Hierarchical state machines:
-
A state in a top-level FSM can be implemented ("refined") as an internal/embedded state machine
- The top level state = “super-state”
- An internal state inside it = “sub-state”
Problems:
- Which sub-state is entered?
- What transitions are executed and in what order?
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Question:
- Suppose the FSM is in sub-state D of B
- If g1 and g3 are both true, which reacts first? The inner FSM or the outer FSM?
Two solutions:
-
[Statecharts language] Inner FSM reacts first, outer FSM reacts later
- The two reactions are considered simultaneous
- The output actions are required to not conflict
In this example:
- starting from D, inside B
- check inner transition,
$g_3$ is True$\rightarrow$ $a_3$ is executed - check outer transition,
$g_1$ is True$\rightarrow$ $a_1$ is executed - ending state is A
-
[Stateflow, Matlab] Outer FSM reacts first, inner FSM reacts later (if at all)
- If state is left, the inner FSM will not react at all
In this example:
- starting from D, inside B
- check outer transition,
$g_1$ is True$\rightarrow$ $a_1$ is executed - ending state is A
- (action
$a_3$ is not executed)
When entering a super-state, which sub-state is entered?
Two solutions:
-
Enter the last sub-state you were in, when you last left the super-state
- Represented as a history transition (marked with a full black arrow on these schematics / a H sign in Matlab)
-
Enter the default sub-state every time
- Known as a reset transition (marked with a white arrow on these schematics / default behavior in Matlab)
-
Any hierarchical FSM can be "flattened", e.g. converted into an equivalent model with no super-states
- e.g. Super-state A with two substates B and C is split into to substates AB and AC, transitions from A now leaving from both AB and AC
-
Hierarchy in models brings representation efficiency
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