Button reading with pull-up resistor in Arduino - Time & Space Complexity

Time Complexity: Button reading with pull-up resistor

O(n)

Understanding Time Complexity

When reading a button with a pull-up resistor on Arduino, the program checks the button state repeatedly.

We want to know how the time to check the button grows as the program runs longer or with more checks.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

const int buttonPin = 2;

void setup() {
  pinMode(buttonPin, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  int buttonState = digitalRead(buttonPin);
  Serial.println(buttonState);
  delay(100);
}
    

This code sets up a button with a pull-up resistor and repeatedly reads its state every 100 milliseconds, printing the result.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The loop() function runs forever, reading the button state once each time.
• How many times: It repeats indefinitely, once every 100 milliseconds.

How Execution Grows With Input

Each loop cycle does a fixed amount of work: one button read and one print.

Input Size (n)	Approx. Operations
10 (loops)	10 reads and prints
100 (loops)	100 reads and prints
1000 (loops)	1000 reads and prints

Pattern observation: The number of operations grows directly with the number of loops. More loops mean more reads and prints.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the program grows linearly with how many times the button is read.

Common Mistake

[X] Wrong: "Reading the button once takes longer if the program runs longer."

[OK] Correct: Each button read takes the same short time, no matter how many times it has run before. The total time grows because you do more reads, not because each read gets slower.

Interview Connect

Understanding how repeated actions add up over time helps you explain how programs handle inputs and timing, a useful skill in many projects and interviews.

Self-Check

"What if we added another button and read both in each loop? How would the time complexity change?"

Practice

1. What does setting pinMode(pin, INPUT_PULLUP) do in Arduino when reading a button?

easy

A. Enables the internal pull-up resistor to keep the pin HIGH when button is not pressed

B. Disables the pin to save power

C. Sets the pin as an output to drive an LED

D. Connects the pin directly to ground

Solution

1. Step 1: Understand pinMode with INPUT_PULLUP

   Using INPUT_PULLUP activates the internal pull-up resistor on the pin, so it reads HIGH by default.

2. Step 2: Effect on button reading

   When the button is pressed, it connects the pin to ground, making the reading LOW. When not pressed, the pull-up resistor keeps it HIGH.

3. Final Answer:

   Enables the internal pull-up resistor to keep the pin HIGH when button is not pressed -> Option A

4. Quick Check:

   INPUT_PULLUP means pin reads HIGH unless grounded [OK]

Hint: INPUT_PULLUP means pin is HIGH until button grounds it [OK]

Common Mistakes:

• Thinking INPUT_PULLUP sets pin as output
• Assuming pin reads LOW when button is not pressed
• Confusing pull-up with pull-down resistor

2. Which of the following is the correct syntax to set pin 7 as input with internal pull-up resistor in Arduino?

easy

A. pinMode(7, INPUT_PULLDOWN);

B. pinMode(7, OUTPUT_PULLUP);

C. pinMode(7, INPUT_PULLUP);

D. pinMode(7, INPUT); digitalWrite(7, LOW);

Solution

1. Step 1: Recall correct pinMode usage

   The correct way to enable internal pull-up resistor is pinMode(pin, INPUT_PULLUP);.

2. Step 2: Check each option

   pinMode(7, OUTPUT_PULLUP); uses OUTPUT_PULLUP which does not exist. pinMode(7, INPUT_PULLDOWN); uses INPUT_PULLDOWN which Arduino does not support internally. pinMode(7, INPUT); digitalWrite(7, LOW); sets pin as INPUT but digitalWrite LOW disables pull-up (equivalent to plain INPUT, floating pin), does not enable internal pull-up resistor.

3. Final Answer:

   pinMode(7, INPUT_PULLUP); -> Option C

4. Quick Check:

   Use INPUT_PULLUP to enable pull-up resistor [OK]

Hint: Use INPUT_PULLUP exactly to enable pull-up resistor [OK]

Common Mistakes:

• Using OUTPUT_PULLUP which is invalid
• Trying INPUT_PULLDOWN which Arduino lacks
• digitalWrite(7, LOW) after INPUT (floating input, no pull-up)

3. What will be the output on the Serial Monitor when the button connected to pin 4 is pressed, given this code?

void setup() {
  pinMode(4, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  int state = digitalRead(4);
  Serial.println(state);
  delay(500);
}

medium

A. Prints 1 when button pressed, 0 when released

B. Always prints 1 regardless of button state

C. Always prints 0 regardless of button state

D. Prints 0 when button pressed, 1 when released

Solution

1. Step 1: Understand INPUT_PULLUP behavior

   With INPUT_PULLUP, the pin reads HIGH (1) when button is not pressed and LOW (0) when pressed because button connects pin to ground.

2. Step 2: Analyze Serial output

   The code prints the pin state every 500ms. When pressed, it prints 0; when released, it prints 1.

3. Final Answer:

   Prints 0 when button pressed, 1 when released -> Option D

4. Quick Check:

   Pressed = LOW (0), Released = HIGH (1) [OK]

Hint: Pressed button reads LOW (0) with pull-up resistor [OK]

Common Mistakes:

• Assuming pressed reads HIGH (1)
• Confusing pull-up with pull-down logic
• Ignoring that button grounds the pin when pressed

4. The following code is intended to read a button with internal pull-up resistor on pin 2, but it does not work correctly. What is the error?

void setup() {
  pinMode(2, INPUT);
  Serial.begin(9600);
}

void loop() {
  int val = digitalRead(2);
  Serial.println(val);
  delay(200);
}

medium

A. Missing INPUT_PULLUP mode to enable pull-up resistor

B. Serial.begin should be in loop, not setup

C. digitalRead cannot be used on pin 2

D. delay(200) is too short for button reading

Solution

1. Step 1: Check pinMode configuration

   The code uses pinMode(2, INPUT); which does not enable the internal pull-up resistor, so the pin may float and give unreliable readings.

2. Step 2: Correct usage for button with pull-up

   To use the internal pull-up resistor, the pinMode should be INPUT_PULLUP. This prevents floating and ensures stable readings.

3. Final Answer:

   Missing INPUT_PULLUP mode to enable pull-up resistor -> Option A

4. Quick Check:

   Use INPUT_PULLUP to avoid floating input [OK]

Hint: Always use INPUT_PULLUP for button input to avoid floating [OK]

Common Mistakes:

• Using INPUT without pull-up resistor
• Moving Serial.begin to loop unnecessarily
• Thinking delay affects button reading correctness

5. You want to detect a button press using pin 8 with internal pull-up resistor. Which code snippet correctly reads the button and turns on an LED on pin 13 only when the button is pressed?

hard

A. pinMode(8, INPUT_PULLUP);
pinMode(13, OUTPUT);
if(digitalRead(8) == HIGH) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

B. pinMode(8, INPUT_PULLUP);
pinMode(13, OUTPUT);
if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

C. pinMode(8, INPUT);
pinMode(13, OUTPUT);
if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

D. pinMode(8, OUTPUT);
pinMode(13, INPUT_PULLUP);
if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW);

Solution

1. Step 1: Set pin modes correctly

   Pin 8 must be input with internal pull-up resistor: INPUT_PULLUP. Pin 13 is output for LED.

2. Step 2: Understand button logic with pull-up

   Button press connects pin 8 to ground, so digitalRead(8) returns LOW when pressed.

3. Step 3: Write correct if condition

   Turn LED on when button is pressed (pin reads LOW), so condition is if(digitalRead(8) == LOW).

4. Final Answer:

   pinMode(8, INPUT_PULLUP); pinMode(13, OUTPUT); if(digitalRead(8) == LOW) digitalWrite(13, HIGH); else digitalWrite(13, LOW); -> Option B

5. Quick Check:

   Pressed = LOW, LED ON when LOW [OK]

Hint: Button pressed reads LOW; turn LED ON when reading is LOW [OK]

Common Mistakes:

• Checking for HIGH instead of LOW on button press
• Setting button pin as OUTPUT
• Not enabling INPUT_PULLUP resistor