User`s guide

3 Writing S-Functions As C-MEX files

3-56

Examples of C MEX-File S-Function Blocks

Most S-Function blocks require the handling of states, continuous or discrete.

The following sections discuss common types of systems that you can model in

Simulink with S-functions:

• Continuous state

• Discrete state

• Hybrid

• Variable step sample time

• Zero crossings

• Time varying continuous transfer function

All e xamples are based on the C MEX-file S-function template,

sfuntmpl.c,

and sfuntmpl.doc, which contains a discussion of the S-function template.

Example - Continuous State S-Function

The matlabroot/simulink/src/csfunc.c example shows how to model a

continuous system with states in a C MEX S-function. In continuous state

integration, there is a set of states that Simulink’s solvers integrate using the

equations:

S-functions that contain continuous states implement a state-space equation.

The output portion is placed in

mdlOutputs and the derivative portion in

mdlDerivatives. To visualize how the integration works, refer back to the

flowchart on page 3-16. The output equation above corresponds to the

mdlOutputs in the major time step. Next, the example enters the integration

sectionofthe flowchart. Here Simulink performs a number ofminor time steps

(output)

(derivative)

u,,()=

(states)

(input) (output)