millis() for non-blocking timing in Arduino - Time & Space Complexity

Time Complexity: millis() for non-blocking timing

O(1)

Understanding Time Complexity

When using millis() for timing in Arduino, it's important to see how the program's speed changes as it checks time repeatedly.

We want to know how the time spent grows when the program runs longer or checks more often.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    // Do something every interval
  }
}
    

This code checks the time repeatedly and performs an action every set interval without stopping the program.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loop() function runs continuously, checking millis() and comparing times.
• How many times: It runs as many times as the Arduino is powered on, effectively infinite.

How Execution Grows With Input

Each loop cycle does a fixed amount of work regardless of how long the program runs.

Input Size (n)	Approx. Operations
10	10 checks of millis() and if-condition
100	100 checks of millis() and if-condition
1000	1000 checks of millis() and if-condition

Pattern observation: The number of operations grows linearly with the number of loop cycles, but each cycle's work stays the same.

Final Time Complexity

Time Complexity: O(1)

This means each loop cycle takes the same small amount of time no matter how long the program runs.

Common Mistake

[X] Wrong: "Using millis() makes the program slower as time passes because it checks time constantly."

[OK] Correct: The check is very fast and does not get slower over time; it always takes the same small step each loop.

Interview Connect

Understanding how millis() works for timing without stopping the program shows you can write efficient, responsive code that handles tasks smoothly.

Self-Check

"What if we added a nested loop inside the if-condition that runs n times? How would the time complexity change?"

Practice

1. What does the millis() function in Arduino return?

easy

A. The current date and time

B. The number of milliseconds since the Arduino board started running the current program

C. The number of microseconds since the last reset

D. The current time in seconds

Solution

1. Step 1: Understand what millis() measures

   millis() returns the time in milliseconds since the Arduino started running the program.

2. Step 2: Compare options with the definition

   Only The number of milliseconds since the Arduino board started running the current program correctly describes this behavior; others mention seconds, microseconds, or date/time which are incorrect.

3. Final Answer:

   The number of milliseconds since the Arduino board started running the current program -> Option B

4. Quick Check:

   millis() = milliseconds since start [OK]

Hint: Remember millis() counts milliseconds since start [OK]

Common Mistakes:

• Confusing millis() with delay()
• Thinking millis() returns seconds
• Assuming millis() gives current date/time

2. Which of the following is the correct way to store the current time using millis() in Arduino?

easy

A. char currentTime = millis();

B. int currentTime = millis();

C. float currentTime = millis();

D. unsigned long currentTime = millis();

Solution

1. Step 1: Identify the correct data type for millis()

   millis() returns an unsigned long value representing milliseconds.

2. Step 2: Match the data type with variable declaration

   Only unsigned long currentTime = millis(); uses unsigned long, which can hold large millisecond values without overflow.

3. Final Answer:

   unsigned long currentTime = millis(); -> Option D

4. Quick Check:

   Use unsigned long for millis() values [OK]

Hint: Use unsigned long to store millis() time [OK]

Common Mistakes:

• Using int which can overflow quickly
• Using float or char which are incorrect types
• Not declaring variable before assignment

3. What will the following Arduino code print to the Serial Monitor?

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

void setup() {
  Serial.begin(9600);
}

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

medium

A. Prints "Tick" once and stops

B. Prints "Tick" continuously without delay

C. Prints "Tick" every 1000 milliseconds without stopping the program

D. Causes a compile error due to variable scope

Solution

1. Step 1: Understand the timing logic

   The code checks if 1000 milliseconds have passed since last print using millis() and updates previousMillis accordingly.

2. Step 2: Analyze the output behavior

   When 1000 ms pass, it prints "Tick" and continues looping without blocking, so it prints every second repeatedly.

3. Final Answer:

   Prints "Tick" every 1000 milliseconds without stopping the program -> Option C

4. Quick Check:

   Non-blocking timing prints "Tick" every second [OK]

Hint: Check millis() difference to print repeatedly [OK]

Common Mistakes:

• Thinking it prints only once
• Confusing with delay() causing blocking
• Assuming compile error due to variable scope

4. Identify the error in this Arduino code using millis() for timing:

unsigned long previousMillis;
const long interval = 2000;

void setup() {
  Serial.begin(9600);
}

void loop() {
  if (millis() - previousMillis >= interval) {
    Serial.println("Hello");
  }
}

medium

A. previousMillis is never updated, so "Hello" prints continuously

B. interval should be unsigned long, not long

C. Serial.begin() is missing in setup()

D. millis() cannot be used in loop()

Solution

1. Step 1: Check how previousMillis is used

   The code checks the time difference but never updates previousMillis after printing.

2. Step 2: Understand the effect of missing update

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

3. Final Answer:

   previousMillis is never updated, so "Hello" prints continuously -> Option A

4. Quick Check:

   Update previousMillis to avoid continuous printing [OK]

Hint: Always update previousMillis after action [OK]

Common Mistakes:

• Forgetting to update previousMillis
• Thinking interval type causes error
• Assuming Serial.begin() is missing

5. You want to blink an LED every 500 milliseconds without stopping other code from running. Which code snippet correctly uses millis() for this non-blocking timing? A)

unsigned long previousMillis = 0;
const long interval = 500;
void loop() {
  if (millis() - previousMillis >= interval) {
    previousMillis = millis();
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
  }
  // other code runs here
}

B)

void loop() {
  delay(500);
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
}

C)

unsigned long previousMillis = 0;
const long interval = 500;
void loop() {
  if (millis() >= previousMillis + interval) {
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
    previousMillis = millis();
  }
}

D)

unsigned long previousMillis = 0;
const long interval = 500;
void loop() {
  if (millis() - previousMillis > interval) {
    digitalWrite(LED_BUILTIN, HIGH);
    delay(500);
    digitalWrite(LED_BUILTIN, LOW);
  }
}

hard

A. Correct non-blocking blink using millis() and toggling LED

B. Uses delay(), which blocks other code from running

C. Correct logic but may cause overflow issues with addition

D. Uses delay() inside if, causing blocking and incorrect blink

Solution

1. Step 1: Identify non-blocking timing usage

   Correct non-blocking blink using millis() and toggling LED uses millis() difference and updates previousMillis correctly, toggling LED without delay.

2. Step 2: Compare other options for blocking or logic issues

   The other snippets either use delay(), which blocks other code from running, or use addition in the condition, which can cause overflow issues with large millisecond values.

3. Final Answer:

   Correct non-blocking blink using millis() and toggling LED -> Option A

4. Quick Check:

   Use millis() difference and update previousMillis [OK]

Hint: Toggle LED using millis() difference, avoid delay() [OK]

Common Mistakes:

• Using delay() causing blocking
• Not updating previousMillis properly
• Using addition risking overflow bugs