Writing S-Functions    

S-Function Examples

Simulink comes with a library of S-function examples.

To run an example:

  1. Enter sfundemos at the MATLAB command line.
  1. MATLAB displays the S-function demo library.

    Each block represents an S-function example.

  1. Click a block to open and run the example that it represents.

It might be helpful to examine some sample S-functions as you read the next chapters. Code for the examples is stored in these subdirectories under the MATLAB root directory:

M-files
 toolbox/simulink/blocks
C, C++, and Fortran
 simulink/src
Ada
 simulink/ada/examples

M-File S-Function Examples

The simulink/blocks directory contains many M-file S-functions. Consider starting off by looking at these files.

Filename
 Description
csfunc.m
 Define a continuous system in state-space format.
dsfunc.m
 Define a discrete system in state-space format.
vsfunc.m
 Illustrates how to create a variable sample time block. This block implements a variable step delay in which the first input is delayed by an amount of time determined by the second input.
mixed.m
 Implement a hybrid system consisting of a continuous integrator in series with a unit delay.
vdpm.m
 Implement the Van der Pol equation (similar to the demo model, vdp).
simom.m
 Example state-space M-file S-function with internal A, B, C, and D matrices. This S-function implements
dx/at = Ax + By
y = Cx + Du
where x is the state vector, u is the input vector, and y is the output vector. The A, B, C, and D matrices are embedded in the M-file.
simom2.m
 Example state-space M-file S-function with external A, B, C, and D matrices. The state-space structure is the same as in simom.m, but the A, B, C, and D matrices are provided externally as parameters to this file.
limintm.m
 Implement a continuous limited integrator where the output is bounded by lower and upper bounds and includes initial conditions.
sfun_varargm.m

Example M-file S-function showing how to use the MATLAB vararg facility.
vlimintm.m

Example of a continuous limited integrator S-function. This illustrates how to use the size entry of -1 to build an S-function that can accommodate a dynamic input/state width.
vdlimintm.m

Example of a discrete limited integrator S-function. This example is identical to vlimint.m, except that the limited integrator is discrete.

C S-Function Examples

The simulink/src directory also contains examples of C MEX S-functions, many of which have an M-file S-function counterpart. These C MEX S-functions are listed in this table.

Filename
 Description
barplot.c

Access simulink signals without using the standard block inputs.
csfunc.c

Example C MEX S-function for defining a continuous system.
dlimint.c
 Implement a discrete-time limited integrator.
dsfunc.c
 Example C MEX S-function for defining a discrete system.
fcncallgen.c
 Execute function-call subsystems n times at the designated rate (sample time).
limintc.c
 Implement a limited integrator.
mixedm.c
 Implement a hybrid dynamic system consisting of a continuous integrator (1/s) in series with a unit delay (1/z).
mixedmex.c
 Implement a hybrid dynamic system with a single output and two inputs.
quantize.c

Example MEX-file for a vectorized quantizer block. Quantizes the input into steps as specified by the quantization interval parameter, q.
resetint.c
 A reset integrator.
sdotproduct
 Compute dot product (multiply-accumulate) of two real or complex vectors.
sftable2.c
 Two-dimensional table lookup in S-function form.
sfun_atol.c
 Set different absolute tolerances for each continuous state.
sfun_bitop.c
 Perform the bitwise operations AND, OR, XOR, left shift, right shift, and one's complement on uint8, uint16, and uint32 inputs.
sfun_cplx.c
 Complex signal add with one input port and one parameter.
sfun_directlook.c
 Direct 1-D lookup.
sfun_dtype_io.c
 Example of the use of Simulink data types for inputs and outputs.
sfun_dtype_param.c
 Example of the use of Simulink data types for parameters.
sfun_dynsize.c
 Simple example of how to size outputs of an S-function dynamically.
sfun_errhdl.c
 Simple example of how to check parameters using the mdlCheckParams S-function routine.
sfun_fcncall.c
 Example of an S-function that is configured to execute function-call subsystems on the first and third output elements.
sfun_frmad.c
 Frame-based A/D converter.
sfun_frmda.c
 Frame-based D/A converter.
sfun_frmdft.c
 Multichannel frame-based Discrete-Fourier transformation (and its inverse).
sfun_frmunbuff.c
 Frame-based unbuffer block.
sfun_multiport.c

S-function that has multiple input and output ports.
sfun_manswitch.c

Manual switch.
sfun_matadd.c

Matrix add with one input port, one output port, and one parameter.
sfun_multirate.c

Demonstrate how to specify port-based sample times.
sfun_psbbreaker.c

Implement the logic for the breaker block in the Power System Blockset.
sfun_psbcontc.c

Continuous implementation of state-space system.
sfun_psbdiscc.c

Discrete implementation of state-space system.
sfun_runtime1.c

Run-time parameter example.
sfun_runtime2.c

Run-time parameter example.
sfun_zc.c
 Demonstrate use of nonsampled zero crossings to implement abs(u). This S-function is designed to be used with a variable-step solver.
sfun_zc_sat.c
 Saturation example that uses zero crossings.
sfunmem.c
 A one-integration-step delay and hold memory function.
simomex.c

Implements a single-output, two-input state-space dynamic system described by these state-space equations 

  • dx/dt = Ax + Bu
y = Cx + Du

where x is the state vector, u is vector of inputs, and y is the vector of outputs.

smatrxcat.c

Matrix concatenation.
sreshape.c

Reshape the input signal.
stspace.c

Implement a set of state-space equations. You can turn this into a new block by using the S-Function block and mask facility. This example MEX-file performs the same function as the built-in State-Space block. This is an example of a MEX-file where the number of inputs, outputs, and states is dependent on the parameters passed in from the workspace. Use this as a template for other MEX-file systems.
stvctf.c

Implement a continuous-time transfer function whose transfer function polynomials are passed in via the input vector. This is useful for continuous time adaptive control applications.
stvdct.f

Implement a discrete-time transfer function whose transfer function polynomials are passed in via the input vector. This is useful for discrete-time adaptive control applications.
stvmgain.c

Time-varying matrix gain.
table3.c

3-D lookup table.
timestwo.c

Basic C MEX S-function that doubles its input.
vdlmint.c
Implement a discrete-time vectorized limited integrator.
vdpmex.c
 Implement the Van der Pol equation.
vlimint.c

Implement a vectorized limited integrator.
vsfunc.c

Illustrate how to create a variable sample time block in Simulink. This block implements a variable-step delay in which the first input is delayed by an amount of time determined by the second input.

Fortran S-Function Examples

Filename
 Description
sfun_timestwo_for.for
 Sample Level 1 Fortran representation of a C timestwo S-function.
sfun_atmos.c
 Calculation of the 1976 standard atmosphere to 86 km.
vdpmexf.for
 Van der Pol system.
The following table lists sample Fortran S-functions.

C++ S-Function Examples

Filename
 Description
sfun_counter_cpp.cpp
 Store a C++ object in the pointers vector PWork.
The following table lists sample C++ S-functions.

Ada S-Function Examples

The simulink/ada/examples directory contains the following examples of S-functions implemented in Ada.

Directory Name
 Description
matrix_gain
 Implement a Matrix Gain block.
multi_port
 Multiport block.
simple_lookup
 Lookup table. Illustrates use of a wrapper S-function that wraps stand-alone Ada code (i.e., Ada packages and procedures) both for use with Simulink as an S-function and directly with Ada code generated using the RTW Ada Coder.
times_two
 Output twice its input.


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