Writing S-Functions

Example - Hybrid System S-Functions

The S-function, matlabroot/simulink/src/mixedm.c, is an example of a hybrid (a combination of continuous and discrete states) system. mixedm.c combines elements of csfunc.c and dsfunc.c. If you have a hybrid system, place your continuous equations in mdlDerivative and your discrete equations in mdlUpdate. In addition, you need to check for sample hits to determine at what point your S-function is being called.

In Simulink block diagram form, the S-function, mixedm.c looks like

which implements a continuous integrator followed by a discrete unit delay.

Since there are no tasks to complete at termination, mdlTerminate is an empty function. mdlDerivatives calculates the derivatives of the continuous states of the state vector x, and mdlUpdate contains the equations used to update the discrete state vector, x.

matlabroot/simulink/src/mixedm.c

/*  File    : mixedm.c
 *  Abstract:
 *
 *      An example S-function illustrating multiple sample times by implementing
 *         integrator -> ZOH(Ts=1second) -> UnitDelay(Ts=1second) 
 *      with an initial condition of 1.
 *  (e.g. an integrator followed by unit delay operation).
 *
 *      For more details about S-functions, see simulink/src/sfuntmpl_doc.c
 *
 *  Copyright 1990-2000 The MathWorks, Inc.
 *  $Revision: 1.1 $
 */

#define S_FUNCTION_NAME mixedm
#define S_FUNCTION_LEVEL 2

#include "simstruc.h"

#define U(element) (*uPtrs[element])  /* Pointer to Input Port0 */

/*====================*
 * S-function methods *
 *====================*/

/* Function: mdlInitializeSizes ===============================================
 * Abstract:
 *    The sizes information is used by Simulink to determine the S-function
 *    block's characteristics (number of inputs, outputs, states, etc.).
 */
static void mdlInitializeSizes(SimStruct *S)
{
    ssSetNumSFcnParams(S, 0);  /* Number of expected parameters */
    if (ssGetNumSFcnParams(S) != ssGetSFcnParamsCount(S)) {
        return; /* Parameter mismatch will be reported by Simulink */
    }

    ssSetNumContStates(S, 1);
    ssSetNumDiscStates(S, 1);
    ssSetNumRWork(S, 1);  /* for zoh output feeding the delay operator */

    if (!ssSetNumInputPorts(S, 1)) return;
    ssSetInputPortWidth(S, 0, 1);
    ssSetInputPortDirectFeedThrough(S, 0, 1);
    ssSetInputPortSampleTime(S, 0, CONTINUOUS_SAMPLE_TIME);
    ssSetInputPortOffsetTime(S, 0, 0.0);

    if (!ssSetNumOutputPorts(S, 1)) return;
    ssSetOutputPortWidth(S, 0, 1);
    ssSetOutputPortSampleTime(S, 0, 1.0);
    ssSetOutputPortOffsetTime(S, 0, 0.0);

    ssSetNumSampleTimes(S, 2);

    /* Take care when specifying exception free code - see sfuntmpl_doc.c. */
    ssSetOptions(S, (SS_OPTION_EXCEPTION_FREE_CODE |
                     SS_OPTION_PORT_SAMPLE_TIMES_ASSIGNED));

} /* end mdlInitializeSizes */



/* Function: mdlInitializeSampleTimes =========================================
 * Abstract:
 *    Two tasks: One continuous, one with discrete sample time of 1.0.
 */
static void mdlInitializeSampleTimes(SimStruct *S)
{
    ssSetSampleTime(S, 0, CONTINUOUS_SAMPLE_TIME);
    ssSetOffsetTime(S, 0, 0.0);

    ssSetSampleTime(S, 1, 1.0);
    ssSetOffsetTime(S, 1, 0.0);

} /* end mdlInitializeSampleTimes */



#define MDL_INITIALIZE_CONDITIONS
/* Function: mdlInitializeConditions ==========================================
 * Abstract:
 *    Initialize both continuous states to one.
 */
static void mdlInitializeConditions(SimStruct *S)
{
    real_T *xC0 = ssGetContStates(S);
    real_T *xD0 = ssGetRealDiscStates(S);

    xC0[0] = 1.0;
    xD0[0] = 1.0;

} /* end mdlInitializeConditions */



/* Function: mdlOutputs =======================================================
 * Abstract:
 *      y = xD, and update the zoh internal output.
 */
static void mdlOutputs(SimStruct *S, int_T tid)
{
    /* update the internal "zoh" output */
    if (ssIsContinuousTask(S, tid)) {
        if (ssIsSpecialSampleHit(S, 1, 0, tid)) {
            real_T *zoh = ssGetRWork(S);
            real_T *xC  = ssGetContStates(S);
            *zoh = *xC;
        }
    }

    /* y=xD */
    if (ssIsSampleHit(S, 1, tid)) {
        real_T *y   = ssGetOutputPortRealSignal(S,0);
        real_T *xD  = ssGetRealDiscStates(S);
        y[0]=xD[0];
    }


} /* end mdlOutputs */



#define MDL_UPDATE
/* Function: mdlUpdate ======================================================
 * Abstract:
 *      xD = xC
 */
static void mdlUpdate(SimStruct *S, int_T tid)
{
    UNUSED_ARG(tid); /* not used in single tasking mode */

    /* xD=xC */
    if (ssIsSampleHit(S, 1, tid)) {
        real_T *xD = ssGetRealDiscStates(S);
        real_T *zoh = ssGetRWork(S);
        xD[0]=*zoh;
    }

} /* end mdlUpdate */



#define MDL_DERIVATIVES
/* Function: mdlDerivatives =================================================
 * Abstract:
 *      xdot = U
 */
static void mdlDerivatives(SimStruct *S)
{
    real_T            *dx   = ssGetdX(S);
    InputRealPtrsType uPtrs = ssGetInputPortRealSignalPtrs(S,0);

    /* xdot=U */
    dx[0]=U(0);

} /* end mdlDerivatives */



/* Function: mdlTerminate =====================================================
 * Abstract:
 *    No termination needed, but we are required to have this routine.
 */
static void mdlTerminate(SimStruct *S)
{
    UNUSED_ARG(S); /* unused input argument */
}

#ifdef  MATLAB_MEX_FILE    /* Is this file being compiled as a MEX-file? */
#include "simulink.c"      /* MEX-file interface mechanism */
#else
#include "cg_sfun.h"       /* Code generation registration function */
#endif

Example - Discrete State S-Function

Example - Variable Step S-Function