Writing S-Functions

Run-Time Parameters

Simulink allows an S-function to create internal representations of external dialog parameters called run-time parameters. Every run-time parameter corresponds to one or more dialog parameters and can have the same value and data type as its corresponding external parameters or a different value or data type. If a run-time parameter differs in value or data type from its external counterpart, the dialog parameter is said to have been transformed to create the run-time parameter. The value of a run-time parameter that corresponds to multiple dialog parameters is typically a function of the values of the dialog parameters. Simulink allocates and frees storage for run-time parameters and provides functions for updating and accessing them, thus eliminating the need for S-functions to perform these tasks.

Run-time parameters facilitate the following kinds of S-function operations:

• Computed parameters

• Often the output of a block is a function of the values of several dialog parameters. For example, suppose a block has two parameters, the volume and density of some object, and the output of the block is a function of the input signal and the weight of the object. In this case, the weight can be viewed as a third internal parameter computed from the two external parameters, volume and density. An S-function can create a run-time parameter corresponding to the computed weight, thereby eliminating the need to provide special case handling for weight in the output computation.

• Data type conversions

• Often a block needs to change the data type of a dialog parameter to facilitate internal processing. For example, suppose that the output of the block is a function of the input and a parameter and the input and parameter are of different data types. In this case, the S-function can create a run-time parameter that has the same value as the dialog parameter but has the data type of the input signal, and use the run-time parameter in the computation of the output.

• Code generation

• During code generation, Real-Time Workshop writes all run-time parameters automatically to the model.rtw file, eliminating the need for the S-function to perform this task via an mdlRTW method.

Creating Run-Time Parameters

An S-function can create run-time parameters all at once or one by one.

Creating Run-Time Parameters All at Once

Use the SimStruct function ssRegAllTunableParamsAsRunTimeParams in mdlSetWorkWidths to create run-time parameters corresponding to all tunable parameters. This function requires that you pass it an array of names, one for each run-time parameter. Real-Time Workshop uses this name as the name of the parameter during code generation.

Note    The first four characters of the names of a block's run-time parameters must be unique. If they are not, Simulink signals an error. For example, trying to register a parameter named param2 triggers an error if a parameter named param1 already exists. This restriction allows Real-time Workshop to generate variable names that are unique within a pre-specified number of characters.

This approach to creating run-time parameters assumes that there is a one-to-one correspondence between an S-function's run-time parameters and its tunable dialog parameters. This might not be the case. For example, an S-function might want to use a computed parameter whose value is a function of several dialog parameters. In such cases, the S-function might need to create the run-time parameters individually.

Creating Run-Time Parameters Individually

    To create run-time parameters individually, the S-function's mdlSetWorkWidths method should

1. Specify the number of run-time parameters it intends to use, using ssSetNumRunTimeParams.
2. Use ssRegDlgParamAsRunTimeParam to register a run-time parameter that corresponds to a single, untransformed dialog parameter or ssSetRunTimeParamInfo to set the attributes of a run-time parameter that corresponds to more than one dialog parameter or a transformed dialog parameter.

Tunable Parameters

Updating Run-Time Parameters

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