System Objects in MATLAB Code Generation

System Objects in Generated Code

You can generate C/C++ code in MATLAB® from your system that contains System objects by using MATLAB Coder™. You can generate efficient and compact code for deployment in desktop and embedded systems and accelerate fixed-point algorithms.

System Objects Code with Persistent Objects for Code Generation

This example shows how to use System objects to make MATLAB code suitable for code generation. The example highlights key factors to consider, such as passing property values and using extrinsic functions. It also shows that by using persistent objects, the object states are maintained between calls.

function w = lmssystem(x, d)
% LMSSYSTEMIDENTIFICATION System identification using 
% LMS adaptive filter
% #codegen

    % Declare System objects as persistent
    persistent hlms;

    % Initialize persistent System objects only once.
    % Do this with 'if isempty(persistent variable).'
    % This condition will be false after the first time. 

    if isempty(hlms)
        % Create LMS adaptive filter used for system 
        % identification. Pass property value arguments
        % as constructor arguments. Property values must
        % be constants during compile time.

        hlms = dsp.LMSFilter(11,'StepSize',0.01);
    end

    [~,~,w] = hlms(x,d);      % Filter weights
end

This example shows how to compile the lmssystem function and produce a MEX file with the same name in the current directory.

% LMSSYSTEMIDENTIFICATION System identification using 
% LMS adaptive filter

coefs = fir1(10,.25);
hfilt = dsp.FIRFilter('Numerator', coefs);

x = randn(1000,1);                 % Input signal
hSrc = dsp.SignalSource(x,100);    % Use x as input-signal with
                                   % 100 samples per frame

% Generate code for lmssystem
codegen lmssystem -args {ones(100,1),ones(100,1)}

while ~isDone(hSrc)
    in = hSrc();
    d  = hfilt(in) + 0.01*randn(100,1);  % Desired signal
    w  = lmssystem_mex(in,d);           % Call generated mex file
    stem([coefs.',w]);
end

For another detailed code generation example, see Generate Code for MATLAB Handle Classes and System Objects in the MATLAB Coder product documentation.

System Objects Code Without Persistent Objects for Code Generation

The following example, using System objects, does not use the persistent keyword because calling a persistent object with different data types causes a data type mismatch error. This example filters the input and then performs a discrete cosine transform on the filtered output. Each call to the FilterAndDCTLib function is independent and state information is not retained between calls.

function [out] = FilterAndDCTLib(in)
   hFIR = dsp.FIRFilter('Numerator',fir1(10,0.5));
   hDCT = dsp.DCT;

   % Run the objects to get the filtered spectrum
   firOut = hFIR(in);
   out = hDCT(firOut);


function [out1, out2] = CompareRealInt(in1)
   % Call the library function, FilterAndDCTLib, which can 
   % generate code for multiple calls each with a different data type. 

   % Convert input data from double to int16
   in2 = int16(in1);

   % Call the library function for both data types, double and int16
   out1 = FilterAndDCTLib(in1);
   out2 = FilterAndDCTLib(in2);


function RunDCTExample
   % Execute everything needed at the command line to run the example

   warnState = warning('off','SimulinkFixedPoint:util:fxpParameterUnderflow');

   % Create vector, length 256, of data containing noise and sinusoids
   dataLength = 256;
   sampleData = rand(dataLength,1) + 3*sin(2*pi*[1:dataLength]*.085)' ...
             + 2*cos(2*pi*[1:dataLength]*.02)';

   % Generate code and run generated file
   codegen CompareRealInt -args {sampleData}
   [out1,out2] = CompareRealInt_mex(sampleData);

   % Compare the floating point results, in blue
   % with the int16 results, in red
   plot(out1,'b')
   hold on
   plot(out2,'r')
   hold off

   warning(warnState.state,warnState.identifier);
end

Usage Rules and Limitations for System Objects for Generating Code

The following usage rules and limitations apply to using System objects in code generated from MATLAB.

Object Construction and Initialization

  • If objects are stored in persistent variables, initialize System objects once by embedding the object handles in an if statement with a call to isempty().

  • Set arguments to System object™ constructors as compile-time constants.

  • You cannot initialize System objects properties with other MATLAB class objects as default values in code generation. You must initialize these properties in the constructor.

Inputs and Outputs

  • System objects accept a maximum of 1024 inputs. A maximum of 8 dimensions per input is supported.

  • The data type of the inputs should not change.

  • If you want the size of inputs to change, verify that variable-size is enabled. Code generation support for variable-size data also requires that the Enable variable sizing option is enabled, which is the default in MATLAB.

      Note:   Variable-size properties in MATLAB Function block in Simulink® are not supported. System objects predefined in the software do not support variable-size if their data exceeds the DynamicMemoryAllocationThreshold value.

  • Do not set System objects to become outputs from the MATLAB Function block.

  • Do not use the Save and Restore Simulation State as SimState option for any System object in a MATLAB Function block.

  • Do not pass a System object as an example input argument to a function being compiled with codegen.

  • Do not pass a System object to functions declared as extrinsic (functions called in interpreted mode) using the coder.extrinsic function. System objects returned from extrinsic functions and scope System objects that automatically become extrinsic can be used as inputs to another extrinsic function, but do not generate code.

Tunable and Nontunable Properties

  • The value assigned to a nontunable property must be a constant and there can be at most one assignment to that property (including the assignment in the constructor).

  • For most System objects, the only time you can set their nontunable properties during code generation is when you construct the objects.

    • For System objects that are predefined in the software, you can set their tunable properties at construction time or using dot notation after the object is locked.

    • For System objects that you define, you can change their tunable properties at construction time or using dot notation during code generation. For getNumInputsImpl and getNumOutputsImpl methods, if you set the return argument from an object property, that object property must have the Nontunable attribute.

  • Objects cannot be used as default values for properties.

Global Variables

  • Global variables are allowed in a System object, unless you will be using that System object in Simulink via the MATLAB System block. To avoid syncing global variables between a MEX file and the workspace, use a coder configuration object. For example: 

    f = coder.MEXConfig; 
f.GlobalSyncMethod = 'NoSync'
    Then, include '-config f' in your codegen command.

Methods

  • Code generation support is available only for these System object methods:

    • get

    • getNumInputs

    • getNumOutputs

    • isDone (for sources only)

    • isLocked

    • release

    • reset

    • set (for tunable properties)

    • step

  • For System objects that you define,

    Code generation support is available only for these methods:

    • getDiscreteStateImpl

    • getNumInputsImpl

    • getNumOutputsImpl

    • infoImpl

    • isDoneImpl

    • isInputDirectFeedThroughImpl

    • outputImpl

    • processTunedPropertiesImpl

    • releaseImpl — Code is not generated automatically for this method. To release an object, you must explicitly call the release method in your code.

    • resetImpl

    • setupImpl

    • stepImpl

    • updateImpl

    • validateInputsImpl

    • validatePropertiesImpl

System Objects in codegen

You can include System objects in MATLAB code in the same way you include any other elements. You can then compile a MEX file from your MATLAB code by using the codegen command, which is available if you have a MATLAB Coder license. This compilation process, which involves a number of optimizations, is useful for accelerating simulations. See Getting Started with MATLAB Coder and MATLAB Classes for more information.

    Note:   Most, but not all, System objects support code generation. Refer to the particular object's reference page for information.

System Objects in the MATLAB Function Block

Using the MATLAB Function block, you can include any System object and any MATLAB language function in a Simulink model. This model can then generate embeddable code. System objects provide higher-level algorithms for code generation than do most associated blocks. For more information, see What Is a MATLAB Function Block? in the Simulink documentation.

System Objects in the MATLAB System Block

Using the MATLAB System block, you can include in a Simulink model individual System objects that you create with a class definition file. The model can then generate embeddable code. For more information, see MATLAB System Block in the Simulink documentation.

System Objects and MATLAB Compiler Software

MATLAB Compiler™ software supports System objects for use inside MATLAB functions. The compiler product does not support System objects for use in MATLAB scripts.

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