Stateflow

What Is a Finite State Machine?

A finite state machine is a representation of an event-driven (reactive) system. In an event-driven system, the system makes a transition from one state (mode) to another prescribed state, provided that the condition defining the change is true.

For example, you can use a state machine to represent a car's automatic transmission. The transmission has a number of operating states: park, reverse, neutral, drive, and low. As the driver shifts from one position to another the system makes a transition from one state to another, for example, from park to reverse.

Finite State Machine Representations

Traditionally, designers used truth tables to represent relationships among the inputs, outputs, and states of a finite state machine. The resulting table describes the logic necessary to control the behavior of the system under study. Another approach to designing event-driven systems is to model the behavior of the system by describing it in terms of transitions among states. The state that is active is determined based on the occurrence of events under certain conditions. State-transition diagrams and bubble diagrams are graphical representations based on this approach.

Stateflow Representations

Stateflow uses a variant of the finite state machine notation established by Harel [1]. Using Stateflow, you create Stateflow diagrams. A Stateflow diagram is a graphical representation of a finite state machine, where states and transitions form the basic building blocks of the system. You can also represent flow (stateless) diagrams using Stateflow. Stateflow provides a block that you include in a Simulink model. The collection of Stateflow blocks in a Simulink model is the Stateflow machine.

Additionally, Stateflow enables the representation of hierarchy, parallelism, and history. Hierarchy enables you to organize complex systems by defining a parent/offspring object structure. For example, you can organize states within other higher-level states. A system with parallelism can have two or more orthogonal states active at the same time. History provides the means to specify the destination state of a transition based on historical information. These characteristics enhance the usefulness of this approach and go beyond what state-transition diagrams and bubble diagrams provide.

Notation

Notation defines a set of objects and the rules that govern the relationships between those objects. Stateflow notation provides a common language to communicate the design information conveyed by a Stateflow diagram.

Stateflow notation consists of the following:

• A set of graphical objects
• A set of nongraphical text-based objects
• Defined relationships between those objects

See Stateflow Notation for detailed information on Stateflow notations.

Semantics

Semantics describe how the notation is interpreted and implemented. A completed Stateflow diagram illustrates how the system will behave. A Stateflow diagram contains actions associated with transitions and states. The semantics describe in what sequence these actions take place during Stateflow diagram execution.

Knowledge of the semantics is important to make sound Stateflow diagram design decisions for code generation. Different use of notations results in different ordering of simulation and generated code execution.

The default semantics provided with the product are described in Stateflow Semantics.

References

For more information on finite state machine theory, consult these sources:

[1] Harel, David, "Statecharts: A Visual Formalism for Complex Systems," Science of Computer Programming 8, 1987, pages 231-274.

[2] Hatley, Derek J., and Imtiaz A. Pirbhai, Strategies for Real-Time System Specification, Dorset House Publishing Co., Inc., NY, 1988.

Finite State Machine Concepts

Stateflow and Simulink