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Tunable Expressions

The Real-Time Workshop product supports the use of tunable variables in expressions. An expression that contains one or more tunable parameters is called a tunable expression.

Tunable Expressions in Masked Subsystems

Tunable expressions are allowed in masked subsystems. You can use tunable parameter names or tunable expressions in a masked subsystem dialog box. When referenced in lower-level subsystems, such parameters remain tunable.

As an example, consider the masked subsystem in the next figure. The masked variable k sets the gain parameter of theGain.

Suppose that the base workspace variable b is declared tunable with SimulinkGlobal (Auto) storage class. The next figure shows the tunable expression b*3 in the subsystem's mask dialog box.

Tunable Expression in Subsystem Mask Dialog Box

The Real-Time Workshop product produces the following output computation for theGain. The variable b is represented as a member of the global parameters structure, model_P. (For clarity in showing the individual Gain block computation, expression folding is off in this example.)

/* Gain: '<S1>/theGain' */
  rtb_theGain_C = rtb_SineWave_n * ((subsys_mask_P.b * 3.0));

  /* Outport: '<Root>/Out1' */
  subsys_mask_Y.Out1 = rtb_theGain_C;

As this example shows, for GRT targets, the parameter structure is mangled to create the structure identifier model_P (subject to the identifier length constraint). This is done to avoid namespace clashes in combining code from multiple models using model reference. ERT-based targets provide ways to customize identifier names.

When expression folding is on, the above code condenses to

/* Outport: '<Root>/Out1' incorporates:
   *  Gain: '<S1>/theGain'
   */
 subsys_mask_Y.Out1 = rtb_SineWave_n * ((subsys_mask_P.b * 3.0));

Expressions that include variables that were declared or modified in mask initialization code are not tunable.

As an example, consider the subsystem above, modified as follows:

• The mask initialization code is

  t = 3 * k;
  

• The parameter k of the myGain block is 4 + t.

• Workspace variable b = 2. The expression b * 3 is plugged into the mask dialog box as in the preceding figure.

Since the mask initialization code can run only once, k is evaluated at code generation time as

4 + (3 * (2 * 3) )

The Real-Time Workshop product inlines the result. Therefore, despite the fact that b was declared tunable, the code generator produces the following output computation for theGain. (For clarity in showing the individual Gain block computation, expression folding is off in this example.)

/* Gain Block: <S1>/theGain */
rtb_temp0 *= (22.0);

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Tunable Expression Limitations

Currently, there are certain limitations on the use of tunable variables in expressions. When an unsupported expression is encountered during code generation a warning is issued and the equivalent numeric value is generated in the code. The limitations on tunable expressions are

• Complex expressions are not supported, except where the expression is simply the name of a complex variable.

• The use of certain operators or functions in expressions containing tunable operands is restricted. Restrictions are applied to four categories of operators or functions, classified in the following table:

  Category	Operators or Functions
  1	+ - .* ./ < > <= >= == ~= & |
  2	* /
  3	abs, acos, asin, atan, atan2, boolean, ceil, cos, cosh, exp, floor, log, log10, sign, sin, sinh, sqrt, tan, tanh,
  4	single, int8, int16, int32, uint8, uint16, uint32
  5	: .^ ^ [] {} . \ .\ ' .' ; ,

  The rules applying to each category are as follows:

  • Category 1 is unrestricted. These operators can be used in tunable expressions with any combination of scalar or vector operands.

  • Category 2 operators can be used in tunable expressions where at least one operand is a scalar. That is, scalar/scalar and scalar/matrix operand combinations are supported, but not matrix/matrix.

  • Category 3 lists all functions that support tunable arguments. Tunable arguments passed to these functions retain their tunability. Tunable arguments passed to any other functions lose their tunability.

  • Category 4 lists the casting functions that do not support tunable arguments. Tunable arguments passed to these functions lose their tunability.

    Note   The Real-Time Workshop product casts values using MATLAB typecasting rules. The MATLAB typecasting rules are different from C code typecasting rules. For example, using the MATLAB typecasting rules, int8(3.7) returns the result 4, while in C code int8(3.7) returns the result 3. See Data Type Conversion in the MATLAB reference documentation for more information on MATLAB typecasting.

  • Category 5 operators are not supported.

    Note   The "dot" (structure membership) operator is not supported. This means that expressions that include a structure member are not tunable.

• Expressions that include variables that were declared or modified in mask initialization code are not tunable.

• The Fcn block does not support tunable expressions in code generation.

• Model workspace parameters can take on only the Auto storage class, and thus are not tunable. See Parameterizing Model References for tuning techniques that work with referenced models.

• Non-double expressions are not supported.

• Blocks that access parameters only by address support the use of tunable parameters, if the parameter expression is a simple variable reference. When an operation such as a data type conversion or a math operation is applied, the Real-Time Workshop product creates a nontrivial expression that cannot be accessed by address, resulting in an error during the build process.

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Using the Model Parameter Configuration Dialog Box

Linear Block Parameter Tunability

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