Write Level-2 MATLAB S-Functions
Setup Working Environment to Write Level-2 MATLAB S-Function
This example opens up a directory containing the following files required fro this topic.
About Level-2 MATLAB S-Functions
The Level-2 MATLAB® S-function API allows you to use the MATLAB language to create custom blocks with multiple input and output ports and capable of handling any type of signal produced by a Simulink® model, including matrix and frame signals of any data type. The Level-2 MATLAB S-function API corresponds closely to the API for creating C MEX S-functions. Much of the documentation for creating C MEX S-functions applies also to Level-2 MATLAB S-functions. To avoid duplication, this section focuses on providing information that is specific to writing Level-2 MATLAB S-functions.
A Level-2 MATLAB S-function is MATLAB function that defines the properties and behavior of an instance of a Level-2 MATLAB S-Function block that references the MATLAB function in a Simulink model. The MATLAB function itself comprises a set of callback methods (see Level-2 MATLAB S-Function Callback Methods) that the Simulink engine invokes when updating or simulating the model. The callback methods perform the actual work of initializing and computing the outputs of the block defined by the S-function.
To facilitate these tasks, the engine passes a run-time object to the callback methods as an argument. The run-time object effectively serves as a MATLAB proxy for the S-Function block, allowing the callback methods to set and access the block properties during simulation or model updating.
About Run-Time Objects
When the Simulink engine invokes a Level-2 MATLAB S-function callback method,
it passes an instance of the
to the method as an argument. This instance, known as the run-time
object for the S-Function block, serves the same purpose for Level-2 MATLAB S-function callback methods
SimStruct structure serves for C MEX S-function
callback methods. The object enables the method to provide and obtain
information about various elements of the block ports, parameters,
states, and work vectors. The method does this by getting or setting
properties or invoking methods of the block run-time object. See the
documentation for the
for information on getting and setting run-time object properties
and invoking run-time object methods.
Run-time objects do not support MATLAB sparse matrices. For example, if
block is a run-time object, the following
line in a Level-2 MATLAB S-function
produces an error:
block.Outport(1).Data = speye(10);
speye command forms a sparse identity
Other MATLAB programs besides MATLAB S-functions can use run-time objects to obtain information about a MATLAB S-function in a model that is simulating. See Access Block Data During Simulation in Using Simulink for more information.
Level-2 MATLAB S-Function Template
Use the basic Level-2 MATLAB S-function template
msfuntmpl_basic.m to get a head start on creating a new Level-2
MATLAB S-function. The template contains
skeleton implementations of the required callback methods defined by the Level-2 MATLAB S-function API. To write a more complicated
S-function, use the annotated template
To create a MATLAB S-function, make a copy of the template and edit the copy as necessary to reflect the desired behavior of the S-function you are creating. The following two sections describe the contents of the MATLAB code template. The section Example of Writing a Level-2 MATLAB S-Function describes how to write a Level-2 MATLAB S-function that models a unit delay.
Level-2 MATLAB S-Function Callback Methods
The Level-2 MATLAB S-function API defines the signatures and general purposes of the callback methods that constitute a Level-2 MATLAB S-function. The S-function itself provides the implementations of these callback methods. The implementations in turn determine the block attributes (e.g., ports, parameters, and states) and behavior (e.g., the block outputs as a function of time and the block inputs, states, and parameters). By creating an S-function with an appropriate set of callback methods, you can define a block type that meets the specific requirements of your application.
A Level-2 MATLAB S-function must include the following callback methods:
setupfunction to initialize the basic S-function characteristics
Outputsfunction to calculate the S-function outputs
Your S-function can contain other methods, depending on the requirements of the block that the S-function defines. The methods defined by the Level-2 MATLAB S-function API generally correspond to similarly named methods defined by the C MEX S-function API. For information on when these methods are called during simulation, see Process View in Simulink Engine Interaction with C S-Functions.
The following table lists all the Level-2 MATLAB S-function callback methods and their C MEX counterparts.
The body of the
setup method in a Level-2 MATLAB S-function initializes the instance
of the corresponding Level-2 MATLAB S-Function
block. In this respect, the
setup method is similar
mdlInitializeSampleTimes callback methods
implemented by C MEX S-functions. The
performs the following tasks:
Initializing the number of input and output ports of the block.
Setting attributes such as dimensions, data types, complexity, and sample times for these ports.
Specifying the block sample time. See Specify Sample Time in Using Simulink for more information on how to specify valid sample times.
Setting the number of S-function dialog parameters.
Registering S-function callback methods by passing the handles of local functions in the MATLAB S-function to the
RegBlockMethodmethod of the S-Function block's run-time object. See the documentation for
Simulink.MSFcnRunTimeBlockfor information on using the
Example of Writing a Level-2 MATLAB S-Function
The following steps illustrate how to write a simple Level-2 MATLAB S-function. When applicable, the steps include examples from the
msfcn_unit_delay.m used in the model
msfcndemo_sfundsc2. All lines of code use the variable name
block for the S-function run-time object.
Open MATLAB S-function template
msfuntmpl_basic.mfrom the working folder. If you change the file name when you copy the file, change the function name in the
functionline to the same name.
setupmethod to initialize the S-function's attributes. For this example:
Set the run-time object's
1in order to initialize one input port and one output port.
Invoke the run-time object's SetPreCompInpPortInfoToDynamic and SetPreCompOutPortInfoToDynamic methods to indicate that the input and output ports inherit their compiled properties (dimensions, data type, complexity, and sampling mode) from the model.
DirectFeedthroughproperty of the run-time object's
falsein order to indicate the input port does not have direct feedthrough. Retain the default values for all other input and output port properties that are set in your copy of the template file. The values set for the
Complexityproperties override the values inherited using the
Set the run-time object's
1in order to initialize one S-function dialog parameter.
Specify that the S-function has an inherited sample time by setting the value of the runtime object's
Call the run-time object's
RegBlockMethodmethod to register the following four callback methods used in this S-function.
Remove any other registered callback methods from your copy of the template file. In the calls to
RegBlockMethod, the first input argument is the name of the S-function API method and the second input argument is the function handle to the associated local function in the MATLAB S-function.
msfcn_unit_delay.mperforms the previous list of steps:
function setup(block) %% Register a single dialog parameter block.NumDialogPrms = 1; %% Register number of input and output ports block.NumInputPorts = 1; block.NumOutputPorts = 1; %% Setup functional port properties to dynamically %% inherited. block.SetPreCompInpPortInfoToDynamic; block.SetPreCompOutPortInfoToDynamic; %% Hard-code certain port properties block.InputPort(1).Dimensions = 1; block.InputPort(1).DirectFeedthrough = false; block.OutputPort(1).Dimensions = 1; %% Set block sample time to [0.1 0] block.SampleTimes = [0.1 0]; %% Register methods block.RegBlockMethod('PostPropagationSetup',@DoPostPropSetup); block.RegBlockMethod('InitializeConditions',@InitConditions); block.RegBlockMethod('Outputs', @Output); block.RegBlockMethod('Update', @Update);
If your S-function needs continuous states, initialize the number of continuous states in the
setupmethod using the run-time object's
NumContStatesproperty. Do not initialize discrete states in the
Initialize the discrete states in the
PostPropagationSetupmethod. A Level-2 MATLAB S-function stores discrete state information in a DWork vector. The default
PostPropagationSetupmethod in the template file suffices for this example.
DoPostPropSetup, initializes one DWork vector with the name
function DoPostPropSetup(block) %% Setup Dwork block.NumDworks = 1; block.Dwork(1).Name = 'x0'; block.Dwork(1).Dimensions = 1; block.Dwork(1).DatatypeID = 0; block.Dwork(1).Complexity = 'Real'; block.Dwork(1).UsedAsDiscState = true;
If your S-function uses additional DWork vectors, initialize them in the
PostPropagationSetupmethod, as well (see Using DWork Vectors in Level-2 MATLAB S-Functions).
Initialize the values of discrete and continuous states or other DWork vectors in the
Startcallback methods. Use the
Startcallback method for values that are initialized once at the beginning of the simulation. Use the
InitializeConditionsmethod for values that need to be reinitialized whenever an enabled subsystem containing the S-function is reenabled.
For this example, use the
InitializeConditionsmethod to set the discrete state's initial condition to the value of the S-function's dialog parameter. For example, the
function InitConditions(block) %% Initialize Dwork block.Dwork(1).Data = block.DialogPrm(1).Data;
For S-functions with continuous states, use the
ContStatesrun-time object method to initialize the continuous state data. For example:
block.ContStates.Data(1) = 1.0;
Calculate the S-function's outputs in the
Outputscallback method. For this example, set the output to the current value of the discrete state stored in the DWork vector.
function Output(block) block.OutputPort(1).Data = block.Dwork(1).Data;
For an S-function with continuous states, calculate the state derivatives in the
Derivativescallback method. Run-time objects store derivative data in their
Derivativesproperty. For example, the following line sets the first state derivative equal to the value of the first input signal.
block.Derivatives.Data(1) = block.InputPort(1).Data;
This example does not use continuous states and, therefore, does not implement the
Update any discrete states in the
Updatecallback method. For this example, set the value of the discrete state to the current value of the first input signal.
function Update(block) block.Dwork(1).Data = block.InputPort(1).Data;
Perform any cleanup, such as clearing variables or memory, in the
Terminatemethod. Unlike C MEX S-functions, Level-2 MATLAB S-function are not required to have a
For information on additional callback methods, see Level-2 MATLAB S-Function Callback Methods.
For a list of run-time object properties, see the reference page for
Simulink.MSFcnRunTimeBlock and the
Instantiating a Level-2 MATLAB S-Function
To use a Level-2 MATLAB S-function in a model, copy an instance of the Level-2 MATLAB S-Function block into the model. Open the Block Parameters dialog box for the block and enter the name of the MATLAB file that implements your S-function into the S-function name field. If your S-function uses any additional parameters, enter the parameter values as a comma-separated list in the Block Parameters dialog box Parameters field.
Operations for Variable-Size Signals
Following are modifications to the Level-2 MATLAB
S-functions template (
msfuntmpl_basic.m) and additional operations that allow you to use
function setup(block) % Register the properties of the output port block.OutputPort(1).DimensionsMode = 'Variable'; block.RegBlockMethod('SetInputPortDimensionsMode', @SetInputDimsMode); function DoPostPropSetup(block) %Register dependency rules to update current output size of output port a depending on %input ports b and c block.AddOutputDimsDependencyRules(a, [b c], @setOutputVarDims); %Configure output port b to have the same dimensions as input port a block.InputPortSameDimsAsOutputPort(a,b); %Configure DWork a to have its size reset when input size changes. block.DWorkRequireResetForSignalSize(a,true); function SetInputDimsMode(block, port, dm) % Set dimension mode block.InputPort(port).DimensionsMode = dm; block.OutputPort(port).DimensionsMode = dm; function setOutputVarDims(block, opIdx, inputIdx) % Set current (run-time) dimensions of the output outDimsAfterReset = block.InputPort(inputIdx(1)).CurrentDimensions; block.OutputPort(opIdx).CurrentDimensions = outDimsAfterReset;
Generating Code from a Level-2 MATLAB S-Function
Generating code for a model containing a Level-2 MATLAB S-function requires that you provide a corresponding Target Language Compiler (TLC) file. You do not need a TLC file to accelerate a model containing a Level-2 MATLAB S-function. The Simulink Accelerator™ software runs Level-2 MATLAB S-functions in interpreted mode. However, M-file S-functions do not work with accelerated mode if the M-file S-function is in a model reference. For more information on writing TLC files for MATLAB S-functions, see Inlining S-Functions (Simulink Coder) and Inline MATLAB File S-Functions (Simulink Coder).
MATLAB S-Function Examples
The Level-2 MATLAB S-function
examples provide a set of self-documenting models that illustrate
the use of Level-2 MATLAB S-functions.
the MATLAB command prompt to view the examples.
MATLAB S-Function Limitations
Level-2 MATLAB S-functions do not support zero-crossing detection.
You cannot trigger a function-call subsystem from a Level-2 MATLAB S-function.
Level-2 MATLAB S-Function | S-Function Builder | S-Function | MATLAB Function