Writing S-Functions

Synchronizing Multirate S-Function Blocks

If tasks running at different rates need to share data, you must ensure that data generated by one task is valid when accessed by another task running at a different rate. You can use the ssIsSpecialSampleHit macro in the mdlUpdate or mdlOutputs routine of a multirate S-function to ensure that the shared data is valid. This macro returns true if a sample hit has occurred at one rate and a sample hit has also occurred at another rate in the same time step. It thus permits a higher rate task to provide data needed by a slower rate task at a rate the slower task can accommodate.

Suppose, for example, that your model has an input port operating at one rate, 0, and an output port operating at a slower rate, 1. Further, suppose that you want the output port to output the value currently on the input. The following example illustrates usage of this macro.

• if (ssISampleHit(S, 0, tid) {
    if (ssIsSpecialSampleHit(S, 0, 1, tid) {
       /* Transfer input to output memory. */
       ...
    }
  }
  
  if (ssIsSampleHit(S, 1, tid) {
     /* Emit output. */
     ...
  }
  

In this example, the first block runs when a sample hit occurs at the input rate. If the hit also occurs at the output rate, the block transfers the input to the output memory. The second block runs when a sample hit occurs at the output rate. It transfers the output in its memory area to the block's output.

Note that higher-rate tasks always run before slower-rate tasks. Thus, the input task in the preceding example always runs before the output task, ensuring that valid data is always present at the output port.

Multirate S-Function Blocks

Work Vectors