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Initialize Persistent Variables in MATLAB Functions

A persistent variable is a local variable in a MATLAB® function that retains its value in memory between calls to the function. If you generate code from your model, you must initialize a persistent variable for your MATLAB functions. For more information, see persistent.

When using MATLAB functions that contain persistent variables in Simulink® models, you should follow these guidelines:

  • Initialize the persistent variables in functions only by accessing constants.

  • Ensure the control flow of the function does not depend on whether the initialization occurs.

If you do not follow these guidelines, several conditions produce an initialization error:

  • MATLAB Function blocks with persistent variables where the Allow direct feedthrough property is cleared

  • MATLAB Function blocks with persistent variables in models with State Control blocks where State control is set to Synchronous

  • Stateflow® charts that implement Moore machine semantics and that use MATLAB functions with persistent variables

For example, the function fcn below uses a persistent variable, n. fcn violates both guidelines. The initial value of n depends on the input u and the return statement interrupts the normal control flow of the function. Consequently, this code produces an error when used in a model that has one of the conditions described above.

function y = fcn(u)
    persistent n
        
    if isempty(n)
        n = u;
        y = 1;
        return
    end
    
    y = n;
    n = n + u; 
end

To prevent the error, initialize the persistent variable by setting it to a constant value and removing the return statement. This modified version of fcn initializes the persistent variable without producing an error:

function y = fcn(u)
    persistent n
        
    if isempty(n)
        n = 1;
    end
    
    y = n;
    n = n + u; 
end

MATLAB Function Block with No Direct Feedthrough

This model contains a MATLAB Function block that uses the first version of fcn, described previously. The MATLAB Function block input is a square wave, which is provided by a Sign and Sine Wave block. The MATLAB Function block adds the value of u to the persistent variable n at each time step.

Simulate the model. The simulation returns an error because:

  • The initial value of the persistent variable n depends on the input u.

  • The return statement interrupts the normal control flow of the function.

  • The Allow direct feedthrough property of the MATLAB Function block is cleared.

Modify the MATLAB Function block code, as shown in the corrected version of fcn. Simulate the model again.

State Control Block in Synchronous Mode

This model contains a MATLAB Function block that uses the first version of fcn, described previously. The MATLAB Function block input is a square wave, which is provided by a Sign and Sine Wave block. The MATLAB Function block adds the value of u to the persistent variable n at each time step. The model contains a State Control block where State control is set to Synchronous.

Simulate the model. The simulation returns an error because:

  • The initial value of the persistent variable n depends on the input u.

  • The return statement interrupts the normal control flow of the function.

  • the model contains a State Control block where State control is set to Synchronous.

Modify the MATLAB Function block code, as shown in the corrected version of fcn. Simulate the model again.

Stateflow Chart Implementing Moore Semantics

This model contains a Stateflow Chart with a MATLAB function that uses the first version of fcn, described previously. The MATLAB function adds the value (1 or -1) determined by the active state to the persistent variable n at each time step.

Simulate the model. The simulation returns an error because:

  • The initial value of the persistent variable n depends on the input u.

  • The return statement interrupts the normal control flow of the function.

  • The chart implements Moore semantics.

Modify the MATLAB function code, as shown in the corrected version of fcn. Simulate the model again.

See Also

| (Simulink) | (HDL Coder) |

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