Main Content

Maintain Level-1 MATLAB S-Functions

About the Maintenance of Level-1 MATLAB S-Functions


The information provided in this section is intended only for use in maintaining existing Level-1 MATLAB® S-functions. Use the more capable Level-2 API to develop new MATLAB S-functions (see Write Level-2 MATLAB S-Functions). Level-1 MATLAB S-functions support a much smaller subset of the S-function API then Level-2 MATLAB S-functions, and their features are limited compared to built-in blocks.

A Level-1 MATLAB S-function is a MATLAB function of the following form


where f is the name of the S-function. During simulation of a model, the Simulink® engine repeatedly invokes f, using the flag argument to indicate the task (or tasks) to be performed for a particular invocation. The S-function performs the task and returns the results in an output vector.

Level-1 MATLAB S-Function Arguments

The Simulink engine passes the following arguments to a Level-1 MATLAB S-function:

tCurrent time
xState vector
uInput vector
flagInteger value that indicates the task to be performed by the S-function

The following table describes the values that flag can assume and lists the corresponding Level-2 MATLAB S-function method for each value.

Flag Argument

Level-1 FlagLevel-2 Callback MethodDescription
0setupDefines basic S-Function block characteristics, including sample times, initial conditions of continuous and discrete states, and the sizes array (see Define S-Function Block Characteristics for a description of the sizes array).
1mdlDerivativesCalculates the derivatives of the continuous state variables.
2mdlUpdateUpdates discrete states, sample times, and major time step requirements.
3mdlOutputsCalculates the outputs of the S-function.
4mdlOutputs method updates the run-time object NextTimeHit propertyCalculates the time of the next hit in absolute time. This routine is used only when you specify a variable discrete-time sample time in the setup method.
9mdlTerminatePerforms any necessary end-of-simulation tasks.

Level-1 MATLAB S-Function Outputs

A Level-1 MATLAB S-function returns an output vector containing the following elements:

  • sys, a generic return argument. The values returned depend on the flag value. For example, for flag = 3, sys contains the S-function outputs.

  • x0, the initial state values (an empty vector if there are no states in the system). x0 is ignored, except when flag = 0.

  • str, originally intended for future use. Level-1 MATLAB S-functions must set this to the empty matrix, [].

  • ts, a two-column matrix containing the sample times and offsets of the block (see Specify Sample Time in Using Simulink for information on how to specify a sample times and offsets).

    For example, if you want your S-function to run at every time step (continuous sample time), set ts to [0 0]. If you want your S-function to run at the same rate as the block to which it is connected (inherited sample time), set ts to [-1 0]. If you want it to run every 0.25 seconds (discrete sample time) starting at 0.1 seconds after the simulation start time, set ts to [0.25 0.1].

    You can create S-functions that do multiple tasks, each at a different sample rate (i.e., a multirate S-function). In this case, ts should specify all the sample rates used by your S-function in ascending order by sample time. For example, suppose your S-function performs one task every 0.25 second starting from the simulation start time and another task every 1 second starting 0.1 second after the simulation start time. In this case, your S-function should set ts equal to [.25 0; 1.0 .1]. This will cause the Simulink engine to execute the S-function at the following times: [0 0.1 0.25 0.5 0.75 1 1.1 ...]. Your S-function must decide at every sample time which task to perform at that sample time.

    You can also create an S-function that performs some tasks continuously (i.e., at every time step) and others at discrete intervals.

Define S-Function Block Characteristics

For the Simulink engine to recognize a Level-1 MATLAB S-function, you must provide it with specific information about the S-function. This information includes the number of inputs, outputs, states, and other block characteristics.

To provide this information, call the simsizes function at the beginning of the S-function.

sizes = simsizes;

This function returns an uninitialized sizes structure. You must load the sizes structure with information about the S-function. The table below lists the fields of the sizes structure and describes the information contained in each field.

Fields in the sizes Structure

Field NameDescription
sizes.NumContStatesNumber of continuous states
sizes.NumDiscStatesNumber of discrete states
sizes.NumOutputsNumber of outputs
sizes.NumInputsNumber of inputs
sizes.DirFeedthroughFlag for direct feedthrough
sizes.NumSampleTimesNumber of sample times

After you initialize the sizes structure, call simsizes again:

sys = simsizes(sizes);

This passes the information in the sizes structure to sys, a vector that holds the information for use by the Simulink engine.

Processing S-Function Parameters

When invoking a Level-1 MATLAB S-function, the Simulink engine always passes the standard block parameters, t, x, u, and flag, to the S-function as function arguments. The engine can pass additional block-specific parameters specified by the user to the S-function. The user specifies the parameters in the S-function parameters field of the S-Function Block Parameters dialog box (see Passing Parameters to S-Functions). If the block dialog specifies additional parameters, the engine passes the parameters to the S-function as additional function arguments. The additional arguments follow the standard arguments in the S-function argument list in the order in which the corresponding parameters appear in the block dialog. You can use this block-specific S-function parameter capability to allow the same S-function to implement various processing options.

Convert Level-1 MATLAB S-Functions to Level-2

You can convert Level-1 MATLAB S-functions to Level-2 MATLAB S-functions by mapping the code associated with each Level-1 S-function flag to the appropriate Level-2 S-function callback method. See the Flag Arguments table for a mapping of Level-1 flags to Level-2 callback methods. In addition:

  • Store discrete state information for Level-2 MATLAB S-functions in DWork vectors, initialized in the PostPropagationSetup method.

  • Access Level-2 MATLAB S-function dialog parameters using the DialogPrm run-time object property, instead of passing them into the S-function as function arguments.

  • For S-functions with variable sample times, update the NextTimeHit run-time object property in the Outputs method to set the next sample time hit for the Level-2 MATLAB S-function.

For example, the following table shows how to convert the Level-1 MATLAB S-function sfundsc2.m to a Level-2 MATLAB S-function. The example uses the Level-2 MATLAB S-function template msfuntmpl_basic.m as a starting point when converting the Level-1 MATLAB S-function. The line numbers in the table corresponds to the lines of code in sfundsc2.m.


Line NumberCode in sfundsc2.mCode in Level-2 MATLAB file (sfundsc2_level2.m)
function [sys,x0,str,ts]= ...
function sfundsc2(block)
The syntax for the function line changes to accept one input argument block, which is the Level-2 MATLAB S-Function block's run-time object. The main body of the Level-2 MATLAB S-function contains a single line that calls the local setup function.
13 - 19
switch flag,

case 0,
[sys,x0,str,ts] = ...
function setup(block)
The flag value of zero corresponds to calling the setup method. A Level-2 MATLAB S-function does not use a switch statement to invoke the callback methods. Instead, the local setup function registers callback methods that are directly called during simulation.
24 - 31
case 2,
   sys = mdlUpdate(t,x,u);

case 3,
   sys = mdlOutputs(t,x,u);
The setup function registers two local functions associated with flag values of 2 and 3.
block.RegBlockMethod('Outputs' ,@Output);
block.RegBlockMethod('Update'  ,@Update);
53 - 66
sizes = simsizes;

sizes.NumContStates  = 0;
sizes.NumDiscStates  = 1;
sizes.NumOutputs     = 1;
sizes.NumInputs      = 1;
sizes.DirFeedthrough = 0;
sizes.NumSampleTimes = 1;

sys = simsizes(sizes);

x0  = 0;
str = [];
ts  = [.1 0];
The setup function also initializes the attributes of the Level-2 MATLAB S-function:
block.NumInputPorts  = 1;
block.NumOutputPorts = 1;
block.InputPort(1).Dimensions        = 1;
block.InputPort(1).DirectFeedthrough = false;
block.OutputPort(1).Dimensions       = 1;
block.NumDialogPrms     = 0;
block.SampleTimes = [0.1 0];
Because this S-function has discrete states, the setup method registers the PostPropagationSetup callback method to initialize a DWork vector and the InitializeConditions callback method to set the initial state value.
block.RegBlockMethod('InitializeConditions', ...
sizes.NumDiscStates  = 1;
The PostPropagationSetup method initializes the DWork vector that stores the single discrete state.
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;
x0  = 0;
The InitializeConditions method initializes the discrete state value.
function InitConditions(block)

%% Initialize Dwork 
block.Dwork(1).Data = 0
77 - 78
function sys = ...

sys = u;    
The Update method calculates the next value of the discrete state.
function Update(block)
block.Dwork(1).Data = block.InputPort(1).Data;
88 - 89
function sys = ...
sys = x;
The Outputs method calculates the S-function's output.
function Outputs(block)
block.OutputPort(1).Data = block.Dwork(1).Data;