Entry, during, and exit actions in Simulink - Mini Project: Build & Apply

Entry, During, and Exit Actions in Simulink Stateflow

📖 Scenario: Imagine you are designing a simple traffic light controller using Simulink Stateflow. The traffic light has three states: Red, Green, and Yellow. Each state has specific actions that happen when the state starts (entry), while the state is active (during), and when the state ends (exit).

🎯 Goal: You will create a Stateflow chart with three states (Red, Green, Yellow) and add entry, during, and exit actions to each state. Then, you will simulate the chart and observe the printed messages that show these actions.

📋 What You'll Learn

Create a Stateflow chart with three states named Red, Green, and Yellow.

Add entry, during, and exit actions to each state that print messages.

Configure transitions between states to cycle through Red -> Green -> Yellow -> Red.

Simulate the chart and observe the printed output showing entry, during, and exit actions.

💡 Why This Matters

🌍 Real World

Traffic light controllers use state machines to manage light changes safely and predictably. Entry, during, and exit actions help perform tasks like turning lights on/off and timing.

💼 Career

Understanding state machines and their actions is essential for control system engineers, embedded system developers, and anyone working with Simulink Stateflow for automation and control.

1

Create the Stateflow chart with three states

Create a Stateflow chart named TrafficLight with three states named Red, Green, and Yellow. Do not add any actions yet.

Simulink

% Create a Stateflow chart named TrafficLight
% Add three states: Red, Green, Yellow
% Your code here

Hint

Use sfnew('TrafficLight') to create the chart and State(sfChart) to add states. Set the Name property for each state.

2

Add entry, during, and exit actions to each state

Add entry, during, and exit actions to each state (Red, Green, Yellow) that print messages like disp('Entering Red'), disp('During Red'), and disp('Exiting Red') respectively.

Simulink

% Add entry, during, and exit actions to each state
% Your code here

Hint

Set the Entry, During, and Exit properties of each state to disp commands that print messages.

3

Add transitions to cycle through the states

Add transitions to cycle through the states in this order: Red to Green, Green to Yellow, and Yellow to Red. Use a simple condition true for each transition to allow automatic cycling.

Simulink

% Add transitions to cycle Red -> Green -> Yellow -> Red
% Your code here

Hint

Create Transition objects between states and set their Condition property to 'true' to enable automatic cycling.

4

Simulate the chart and display the output

Simulate the TrafficLight Stateflow chart for a few steps and print the output messages showing the entry, during, and exit actions for each state.

Simulink

% Simulate the TrafficLight chart for 3 steps
% Your code here

Hint

Use a for loop to simulate the chart multiple times and observe the printed messages.

Practice

1. In Simulink Stateflow, what is the purpose of an entry action in a state?

easy

A. To execute code continuously while the state is active

B. To execute code once when the state is entered

C. To execute code once when the state is exited

D. To define transitions between states

Solution

1. Step 1: Understand what entry action means

   An entry action runs only once when the state becomes active, right at the moment of entry.

2. Step 2: Compare with other actions

   During actions run continuously while in the state, exit actions run once when leaving the state, and transitions define movement between states.

3. Final Answer:

   To execute code once when the state is entered -> Option B

4. Quick Check:

   Entry action = runs once on entering state [OK]

Hint: Entry actions run once at state start, not continuously [OK]

Common Mistakes:

• Confusing entry with during actions
• Thinking entry runs multiple times
• Mixing entry with exit actions

2. Which of the following is the correct syntax to define an exit action in a Simulink Stateflow state named Idle?

easy

A. exit() { disp('Exiting Idle'); }

B. on exit: disp('Exiting Idle');

C. exit: disp('Exiting Idle');

D. exit_action: disp('Exiting Idle');

Solution

1. Step 1: Recall exit action syntax in Stateflow

   Exit actions are defined with the keyword exit: followed by the action code.

2. Step 2: Check each option

   exit: disp('Exiting Idle'); uses correct exit: syntax. on exit: disp('Exiting Idle'); uses incorrect on exit:. exit() { disp('Exiting Idle'); } uses function style which is invalid. exit_action: disp('Exiting Idle'); uses exit_action: which is not recognized.

3. Final Answer:

   exit: disp('Exiting Idle'); -> Option C

4. Quick Check:

   Exit action syntax = exit: code [OK]

Hint: Exit actions start with 'exit:' keyword followed by code [OK]

Common Mistakes:

• Using 'on exit:' instead of 'exit:'
• Trying to define exit as a function
• Using incorrect keywords like 'exit_action:'

3. Consider the following Stateflow state Processing with these actions:

entry: count = 0;
during: count = count + 1;
exit: disp(count);

What will be displayed when the state Processing is exited after being active for 3 simulation steps?

medium

A. 3

B. 1

C. 0

D. 4

Solution

1. Step 1: Track variable changes during state

   On entry, count is set to 0. During each step, it increments by 1. After 3 steps, count = 3.

2. Step 2: Determine exit action output

   On exit, disp(count) prints the current count, which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Count increments 3 times, exit prints 3 [OK]

Hint: Count increments during each step, exit prints final count [OK]

Common Mistakes:

• Forgetting entry sets count to 0
• Assuming exit prints 0 or 1
• Counting one extra increment

4. You have this Stateflow state code:

entry: x = 5;
during: x = x + 2;
exit: disp(x);

But when you run the simulation, disp(x) shows 5 instead of increasing values. What is the likely error?

medium

A. The during action is not executing because the state is not active long enough

B. The entry action resets x every step

C. The exit action is placed incorrectly inside the during block

D. The variable x is not declared

Solution

1. Step 1: Analyze why during action might not run

   If the state is active only one step or immediately exited, the during action does not get a chance to increment x.

2. Step 2: Check other options for correctness

   Entry runs once on entry, not every step. Exit action placement is correct. Variable declaration is not required to see this behavior.

3. Final Answer:

   The during action is not executing because the state is not active long enough -> Option A

4. Quick Check:

   During runs only while state active [OK]

Hint: During runs only if state stays active multiple steps [OK]

Common Mistakes:

• Thinking entry runs every step
• Misplacing exit action inside during
• Assuming variable declaration causes issue

5. You want to count how many times a state Active is entered during a simulation. Which approach correctly uses entry, during, and exit actions to achieve this?

hard

A. Use entry: count = 0; to reset count every time state is entered

B. Use during: count = count + 1; to increment count continuously while active

C. Use exit: count = count + 1; to increment count when leaving the state

D. Use entry: count = count + 1; and initialize count outside the state; no during or exit needed

Solution

1. Step 1: Understand counting entries

   To count entries, increment count once each time the state is entered. This is done in the entry action.

2. Step 2: Evaluate other options

   During increments continuously, not suitable for counting entries. Exit increments on leaving, not entering. Resetting count on entry loses previous counts.

3. Final Answer:

   Use entry: count = count + 1; and initialize count outside the state; no during or exit needed -> Option D

4. Quick Check:

   Entry action increments count once per entry [OK]

Hint: Increment count in entry action, initialize count outside [OK]

Common Mistakes:

• Incrementing count in during action
• Resetting count on every entry
• Incrementing count on exit instead of entry