Non-blocking code architecture in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this non-blocking LED blink code?

Consider this Arduino sketch that blinks an LED without using delay(). What will be printed to the Serial Monitor?

Arduino

const int ledPin = 13;
unsigned long previousMillis = 0;
const long interval = 1000;

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

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    digitalWrite(ledPin, !digitalRead(ledPin));
    Serial.println("LED toggled");
  }
}

A LED toggled printed every second

B LED toggled printed continuously without delay

C No output printed because Serial.begin is missing

D LED toggled printed once and then stops

💡 Hint

Think about how millis() and the time check control when the message prints.

The code uses millis() to check elapsed time and toggles the LED every 1000 milliseconds (1 second). The message prints only when the LED toggles, so it prints once every second.

🧠 Conceptual

intermediate

Why is using delay() not recommended in non-blocking code?

Which of the following best explains why delay() is avoided in non-blocking Arduino code?

A <code>delay()</code> uses too much memory

B <code>delay()</code> only works with LEDs

C <code>delay()</code> causes the Arduino to reset

D Because <code>delay()</code> stops all code execution, preventing other tasks from running

💡 Hint

Think about what happens to the program when delay() is called.

delay() pauses the entire program for the specified time, blocking any other code from running. This stops the Arduino from doing multiple things at once, which is why non-blocking code avoids it.

🔧 Debug

advanced

Identify the bug in this non-blocking button press code

This code is supposed to toggle an LED each time a button is pressed without blocking. What is the bug that prevents it from working correctly?

Arduino

const int buttonPin = 2;
const int ledPin = 13;
int ledState = LOW;

void setup() {
  pinMode(buttonPin, INPUT);
  pinMode(ledPin, OUTPUT);
}

void loop() {
  if (digitalRead(buttonPin) == HIGH) {
    ledState = !ledState;
    digitalWrite(ledPin, ledState);
    delay(50);
  }
}

A LED pin is not set as OUTPUT

B Button pin is not set as INPUT_PULLUP

C Using <code>delay(50)</code> blocks code and causes multiple toggles on a single press

D Missing <code>Serial.begin()</code> in setup

💡 Hint

Think about what happens when the button is pressed and the code uses delay().

The delay(50) is used to debounce but it blocks the code, causing the loop to run slowly and the button press to be detected multiple times, toggling the LED rapidly. A non-blocking debounce method is better.

📝 Syntax

advanced

Which option correctly implements a non-blocking timer using millis()?

Choose the code snippet that correctly toggles an LED every 500 milliseconds without blocking.

A

unsigned long lastTime = 0;
void loop() {
  if (millis() - lastTime > 500) {
    lastTime = lastTime + 500;
    digitalWrite(13, !digitalRead(13));
  }
}

B

unsigned long lastTime = 0;
void loop() {
  if (millis() - lastTime >= 500) {
    lastTime += 500;
    digitalWrite(13, !digitalRead(13));
  }
}

C

unsigned long lastTime = 0;
void loop() {
  if (millis() - lastTime >= 500) {
    lastTime = 0;
    digitalWrite(13, !digitalRead(13));
  }
}

D

unsigned long lastTime = 0;
void loop() {
  if (millis() - lastTime >= 500) {
    lastTime = millis();
    digitalWrite(13, !digitalRead(13));
  }
}

💡 Hint

Consider how to update lastTime to keep consistent timing without drift.

Option B adds 500 to lastTime to keep the timing consistent and avoid drift. Option B resets lastTime to current millis(), which can cause drift. Option B uses wrong comparison and update. Option B resets lastTime to zero, causing repeated immediate toggles.

🚀 Application

expert

How many times will the LED toggle in 5 seconds with this code?

Given this non-blocking Arduino code, how many times will the LED toggle in 5 seconds?

Arduino

const int ledPin = 13;
unsigned long previousMillis = 0;
const long interval = 700;

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

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

A 7 times

B 5 times

C 6 times

D 8 times

💡 Hint

Divide the total time by the interval and consider how the code updates previousMillis.

The LED toggles every 700 ms. In 5 seconds (5000 ms), 5000 / 700 ≈ 7.14 intervals. But since previousMillis is incremented by 700 each time, the toggles happen at 700, 1400, 2100, 2800, 3500, 4200, 4900 ms (7 toggles). However, the first toggle happens after 700 ms, so total toggles in 5 seconds is 7. But since the code starts at 0 and toggles after the first interval, the LED toggles 7 times. The closest correct answer is 7 times (option A).