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Implement Function-Call Subsystems with S-Functions

You can create a triggered subsystem whose execution is determined by logic internal to a C MEX S-function instead of by the value of a signal. A subsystem so configured is called a function-call subsystem. To implement a function-call subsystem:

  • In the Trigger block, select function-call as the Trigger type parameter.

  • In the S-function, use the ssEnableSystemWithTid and ssDisableSystemWithTid to enable or disable the triggered subsystem and the ssCallSystemWithTid macro to call the triggered subsystem.

  • In the model, connect the S-Function block output directly to the trigger port.


    Function-call connections can only be performed on the first output port.

Function-call subsystems are not executed directly by the Simulink® engine; rather, the S-function determines when to execute the subsystem. When the subsystem completes execution, control returns to the S-function. This figure illustrates the interaction between a function-call subsystem and an S-function.

In this figure, ssCallSystemWithTid executes the function-call subsystem that is connected to the first output port element. ssCallSystemWithTid returns 0 if an error occurs while executing the function-call subsystem or if the output is unconnected. After the function-call subsystem executes, control is returned to your S-function.

Function-call subsystems can only be connected to S-functions that have been properly configured to accept them.

To configure an S-function to call a function-call subsystem:

  • In mdlInitializeSizes, set the data type of the S-function first output port to function-call by specifying

    ssSetOutputPortDataType(S, 0, SS_FCN_CALL); 

  • Specify the elements that are to execute the function-call subsystem in mdlInitializeSampleTimes. For example:

    ssSetCallSystemOutput(S,0);  /* call on first element */
    ssSetCallSystemOutput(S,1);  /* call on second element */
  • Specify in mdlInitializeSampleTimes whether you want the S-function to be able to enable or disable the function-call subsystem. Only S-functions that explicitly enable and disable the function-call subsystem can reset the states and outputs of the subsystem, as determined by the function-call subsystem's Trigger and Outport blocks. For example, the code

    ssSetExplicitFCSSCtrl(S, 1);

    in mdlInitializeSampleTimes specifies that the S-function can enable and disable the function-call subsystem. In this case, the S-function must invoke ssEnableSystemWithTid before executing the subsystem using ssCallSystemWithTid.

  • Execute the subsystem in the appropriate mdlOutputs or mdlUpdate S-function routine. For example:

    static void mdlOutputs(...)
        if (((int)*uPtrs[0]) % 2 == 1) {
          if (!ssCallSystemWithTid(S,0,tid)) {
            /* Error occurred, which will be reported by */
    				/*the Simulink engine*/
        } else {
          if (!ssCallSystemWithTid(S,1,tid)) {
            /* Error occurred, which will be reported by */
    				/*the Simulink engine*/

See sfun_fcncall.c for an example that executes a function-call subsystem on the first and second elements of the first S-function output. The following Simulink model (sfcndemo_sfun_fcncall) uses this S-function.

The first function-call subsystem provides a sine wave output. The second function-call subsystem is a simple feedback loop containing a Unit Delay block.

When the Pulse Generator emits its upper value, the function-call subsystem connected to the first element of the first S-function output port is triggered. Similarly, when the Pulse Generator emits its lower value, the function-call subsystem connected to the second element is triggered. The simulation output is shown on the following Scope.

Function-call subsystems are a powerful modeling construct. You can configure Stateflow® blocks to execute function-call subsystems, thereby extending the capabilities of the blocks. For more information, see the Stateflow documentation.

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