Hierarchical state machine concept in Embedded C

Introduction

A hierarchical state machine helps organize complex behaviors by grouping related states inside bigger states. It makes programs easier to understand and manage.

When a device has many modes that share common behaviors.

When you want to avoid repeating code for similar states.

When you need to handle events differently depending on a broader context.

When designing embedded systems like washing machines or traffic lights.

When you want clearer structure in your state management.

Syntax

Embedded C

typedef enum {
    STATE_TOP,
    STATE_A,
    STATE_A1,
    STATE_A2,
    STATE_B
} State;

void handle_event(State *current_state, Event event) {
    switch (*current_state) {
        case STATE_TOP:
            // handle top-level events
            break;
        case STATE_A:
            // handle events common to A and its substates
            break;
        case STATE_A1:
            // handle events specific to A1
            break;
        case STATE_A2:
            // handle events specific to A2
            break;
        case STATE_B:
            // handle events for B
            break;
    }
}

States are organized in a tree-like structure, where some states contain other states.

Events can be handled at different levels, allowing shared behavior.

Examples

Defines states where STATE_A1 and STATE_A2 are inside STATE_A.

Embedded C

typedef enum {
    STATE_TOP,
    STATE_A,
    STATE_A1,
    STATE_A2
} State;

Handles events differently for sub-states A1 and A2.

Embedded C

switch (current_state) {
    case STATE_A1:
        // handle A1 events
        break;
    case STATE_A2:
        // handle A2 events
        break;
}

Sample Program

This program shows a simple hierarchical state machine with states inside STATE_A. It moves between states based on events and prints the current state changes.

Embedded C

#include <stdio.h>

typedef enum {
    STATE_TOP,
    STATE_A,
    STATE_A1,
    STATE_A2,
    STATE_B
} State;

typedef enum {
    EVENT_GO_A1,
    EVENT_GO_A2,
    EVENT_GO_B,
    EVENT_BACK
} Event;

void handle_event(State *current_state, Event event) {
    switch (*current_state) {
        case STATE_TOP:
            if (event == EVENT_GO_A1) {
                *current_state = STATE_A1;
                printf("Moved to STATE_A1\n");
            } else if (event == EVENT_GO_B) {
                *current_state = STATE_B;
                printf("Moved to STATE_B\n");
            } else {
                printf("No transition from STATE_TOP\n");
            }
            break;
        case STATE_A1:
            if (event == EVENT_GO_A2) {
                *current_state = STATE_A2;
                printf("Moved to STATE_A2\n");
            } else if (event == EVENT_BACK) {
                *current_state = STATE_TOP;
                printf("Returned to STATE_TOP\n");
            } else {
                printf("No transition from STATE_A1\n");
            }
            break;
        case STATE_A2:
            if (event == EVENT_BACK) {
                *current_state = STATE_TOP;
                printf("Returned to STATE_TOP\n");
            } else {
                printf("No transition from STATE_A2\n");
            }
            break;
        case STATE_B:
            if (event == EVENT_BACK) {
                *current_state = STATE_TOP;
                printf("Returned to STATE_TOP\n");
            } else {
                printf("No transition from STATE_B\n");
            }
            break;
    }
}

int main() {
    State current_state = STATE_TOP;

    handle_event(&current_state, EVENT_GO_A1); // Move to A1
    handle_event(&current_state, EVENT_GO_A2); // Move to A2
    handle_event(&current_state, EVENT_BACK);  // Back to TOP
    handle_event(&current_state, EVENT_GO_B);  // Move to B
    handle_event(&current_state, EVENT_BACK);  // Back to TOP

    return 0;
}

Important Notes

Hierarchical states reduce repeated code by sharing behavior in parent states.

Transitions can happen between sub-states or back to parent states.

Keep the state tree simple to avoid confusion.

Summary

Hierarchical state machines group states inside bigger states for clarity.

They help manage complex behaviors by sharing common actions.

Use events to move between states and sub-states.

Practice

1. What is the main benefit of using a hierarchical state machine in embedded C programming?

easy

A. It removes the need for any state transitions.

B. It eliminates the need for events to change states.

C. It allows states to run in parallel without interaction.

D. It organizes states into groups, making complex behavior easier to manage.

Solution

1. Step 1: Understand hierarchical state machine purpose

   Hierarchical state machines group related states inside bigger states to simplify complex logic.

2. Step 2: Identify the benefit

   This grouping helps share common actions and manage transitions more clearly.

3. Final Answer:

   It organizes states into groups, making complex behavior easier to manage. -> Option D

4. Quick Check:

   Grouping states = easier management [OK]

Hint: Think of states like folders inside folders for clarity [OK]

Common Mistakes:

• Thinking events are not needed in hierarchical state machines
• Believing states run independently without interaction
• Assuming no transitions are required

2. Which of the following is the correct way to define a sub-state inside a parent state in embedded C code for a hierarchical state machine?

easy

A. typedef enum { STATE_PARENT, STATE_CHILD } State;

B. typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD as a separate enum

C. typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD nested inside STATE_PARENT logically

D. typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD declared inside STATE_PARENT block

Solution

1. Step 1: Understand enum usage for states

   In embedded C, states are often defined as enums. Logical nesting is done by naming and design, not by code nesting.

2. Step 2: Identify correct sub-state definition

   typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD nested inside STATE_PARENT logically correctly states that STATE_CHILD is logically nested inside STATE_PARENT, even though enums are flat.

3. Final Answer:

   typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD nested inside STATE_PARENT logically -> Option C

4. Quick Check:

   Logical nesting, not code nesting [OK]

Hint: Enums are flat; nesting is logical, not syntax [OK]

Common Mistakes:

• Trying to nest enums inside enums (not allowed in C)
• Assuming code blocks define sub-states
• Declaring sub-states as separate enums without relation

3. Given the following code snippet for a hierarchical state machine, what will be the output when event EVENT_START occurs in STATE_IDLE?

#define EVENT_START 1
typedef enum { STATE_IDLE, STATE_ACTIVE, STATE_ACTIVE_SUB } State;
State current_state = STATE_IDLE;

void handle_event(int event) {
  switch (current_state) {
    case STATE_IDLE:
      if (event == EVENT_START) {
        current_state = STATE_ACTIVE;
        printf("Entered ACTIVE state\n");
      }
      break;
    case STATE_ACTIVE:
      if (event == EVENT_START) {
        current_state = STATE_ACTIVE_SUB;
        printf("Entered ACTIVE_SUB state\n");
      }
      break;
    default:
      printf("No action\n");
  }
}

handle_event(EVENT_START);

medium

A. No action

B. Entered ACTIVE state

C. Entered ACTIVE_SUB state

D. Compilation error

Solution

1. Step 1: Check initial state and event

   The current_state is STATE_IDLE. The event is EVENT_START.

2. Step 2: Follow switch-case logic for STATE_IDLE

   On EVENT_START in STATE_IDLE, current_state changes to STATE_ACTIVE and prints "Entered ACTIVE state".

3. Final Answer:

   Entered ACTIVE state -> Option B

4. Quick Check:

   STATE_IDLE + EVENT_START = Entered ACTIVE state [OK]

Hint: Trace current state before event handling [OK]

Common Mistakes:

• Assuming it jumps directly to sub-state
• Ignoring initial state value
• Confusing event handling in different states

4. Identify the error in this hierarchical state machine event handler code snippet:

#define EVENT_GO 1
typedef enum { STATE_TOP, STATE_SUB } State;
State current_state = STATE_TOP;

void handle_event(int event) {
  switch (current_state) {
    case STATE_TOP:
      if (event = EVENT_GO) {
        current_state = STATE_SUB;
        printf("Moved to SUB state\n");
      }
      break;
    case STATE_SUB:
      printf("In SUB state\n");
      break;
  }
}

medium

A. Using assignment (=) instead of comparison (==) in if condition

B. Missing default case in switch statement

C. Incorrect enum values for states

D. No return statement in function

Solution

1. Step 1: Examine if condition in STATE_TOP case

   The condition uses single '=' which assigns EVENT_GO to event instead of comparing.

2. Step 2: Understand impact of assignment in if

   This causes the condition to always be true if EVENT_GO is non-zero, leading to unintended behavior.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in if condition -> Option A

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Check if conditions for == vs = errors [OK]

Common Mistakes:

• Confusing assignment and comparison operators
• Ignoring missing default case (not critical here)
• Expecting return in void function

5. You want to design a hierarchical state machine in embedded C where STATE_RUNNING has two sub-states: STATE_RUNNING_FAST and STATE_RUNNING_SLOW. Which approach correctly models this hierarchy and allows shared entry actions for STATE_RUNNING and specific actions for its sub-states?

hard

A. Define separate enums for all states and use nested switch-case to handle parent and sub-states with shared and specific actions.

B. Use a single enum with flat states and duplicate code for shared actions in each sub-state case.

C. Use multiple variables to track parent and sub-state separately without nesting logic.

D. Implement sub-states as independent states without any relation to the parent state.

Solution

1. Step 1: Understand hierarchical state machine design

   Parent states group sub-states to share common behavior and allow specific sub-state actions.

2. Step 2: Identify correct coding approach

   Defining separate enums for parent and sub-states and using nested switch-case allows shared entry actions for parent and specific actions for sub-states efficiently.

3. Final Answer:

   Define separate enums for all states and use nested switch-case to handle parent and sub-states with shared and specific actions. -> Option A

4. Quick Check:

   Nested switch-case models hierarchy best [OK]

Hint: Use nested switch-case for parent and sub-state actions [OK]

Common Mistakes:

• Duplicating shared actions in sub-states
• Tracking states separately without hierarchy
• Ignoring parent-child relationship in code