Timer interrupts with TimerOne library in Arduino - Time & Space Complexity

Time Complexity: Timer interrupts with TimerOne library

O(n)

Understanding Time Complexity

When using timer interrupts with the TimerOne library, it's important to understand how often the interrupt code runs as input changes.

We want to know how the number of operations grows as the timer interval or workload changes.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <TimerOne.h>

volatile int count = 0;

void setup() {
  Timer1.initialize(1000000); // 1 second
  Timer1.attachInterrupt(timerIsr);
}

void loop() {
  // main code runs here
}

void timerIsr() {
  count++;
}
    

This code sets a timer interrupt to run every 1 second and increments a counter each time the interrupt triggers.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The interrupt service routine (ISR) runs repeatedly.
• How many times: It runs once every timer interval (e.g., every 1 second).

How Execution Grows With Input

The number of times the ISR runs depends on the total time the program runs and the timer interval.

Input Size (seconds running)	Approx. ISR Calls
10	10
100	100
1000	1000

Pattern observation: The number of ISR calls grows linearly with the running time.

Final Time Complexity

Time Complexity: O(n)

This means the total number of interrupt calls grows directly in proportion to the running time.

Common Mistake

[X] Wrong: "The interrupt runs only once or a fixed number of times regardless of time."

[OK] Correct: The interrupt triggers repeatedly at each timer interval, so the total calls increase as time passes.

Interview Connect

Understanding how timer interrupts scale with time helps you reason about real-time systems and how often tasks run in embedded programming.

Self-Check

What if we changed the timer interval from 1 second to 0.1 seconds? How would the time complexity change?

Practice

1. What is the main purpose of using the TimerOne library in Arduino programming?

easy

A. To run code at precise time intervals using interrupts

B. To control the speed of the Arduino board

C. To manage power consumption automatically

D. To connect Arduino to the internet

Solution

1. Step 1: Understand TimerOne library role

   The TimerOne library is designed to help run code at exact time intervals by using hardware timer interrupts.

2. Step 2: Compare options with library purpose

   Only To run code at precise time intervals using interrupts correctly describes this purpose. Other options describe unrelated features.

3. Final Answer:

   To run code at precise time intervals using interrupts -> Option A

4. Quick Check:

   TimerOne = precise timed interrupts [OK]

Hint: TimerOne = precise timing with interrupts [OK]

Common Mistakes:

• Thinking TimerOne controls Arduino speed
• Confusing interrupts with internet connection
• Assuming it manages power automatically

2. Which of the following is the correct way to start TimerOne with a 1-second interval?

easy

A. Timer1.initialize(1000000);

B. Timer1.start(1000);

C. Timer1.setInterval(1);

D. Timer1.begin(1000);

Solution

1. Step 1: Recall TimerOne initialization syntax

   The TimerOne library uses initialize() with microseconds as argument to set the timer interval.

2. Step 2: Match 1 second to microseconds

   1 second = 1,000,000 microseconds, so initialize(1000000) is correct.

3. Final Answer:

   Timer1.initialize(1000000); -> Option A

4. Quick Check:

   initialize uses microseconds = 1000000 [OK]

Hint: Use initialize() with microseconds for timing [OK]

Common Mistakes:

• Using milliseconds instead of microseconds
• Using non-existent start() or begin() methods
• Confusing setInterval() with initialize()

3. What will be the output on the Serial Monitor if this code runs?

void blink() {
  Serial.println("Tick");
}

void setup() {
  Serial.begin(9600);
  Timer1.initialize(500000);
  Timer1.attachInterrupt(blink);
}

void loop() {
  // empty
}

medium

A. No output because loop is empty

B. Prints "Tick" once and stops

C. Prints "Tick" every 0.5 seconds repeatedly

D. Prints "Tick" continuously without delay

Solution

1. Step 1: Understand Timer1 setup

   Timer1 is initialized to 500000 microseconds (0.5 seconds) and blink() is attached as interrupt.

2. Step 2: Analyze interrupt behavior

   Every 0.5 seconds, blink() runs and prints "Tick" to Serial Monitor repeatedly.

3. Final Answer:

   Prints "Tick" every 0.5 seconds repeatedly -> Option C

4. Quick Check:

   attachInterrupt triggers blink() repeatedly [OK]

Hint: attachInterrupt runs function repeatedly at interval [OK]

Common Mistakes:

• Thinking loop() must print for output
• Assuming interrupt runs only once
• Confusing timing units causing wrong intervals

4. Identify the error in this TimerOne usage:

void blink() {
  digitalWrite(13, !digitalRead(13));
}

void setup() {
  pinMode(13, OUTPUT);
  Timer1.initialize(1000000);
  Timer1.attachInterrupt(blink());
}

void loop() {}

medium

A. Timer1.initialize() argument should be in milliseconds

B. Missing Serial.begin() in setup()

C. Pin 13 cannot be used with TimerOne interrupts

D. Incorrect call: attachInterrupt(blink()) should be attachInterrupt(blink)

Solution

1. Step 1: Check attachInterrupt usage

   attachInterrupt expects a function pointer, so it should be attachInterrupt(blink), not attachInterrupt(blink()).

2. Step 2: Verify other parts

   Serial.begin() is not needed here, pin 13 can be toggled, and initialize() uses microseconds correctly.

3. Final Answer:

   Incorrect call: attachInterrupt(blink()) should be attachInterrupt(blink) -> Option D

4. Quick Check:

   attachInterrupt needs function name, no parentheses [OK]

Hint: Pass function name without () to attachInterrupt [OK]

Common Mistakes:

• Calling function with () inside attachInterrupt
• Confusing microseconds with milliseconds
• Thinking Serial.begin() is always required

5. You want to toggle an LED every 200ms using TimerOne. Which code snippet correctly sets this up?

hard

A. Timer1.initialize(200);
Timer1.attachInterrupt(blink);

B. Timer1.initialize(200000);
Timer1.attachInterrupt([](){ digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN)); });

C. Timer1.start(200000);
Timer1.attachInterrupt(blink());

D. Timer1.setInterval(200000);
Timer1.attachInterrupt(blink);

Solution

1. Step 1: Convert 200ms to microseconds

   200 milliseconds = 200,000 microseconds, so initialize(200000) is correct.

2. Step 2: Check attachInterrupt syntax and function

   Using a lambda function without parentheses is valid and toggles LED correctly. Other options use wrong methods or call functions incorrectly.

3. Final Answer:

   Timer1.initialize(200000); Timer1.attachInterrupt([](){ digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN)); }); -> Option B

4. Quick Check:

   initialize with microseconds + correct attachInterrupt [OK]

Hint: Use initialize(microseconds) and attachInterrupt with function [OK]

Common Mistakes:

• Using milliseconds instead of microseconds
• Calling function with () inside attachInterrupt
• Using non-existent start() or setInterval() methods