Why does the Simulink Accelerator 6.0 (R14) produce incorrect results when using a C MEX S-function with variable step timing in a Simulink model?

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MathWorks Support Team on 27 Jun 2009

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I am using the following callback in my S-function

mdlGetTimeOfNextVarHit

and I have set the Sample Time of my S-function as follows

static void mdlInitializeSampleTimes(SimStruct *S)
{
            if(ssGetSimMode(S) == SS_SIMMODE_RTWGEN)
            {
ssSetSampleTime(S, 0, INHERITED_SAMPLE_TIME);
ssSetOffsetTime(S, 0, 0.0);
            }
            else
            {
ssSetSampleTime(S, 0, VARIABLE_SAMPLE_TIME);
ssSetOffsetTime(S, 0, 0.0);
             }
}

The output of my S-function is always zero when I use the Simulink Accelerator.

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Answer 1

MathWorks Support Team on 27 Jun 2009

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This is expected behavior when the sample time is set to inherited: S-functions will only get a Variable Sample time if the sample time is set explicitly and never if the sample time is set to be inherited. When the S-function's sample time is inherited, the mdlGetTimeOfNextVarHit is not called and therefore, the S-function produces unexpected results.

Here are a few suggestions for writing variable step S-functions:

1. Set the sample time of the S-function to variable, i.e.,

ssSetSampleTime(S, 0, VARIABLE_SAMPLE_TIME);

2. Instead of using the ssGetSimMode check, check for a variable step solver using the ssIsVariableStepSolver macro, e.g.,

static void mdlInitializeSampleTimes(SimStruct *S)
{
      if(ssIsVariableStepSolver(S))
     {
         ssSetSampleTime(S, 0, VARIABLE_SAMPLE_TIME);
     }
     else
     {
        ssSetSampleTime(S, 0, INHERITED_SAMPLE_TIME);  
     }
     ssSetOffsetTime(S, 0, 0.0);
}

The code generation that works with variable step solvers (Simulink Accelerator and RSim Target) will work with variable sample time S-functions if the S-function sets the sample time explicitly. Therefore, be sure that the sample time of your S-function is not based solely on the results of the ssGetSimMode macro, which does not query the solver being used for simulation.