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.
When the Simulink engine invokes a Level-2 MATLAB S-function callback method,
it passes an instance of the Simulink.MSFcnRunTimeBlock class
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
as the 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 Simulink.MSFcnRunTimeBlock class
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
the variable 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);
where the speye command forms a sparse identity
matrix.
Note
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.
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 msfuntmpl.m.
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.
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:
A setup function to initialize
the basic S-function characteristics
An Outputs function 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.
setup MethodThe 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
to the mdlInitializeSizes and mdlInitializeSampleTimes callback methods
implemented by C MEX S-functions. The setup method
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 RegBlockMethod method of the S-Function
block's run-time object. See the documentation for Simulink.MSFcnRunTimeBlock for information
on using the RegBlockMethod method.
The following steps illustrate how to write a simple Level-2 MATLAB S-function. When applicable,
the steps include examples from the S-function example 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.
Copy the Level-2 MATLAB S-function
template msfuntmpl_basic.m to
your working folder. If you change the file name when you copy the
file, change the function name in the function line
to the same name.
Modify the setup method to initialize
the S-function's attributes. For this example:
Set the run-time object's NumInputPorts and NumOutputPorts properties
to 1 in 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.
Set the DirectFeedthrough property
of the run-time object's InputPort to false in
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 Dimensions, DatatypeID,
and Complexity properties override the values inherited
using the SetPreCompInpPortInfoToDynamic and SetPreCompOutPortInfoToDynamic methods.
Set the run-time object's NumDialogPrms property
to 1 in 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 SampleTimes property
to [-1 0].
Call the run-time object's RegBlockMethod method
to register the following four callback methods used in this S-function.
PostPropagationSetup
InitializeConditions
Outputs
Update
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.
The following setup method from msfcn_unit_delay.m performs
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 setup method using
the run-time object's NumContStates property. Do
not initialize discrete states in the setup method.
Initialize the discrete states in the PostPropagationSetup method.
A Level-2 MATLAB S-function
stores discrete state information in a DWork vector. The default PostPropagationSetup method
in the template file suffices for this example.
The following PostPropagationSetup method
from msfcn_unit_delay.m, named DoPostPropSetup,
initializes one DWork vector with the name x0.
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 PostPropagationSetup method, 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 InitializeConditions or Start callback
methods. Use the Start callback method for values
that are initialized once at the beginning of the simulation. Use
the InitializeConditions method for values that
need to be reinitialized whenever an enabled subsystem containing
the S-function is reenabled.
For this example, use the InitializeConditions method
to set the discrete state's initial condition to the value of the
S-function's dialog parameter. For example, the InitializeConditions method
in msfcn_unit_delay.m is:
function InitConditions(block) %% Initialize Dwork block.Dwork(1).Data = block.DialogPrm(1).Data;
For S-functions with continuous states, use the ContStates run-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 Outputs callback
method. For this example, set the output to the current value of the
discrete state stored in the DWork vector.
The Outputs method in msfcn_unit_delay.m is:
function Output(block) block.OutputPort(1).Data = block.Dwork(1).Data;
For an S-function with continuous states, calculate
the state derivatives in the Derivatives callback
method. Run-time objects store derivative data in their Derivatives property.
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 Derivatives callback method.
Update any discrete states in the Update callback
method. For this example, set the value of the discrete state to the
current value of the first input signal.
The Update method in msfcn_unit_delay.m is:
function Update(block) block.Dwork(1).Data = block.InputPort(1).Data;
Perform any cleanup, such as clearing variables or
memory, in the Terminate method. Unlike C MEX S-functions,
Level-2 MATLAB S-function
are not required to have a Terminate method.
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
parent class Simulink.RunTimeBlock.
To use a Level-2 MATLAB S-function in a model, copy an instance of the Level-2 MATLAB S-Functionblock 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.
Following are modifications to the Level-2 MATLAB S-functions template (msfuntmpl_basic.m)
and additional operations that allow you to use variable-size signals.
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 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).
The Level-2 MATLAB S-function
examples provide a set of self-documenting models that illustrate
the use of Level-2 MATLAB S-functions.
Enter sfundemos at
the MATLAB command prompt to view the examples.
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 | MATLAB Function | S-Function | S-Function Builder