Using the ssSetTNext S-function macro outside of mdlGetTimeOfNextVarHit method

Question

The Simulink documentation for ssSetTNext says that: "A discrete S-function with a variable sample time should use this macro in mdlGetTimeOfNextVarHit to specify the time of the next sample hit".

Now, I'd like to compute the next sample hit for the S-function after the execution of mdlOutputs. However, according to Simulink Engine Interaction with C S-Functions, mdlGetTimeOfNextVarHit is executed before mdlOutputs (see the picture describing the Simulation Loop).

On the other hand, the documentation for mdlUpdate (unfortunately, I'm not allowed to post the link) says that: "The method can also perform any other tasks that the S-function needs to perform at each major time step".

Since mdlUpdate is executed after mdlOutputs, I was wondering if it's safe to ignore the method mdlGetTimeOfNextVarHit and put all the logics defining the next sample hit (and also use ssSetTNext) inside mdlUpdate.

Thank you in advance.

--Matteo

Answer 1

In short, yes.

It is possible to set the variable-rate task's next hit time outside of the mdlGetTimeOfNextVarHit() method. The next sample instant simply must be set before the end of the present simulation loop: it may therefore be set either in the mdlOutputs() or mdlUpdate() methods.

Remember that C-MEX S-Functions configured to use a variable timestep task still require:

• that MDL_GET_TIME_OF_NEXT_VAR_HIT be defined, and
• an implementation/definition of mdlGetTimeOfNextVarHit().

This means that if you wish to set your new sample instant elsewhere, you still need a (potentially empty) definition for mdlGetTimeOfNextVarHit().

Gotcha's:

If MDL_GET_TIME_OF_NEXT_VAR_HIT is not defined for a C-MEX S-Function configured to use a variable timestep task and there is no definition for mdlGetTimeOfNextVarHit(); compilation will succeed, but upon invocation the following error diagnostic will be reported, and the simulation will terminate:

To have a variable sample time, define MDL_GET_TIME_OF_NEXT_VAR_HIT and add a mdlGetTimeOfNextVarHit routine.

Lastly, if MDL_GET_TIME_OF_NEXT_VAR_HIT is not defined but there is a definition of mdlGetTimeOfNextVarHit(), compilation will fail.