Why Timing-based state machines in Arduino? - Purpose & Use Cases

The Big Idea

What if your program could keep running smoothly while still keeping perfect track of time?

The Scenario

Imagine you want to control a traffic light with red, yellow, and green lights turning on and off at specific times. You try to write code that waits for each light to finish before moving to the next, using delay commands everywhere.

The Problem

Using delays stops your whole program from doing anything else while waiting. This means your traffic light can't respond to button presses or sensors during those pauses. It's slow, clunky, and hard to add new features.

The Solution

Timing-based state machines let you track time without stopping the program. You check how much time has passed and switch states only when needed. This keeps your program running smoothly and lets you handle many tasks at once.

Before vs After

✗ Before

delay(1000);
turnOnRed();
delay(1000);
turnOnGreen();

✓ After

if (millis() - lastChange >= interval) {
  changeState();
  lastChange = millis();
}

What It Enables

You can build responsive, multitasking programs that control devices precisely over time without freezing or missing important events.

Real Life Example

Controlling a robot's movements where motors run for set times but sensors must be checked constantly to avoid obstacles.

Key Takeaways

Manual delays pause everything and block other actions.

Timing-based state machines use time checks to switch states smoothly.

This approach keeps programs responsive and easy to expand.

Practice

1.

What is the main purpose of using millis() in a timing-based state machine on Arduino?

easy

A. To pause the program for a fixed time

B. To reset the Arduino board

C. To track elapsed time without stopping the program

D. To read analog sensor values

Solution

1. Step 1: Understand what millis() does

   millis() returns the number of milliseconds since the Arduino started running. It keeps counting without stopping the program.

2. Step 2: Connect millis() to timing-based state machines

   Using millis() lets the program check how much time passed and change states without pausing or blocking other tasks.

3. Final Answer:

   To track elapsed time without stopping the program -> Option C

4. Quick Check:

   millis() tracks time without delay [OK]

Hint: Remember: millis() never stops your code [OK]

Common Mistakes:

• Thinking millis() pauses the program
• Confusing millis() with delay()
• Using millis() to reset Arduino

2.

Which of the following is the correct way to check if 1000 milliseconds have passed using millis()?

unsigned long previousMillis = 0;
unsigned long interval = 1000;

void loop() {
  unsigned long currentMillis = millis();
  // What condition checks if interval passed?
  if (__________) {
    // do something
    previousMillis = currentMillis;
  }
}

easy

A. previousMillis + currentMillis <= interval

B. previousMillis - currentMillis >= interval

C. currentMillis + previousMillis >= interval

D. currentMillis - previousMillis >= interval

Solution

1. Step 1: Understand elapsed time calculation

   Elapsed time is current time minus previous time: currentMillis - previousMillis.

2. Step 2: Check if elapsed time reached interval

   We compare if elapsed time is greater or equal to the interval: currentMillis - previousMillis >= interval.

3. Final Answer:

   currentMillis - previousMillis >= interval -> Option D

4. Quick Check:

   Elapsed time = current - previous [OK]

Hint: Subtract previous from current time to get elapsed [OK]

Common Mistakes:

• Reversing subtraction order
• Adding times instead of subtracting
• Using <= instead of >=

3.

What will be the output of this Arduino code snippet that uses a timing-based state machine?

unsigned long previousMillis = 0;
const long interval = 2000;
int ledState = LOW;

void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }
    digitalWrite(13, ledState);
    Serial.println(ledState);
  }
}

medium

A. Prints alternating 0 and 1 every 2 seconds

B. Prints 1 continuously every 2 seconds

C. Prints 0 continuously every 2 seconds

D. No output because of syntax error

Solution

1. Step 1: Understand the timing and state toggle

   Every 2000 ms, the code toggles ledState between LOW (0) and HIGH (1).

2. Step 2: Check output printed

   Each toggle prints the current ledState (0 or 1) to Serial, alternating every 2 seconds.

3. Final Answer:

   Prints alternating 0 and 1 every 2 seconds -> Option A

4. Quick Check:

   State toggles and prints 0,1 alternately [OK]

Hint: Toggle state and print inside timed if-block [OK]

Common Mistakes:

• Assuming constant output without toggle
• Confusing HIGH/LOW with 1/0
• Missing update of previousMillis

4.

Identify the bug in this timing-based state machine code and choose the fix.

unsigned long previousMillis = 0;
const long interval = 1000;
int ledState = LOW;

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis > interval) {
    ledState = !ledState;
    digitalWrite(13, ledState);
  }
}

medium

A. Add previousMillis = currentMillis; inside the if-block

B. Change int ledState to bool ledState

C. Replace ! with ~ in toggle

D. Remove the if condition to toggle every loop

Solution

1. Step 1: Check timing update logic

   The code never updates previousMillis, so the condition stays true forever after first pass.

2. Step 2: Fix by updating previousMillis

   Adding previousMillis = currentMillis; inside the if-block resets the timer for the next interval.

3. Final Answer:

   Add previousMillis = currentMillis; inside the if-block -> Option A

4. Quick Check:

   Update previousMillis to reset timer [OK]

Hint: Always update previousMillis after interval check [OK]

Common Mistakes:

• Forgetting to update previousMillis
• Using bitwise NOT (~) instead of logical NOT (!)
• Removing timing check causes fast toggling

5.

You want to create a state machine that cycles through three LED states: OFF, RED, GREEN. Each state lasts 3 seconds. Which code snippet correctly implements this using millis()?

unsigned long previousMillis = 0;
const long interval = 3000;
int state = 0;

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    state = (state + 1) % 3;
    switch(state) {
      case 0:
        digitalWrite(RED_PIN, LOW);
        digitalWrite(GREEN_PIN, LOW);
        break;
      case 1:
        digitalWrite(RED_PIN, HIGH);
        digitalWrite(GREEN_PIN, LOW);
        break;
      case 2:
        digitalWrite(RED_PIN, LOW);
        digitalWrite(GREEN_PIN, HIGH);
        break;
    }
  }
}

hard

A. Does not change states due to missing update

B. Correctly cycles OFF, RED, GREEN every 3 seconds

C. Cycles states every 1 second instead of 3

D. Cycles only RED and GREEN, skipping OFF

Solution

1. Step 1: Check timing and state update

   The code uses millis() to check 3 seconds passed, then updates state cycling 0,1,2 with modulo 3.

2. Step 2: Verify LED outputs per state

   State 0 turns both LEDs off, 1 turns RED on, 2 turns GREEN on. This matches the required cycle.

3. Final Answer:

   Correctly cycles OFF, RED, GREEN every 3 seconds -> Option B

4. Quick Check:

   State cycles with modulo and timing [OK]

Hint: Use modulo (%) to cycle states smoothly [OK]

Common Mistakes:

• Forgetting to update previousMillis
• Incorrect modulo causing wrong cycles
• Not turning off LEDs in OFF state