column of the panel. To clear the breakpoints on a block and remove its entry from the panel,

  1. Select the entry.

  2. Click the Remove selected point button.


You cannot set a breakpoint on a virtual block. A virtual block is purely graphical: it indicates a grouping or relationship among the computational blocks of a model. The debugger warns you if you try to set a breakpoint on a virtual block. You can get a listing of nonvirtual blocks of a model by using the slist command (see Displaying a Model's Nonvirtual Blocks).

Setting Breakpoints from the Simulation Loop Pane

To set a breakpoint at a particular invocation of a method displayed in the Simulation Loop pane, select the check box next to the method name of the method in the breakpoint column.

To clear the breakpoint, clear the check box.

Setting Breakpoints from the Command Window

In command-line mode, use the break and bafter commands to set breakpoints before or after a specified method, respectively. Use the clear command to clear breakpoints.

Setting Conditional Breakpoints

You can use the Break on conditions controls group on the debugger Break Points pane.

Use these functions to set conditional breakpoints.

FunctionBreakpoint Behavior
tbreak [t]

Pauses simulation at specified simulation time step


Pauses simulation at a recoverable error in the model


Pauses simulation at the occurrence of an underflow or overflow (NaN) or infinite (Inf) value


Pauses simulation when the simulation reaches the state that determines the simulation step size


Pauses simulation when a zero crossing occurs between simulation time steps

Setting Breakpoints at Time Steps

To set a breakpoint at a time step, enter a time in the debugger Break at time field or enter the time using the tbreak function. The debugger stops the simulation at the Outputs.Major method of the model at the first time step that follows the specified time. For example, starting vdp in debug mode and entering these commands causes the debugger to pause the simulation at the vdp.Outputs.Major method of time step 2.078.

tbreak 2
[Tm = 2.034340153847549      ] vdp.Outputs.Minor
(sldebug @37): 

Breaking on Nonfinite Values

Selecting the debugger NaN values option or entering the nanbreak command causes the simulation to stop when a computed value is infinite or outside the range of values that is supported by the machine running the simulation. This option is useful for locating computational errors in a model.

Breaking on Step-Size Limiting Steps

Selecting the Step size limited by state option or entering the xbreak command causes the debugger to stop the simulation when the model uses a variable-step solver and the solver encounters a state that limits the size of the steps that it can take. This command is useful in debugging models that appear to require an excessive number of simulation time steps to solve.

Breaking at Zero Crossings

Selecting the Zero crossings option or entering the zcbreak command causes the simulation to halt when a nonsampled zero crossing is detected in a model that includes blocks where zero crossings can arise. After halting, the ID, type, and name of the block in which the zero crossing was detected is displayed. The block ID (s:b:p) consists of a system index s, block index b, and port index p separated by colons (see Block ID).

For example, setting a zero-crossing break at the start of execution of the zeroxing example model,

>> sldebug zeroxing
[TM = 0                      ] zeroxing.Simulate
(sldebug @0): >> zcbreak
Break at zero crossing events              : enabled

and continuing the simulation

(sldebug @0): >> continue

results in a zero-crossing break at

Interrupting model execution before running mdlOutputs at the left post of 
(major time step just before) zero crossing event detected at the following location:
  6[-0]  0:5:2  Saturate  'zeroxing/Saturation'
[TzL= 0.3435011087932808     ] zeroxing.Outputs.Major
(sldebug @16): >> 

If a model does not include blocks capable of producing nonsampled zero crossings, the command prints a message advising you of this fact.

Breaking on Solver Errors

Selecting the debugger Solver Errors option or entering the ebreak command causes the simulation to stop if the solver detects a recoverable error in the model. If you do not set or disable this breakpoint, the solver recovers from the error and proceeds with the simulation without notifying you.

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