Real-Time Workshop

Slower to Faster Transitions in Real Time

In models where a slower block drives a faster block, the generated code assigns the faster block a higher priority than the slower block. This means the faster block is executed before the slower block, which requires special care to avoid incorrect results.

Figure 8-9: Time Overlaps in Slower to Faster Transitions

This timing diagram illustrates two problems:

• Execution of the slower block is split over more than one faster block interval. In this case the faster task executes a second time before the slower task has completed execution. This means the inputs to the slower task can change.
• The faster block executes before the slower block (which is backwards from the way Simulink operates). In this case, the 1 second block executes first; but the inputs to the faster task have not been computed. This can cause unpredictable results.

To eliminate these problems, you must insert a Rate Transition block between the slower and faster blocks..

We assume that the Rate Transition block is used in its default (Protected/Deterministic) mode.

The picture below shows the timing sequence that results with the added Rate Transition block.

Three key points about this diagram:

• The Rate Transition block output runs in the 1 second task, but only at its rate (2 seconds). The output of the Rate Transition block feeds the 1 second task blocks.
• The Rate Transition update uses the output of the 2 second task in its update of its internal state.
• The Rate Transition update uses the state of the Rate Transition in the 1 second task.

The output portion of a Rate Transition block is executed at the sample rate of the slower block, but with the priority of the faster block. Since the Rate Transition block drives the faster block and has effectively the same priority, it is executed before the faster block. This solves the first problem.

The second problem is alleviated because the Rate Transition block executes at a slower rate and its output does not change during the computation of the faster block it is driving.

Note    This use of the Rate Transition block changes the model. The output of the slower block is now delayed by one time step compared to the output without a Rate Transition block.

Slower to Faster Transitions in Simulink

Singletasking and Multitasking Execution of a Model: an Example