User`s guide

Introduction
1-7
Simulink makes repeated calls to S-functions in your model. During these
calls, Simulink calls S-function routines (also called methods), which perform
tasks required at e ach stage. These tasks include:
• Initialization — Prior to the first simulation loop, Simulink initializes the
S-function. During this s ta ge, Simulink:
- Initializes the
SimStruct, a simulation structure that contains
information about the S-function.
- Sets the number and size of input and output ports.
- Sets the block sample time(s).
- Allocates storage areas and the
sizes array.
• Calculation of next sample hit — If you’ve selected a variable step
integration routine, this stage calculates the time of the next variable hit,
that is, it calculates the next stepsize.
• Calculation of outputs in the major time step — After this call is complete,
all the output ports of the blo cks are valid for the current time step.
• Updatediscretestates inthe majortime step— In thiscall, allblocks should
perform once-per-time-step activities such a s updating discrete states for
next ti me around the simula tion loop.
• Integration — This applies to models with continuous states and/or
nonsampled zero crossings. If your S-function has continuous states,
Simulink calls theout put and derivative portions ofyourS-functio n at minor
time steps. This is so Simulink can compute the state(s) for your S-function.
If your S-function (C MEX only) ha s nonsampled zero crossings, t hen
Simulink will call the output and zero crossings portion ofyour S-function at
minor time steps, s o that it can locate the zero crossings.
Overview of M-File and C MEX S-Functions
In M-file S-functions, the S-function routines are implemented as M-file
subfunctions. In C MEX S-functions, they are implemented as C functions. All
the S-function routines available to M-file S-functions exist for C MEX
S-functions as well. However, Simulink provides a larger set of S-function
routines for C MEX S-functions. If an S-function routine exists for both M-file
and C MEX S-functions, its name is the same for both.
For an M-file S-function, Simulink passes a
flag parameter to the S-funct ion.
The
flag indicates the current simulation stage. You must write M-code that