Writing S-Functions

S-Function Callback Methods

An S-function comprises a set of S-function callback methods that perform tasks required at each simulation stage. During simulation of a model, at each simulation stage, Simulink calls the appropriate methods for each S-function block in the model. Tasks performed by S-function methods include:

• Initialization -- Prior to the first simulation loop, Simulink initializes the S-function. During this stage, Simulink:
• Initializes the SimStruct, a simulation structure that contains information about the S-function
• Sets the number and dimensions of input and output ports
• Sets the block sample times
• Allocates storage areas and the sizes array
• Calculation of next sample hit -- If you've created a variable sample time block, this stage calculates the time of the next sample hit; that is, it calculates the next step size.
• Calculation of outputs in the major time step -- After this call is complete, all the output ports of the blocks are valid for the current time step.
• Update of discrete states in the major time step -- In this call, all blocks should perform once-per-time-step activities such as updating discrete states for next time around the simulation loop.
• Integration -- This applies to models with continuous states and/or nonsampled zero crossings. If your S-function has continuous states, Simulink calls the output and derivative portions of your S-function at minor time steps. This is so Simulink can compute the states for your S-function. If your S-function (C MEX only) has nonsampled zero crossings, Simulink calls the output and zero-crossings portions of your S-function at minor time steps so that it can locate the zero crossings.

Note    See "How Simulink Works" in the Using Simulin documentation for an explanation of major and minor time steps.

How S-Functions Work

Implementing S-Functions