Hierarchical state machine concept in Embedded C - Step-by-Step Execution

Concept Flow - Hierarchical state machine concept

Start

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Top State

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Substate A

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Sub-substate A1

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Sub-substate A2

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Substate B

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Sub-substate B1

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Sub-substate B2

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Event Occurs

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Check Transitions

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Change State Hierarchically

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Execute Entry/Exit Actions

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Wait for Next Event

The hierarchical state machine starts at a top state, which contains nested substates. When an event occurs, transitions are checked from the current substate up through parent states, changing states and running entry/exit actions accordingly.

Execution Sample

Embedded C

typedef enum {TOP, A, A1, A2, B, B1, B2} State;
State current_state = TOP;

void event_handler(Event e) {
  switch(current_state) {
    case A1: if(e == EVT_X) current_state = B1; break;
  }
}

This code shows a simple hierarchical state machine with states and a transition from substate A1 to B1 on event EVT_X.

Execution Table

Step	Current State	Event	Transition Condition	New State	Entry/Exit Actions
1	TOP	None	No event	TOP	None
2	TOP	Init	Enter initial substate A	A	Enter A
3	A	Init	Enter initial sub-substate A1	A1	Enter A1
4	A1	EVT_X	If EVT_X in A1, go to B1	B1	Exit A1, Exit A, Enter B, Enter B1
5	B1	None	No event	B1	None
6	B1	EVT_Y	No transition	B1	None
7	B1	EVT_Z	Transition to B2	B2	Exit B1, Enter B2
8	B2	None	No event	B2	None
9	B2	EVT_STOP	Transition to TOP	TOP	Exit B2, Exit B, Enter TOP
10	TOP	None	No event	TOP	None

💡 Execution stops when no more events or transitions occur.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 7	After Step 9	Final
current_state	TOP	A	A1	B1	B2	TOP	TOP

Key Moments - 3 Insights

Why do we exit both A1 and A when transitioning from A1 to B1?

Because B1 is under B, a different branch from A, we must exit the current substate A1 and its parent A before entering B and B1, as shown in step 4 of the execution_table.

What happens if an event has no matching transition in the current state?

The state machine stays in the current state without changes or actions, as seen in step 6 where EVT_Y does not cause a transition from B1.

How does the machine know which substate to enter initially?

On entering a parent state, the machine enters its initial substate automatically, like entering A then A1 in steps 2 and 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the current_state after step 4?

A B1

B A1

C A

D TOP

💡 Hint

Check the 'New State' column at step 4 in the execution_table.

At which step does the state machine exit state B1?

A Step 6

B Step 9

C Step 7

D Step 4

💡 Hint

Look for 'Exit B1' in the Entry/Exit Actions column in the execution_table.

If the event EVT_X did not cause a transition in A1, what would happen to current_state after step 4?

A It would change to B1 anyway

B It would remain A1

C It would change to TOP

D It would change to A

💡 Hint

Refer to the key_moments about no matching transition and step 6 where no transition keeps the state unchanged.

Concept Snapshot

Hierarchical State Machine (HSM):
- States can contain nested substates.
- Events trigger transitions checked from current substate up.
- Exiting a state exits all nested substates.
- Entering a state enters its initial substate.
- Entry/Exit actions run on state changes.

Full Transcript

A hierarchical state machine organizes states in a tree structure with parent and nested substates. The machine starts at a top state and enters initial substates automatically. When an event occurs, it checks for transitions starting from the current substate up through parent states. If a transition is found, it exits the current substate and any parent states not shared with the target, then enters the target state and its initial substates, running entry and exit actions accordingly. If no transition matches, the state remains unchanged. This structure helps manage complex behaviors by grouping related states and transitions.