Non-blocking code architecture in Arduino - Time & Space Complexity

Time Complexity: Non-blocking code architecture

O(n)

Understanding Time Complexity

When writing non-blocking code on Arduino, we want to keep the program running smoothly without waiting too long in one place.

We ask: how does the time spent change as the program handles more tasks?

Scenario Under Consideration

Analyze the time complexity of the following non-blocking code snippet.

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

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    // perform task
  }
  // other code runs here without delay
}
    

This code checks time repeatedly and runs a task only when the interval passes, without stopping the rest of the program.

Identify Repeating Operations

Look for repeated actions in the code.

• Primary operation: The loop() function runs continuously, checking the time.
• How many times: It runs as fast as the Arduino can, many thousands of times per second.

How Execution Grows With Input

Imagine the input as how often the task needs to run.

Input Size (task frequency)	Approx. Operations per second
10 times	Loop runs thousands of times, task runs 10 times
100 times	Loop runs thousands of times, task runs 100 times
1000 times	Loop runs thousands of times, task runs 1000 times

Pattern observation: The loop runs very fast and often, but the task runs only when needed, so the main work grows with task frequency, not loop speed.

Final Time Complexity

Time Complexity: O(n)

This means the time spent on the task grows directly with how many times the task runs, while the loop itself runs continuously without delay.

Common Mistake

[X] Wrong: "The loop() function runs only once per task interval."

[OK] Correct: The loop() runs repeatedly and very fast; the task inside runs only when the time condition is met, so the loop keeps the program responsive.

Interview Connect

Understanding non-blocking code shows you can write programs that stay responsive and efficient, a skill useful in many real-world embedded projects.

Self-Check

What if we replaced the time check with a blocking delay? How would the time complexity change?

Practice

1. What is the main advantage of using non-blocking code in Arduino programming?

easy

A. It uses less memory than blocking code.

B. It makes the code run slower but more stable.

C. It allows the Arduino to perform multiple tasks without waiting.

D. It automatically fixes hardware errors.

Solution

1. Step 1: Understand blocking vs non-blocking

   Blocking code stops the program until a task finishes, while non-blocking code lets the program keep running other tasks.

2. Step 2: Identify the benefit of non-blocking

   Non-blocking code allows multitasking and keeps the Arduino responsive.

3. Final Answer:

   It allows the Arduino to perform multiple tasks without waiting. -> Option C

4. Quick Check:

   Non-blocking = multitasking [OK]

Hint: Non-blocking means no waiting, so tasks run together [OK]

Common Mistakes:

• Thinking non-blocking slows code down
• Confusing memory use with blocking behavior
• Believing non-blocking fixes hardware issues

2. Which of the following is the correct way to check elapsed time without blocking in Arduino?

easy

A. delay(1000);

B. if (millis() - previousMillis >= interval) { /* do task */ }

C. while(millis() < 1000) { /* wait */ }

D. delayMicroseconds(1000);

Solution

1. Step 1: Identify blocking vs non-blocking code

   delay() and while loops that wait block the program, stopping other tasks.

2. Step 2: Recognize millis() usage

   Using millis() to check elapsed time allows the program to continue running other code.

3. Final Answer:

   if (millis() - previousMillis >= interval) { /* do task */ } -> Option B

4. Quick Check:

   millis() check = non-blocking [OK]

Hint: Use millis() difference, not delay(), to avoid blocking [OK]

Common Mistakes:

• Using delay() which blocks the program
• Using while loops that wait for time
• Confusing delayMicroseconds with non-blocking

3. What will be the output of this Arduino code snippet?

unsigned long previousMillis = 0;
const long interval = 1000;
void loop() {
  if (millis() - previousMillis >= interval) {
    previousMillis = millis();
    Serial.println("Tick");
  }
}

medium

A. Prints "Tick" every second repeatedly.

B. Prints "Tick" continuously without delay.

C. Prints "Tick" once and stops.

D. Does not print anything due to syntax error.

Solution

1. Step 1: Analyze the millis() timing check

   The code checks if 1000 milliseconds (1 second) have passed since last print.

2. Step 2: Understand the loop behavior

   When 1 second passes, it updates previousMillis and prints "Tick" again, repeating forever.

3. Final Answer:

   Prints "Tick" every second repeatedly. -> Option A

4. Quick Check:

   millis() timer triggers print every 1 second [OK]

Hint: millis() timer resets each print for repeated ticks [OK]

Common Mistakes:

• Thinking it prints only once
• Assuming continuous printing without delay
• Believing there is a syntax error

4. Identify the problem in this non-blocking Arduino code and how to fix it:

unsigned long previousMillis = 0;
const long interval = 1000;
void loop() {
  if (millis() > previousMillis + interval) {
    Serial.println("Hello");
  }
}

medium

A. It never updates previousMillis, causing continuous prints.

B. The comparison operator should be '<' instead of '>'.

C. The interval variable must be unsigned long, not long.

D. Serial.println() cannot be used inside loop.

Solution

1. Step 1: Check the timing logic

   The code checks if millis() is greater than previousMillis + interval but never updates previousMillis.

2. Step 2: Understand the effect

   Without updating previousMillis, the condition stays true, so "Hello" prints continuously without delay.

3. Final Answer:

   It never updates previousMillis, causing continuous prints. -> Option A

4. Quick Check:

   Missing previousMillis update = continuous print [OK]

Hint: Always update previousMillis after action to avoid repeats [OK]

Common Mistakes:

• Forgetting to update previousMillis
• Changing comparison operator incorrectly
• Thinking Serial.println() is invalid in loop

5. You want to blink an LED every 500ms and read a sensor every 200ms without blocking. Which code structure best achieves this?

hard

A. Use while loops to wait 500ms and 200ms before each task.

B. Use delay(500) for LED blink and delay(200) for sensor reading in sequence.

C. Use a single millis() timer for both tasks with combined interval 700ms.

D. Use two millis() timers, one for LED and one for sensor, each updating separately.

Solution

1. Step 1: Understand multitasking with millis()

   Each task needs its own timer to run independently without blocking.

2. Step 2: Evaluate options

   Use two millis() timers, one for LED and one for sensor, each updating separately. This uses two separate millis() timers, allowing LED and sensor to run at different intervals smoothly.

3. Final Answer:

   Use two millis() timers, one for LED and one for sensor, each updating separately. -> Option D

4. Quick Check:

   Separate timers = true multitasking [OK]

Hint: Use separate millis() timers for each task to avoid blocking [OK]

Common Mistakes:

• Using delay() which blocks all tasks
• Combining intervals causing missed timings
• Using while loops that block execution