Reading digital input pin state in Embedded C

Introduction

We read a digital input pin to know if it is ON or OFF, like checking if a button is pressed.

To detect if a button is pressed on a device.

To check if a sensor is triggered or not.

To read a switch position in a control panel.

To monitor if a door is open or closed using a magnetic sensor.

Syntax

Embedded C

int pinState = digitalRead(pinNumber);

pinNumber is the number of the input pin you want to read.

digitalRead() returns HIGH (1) if the pin is ON, or LOW (0) if it is OFF.

Examples

Reads the state of digital pin 2 and stores it in buttonState.

Embedded C

int buttonState = digitalRead(2);

Checks if pin 5 is ON, then runs code inside the if block.

Embedded C

if (digitalRead(5) == HIGH) {
  // do something when pin 5 is ON
}

Sample Program

This program reads the state of a button connected to pin 7. It prints "Button is PRESSED" when the button is pressed, otherwise it prints "Button is NOT pressed" every half second.

Embedded C

#include <Arduino.h>

const int buttonPin = 7;  // input pin number

void setup() {
  pinMode(buttonPin, INPUT);  // set pin as input
  Serial.begin(9600);         // start serial communication
}

void loop() {
  int buttonState = digitalRead(buttonPin);  // read pin state
  if (buttonState == HIGH) {
    Serial.println("Button is PRESSED");
  } else {
    Serial.println("Button is NOT pressed");
  }
  delay(500);  // wait half a second before reading again
}

Important Notes

Make sure to set the pin mode to INPUT before reading the pin.

Use pull-up or pull-down resistors to avoid random readings (floating pins).

digitalRead() only reads digital signals: ON (HIGH) or OFF (LOW).

Summary

Use digitalRead(pin) to check if a digital input pin is ON or OFF.

Always set the pin as INPUT before reading.

Reading digital pins helps detect buttons, switches, and sensors.

Practice

1. What does the function digitalRead(pin) do in embedded C programming?

easy

A. Configures the pin as an analog input.

B. Sets the digital pin to output mode.

C. Reads the current state (HIGH or LOW) of a digital input pin.

D. Writes a HIGH signal to a digital pin.

Solution

1. Step 1: Understand the purpose of digitalRead

   The function digitalRead(pin) is used to check the current electrical state of a digital input pin, which can be either HIGH (on) or LOW (off).

2. Step 2: Differentiate from other pin functions

   Unlike functions that set pin modes or write signals, digitalRead only reads the pin's state without changing it.

3. Final Answer:

   Reads the current state (HIGH or LOW) of a digital input pin. -> Option C

4. Quick Check:

   digitalRead reads pin state = D [OK]

Hint: digitalRead always reads input pin state, not sets it [OK]

Common Mistakes:

• Confusing digitalRead with pinMode or digitalWrite
• Thinking digitalRead sets pin mode
• Assuming digitalRead writes output signals

2. Which of the following is the correct way to set a pin as input before reading its state?

easy

A. pinMode(pin, OUTPUT);

B. pinMode(pin, INPUT);

C. digitalRead(pin);

D. digitalWrite(pin, HIGH);

Solution

1. Step 1: Identify the function to set pin mode

   pinMode(pin, mode) is used to configure a pin as INPUT or OUTPUT.

2. Step 2: Choose the correct mode for reading input

   To read a digital input pin, it must be set as INPUT using pinMode(pin, INPUT).

3. Final Answer:

   pinMode(pin, INPUT); -> Option B

4. Quick Check:

   Set pin as INPUT before reading = A [OK]

Hint: Use pinMode(pin, INPUT) before digitalRead [OK]

Common Mistakes:

• Setting pin as OUTPUT before reading input
• Calling digitalRead before setting pin mode
• Using digitalWrite to set pin mode

3. What will be the output of the following code snippet if the button connected to pin 7 is pressed (assuming pressed means HIGH)?

pinMode(7, INPUT);
int buttonState = digitalRead(7);
if (buttonState == HIGH) {
    printf("Button is pressed\n");
} else {
    printf("Button is not pressed\n");
}

medium

A. Button is pressed

B. Button is not pressed

C. Compilation error

D. No output

Solution

1. Step 1: Analyze pin mode and reading

   The pin 7 is set as INPUT, so digitalRead(7) correctly reads the pin state.

2. Step 2: Check condition for pressed button

   If the button is pressed, the pin reads HIGH, so the if condition is true and prints "Button is pressed".

3. Final Answer:

   Button is pressed -> Option A

4. Quick Check:

   Pressed button = HIGH = prints pressed [OK]

Hint: Pressed button means digitalRead returns HIGH [OK]

Common Mistakes:

• Forgetting to set pinMode before reading
• Confusing HIGH and LOW states
• Assuming digitalRead returns boolean true/false

4. Identify the error in the following code that reads a digital input pin state:

int pin = 5;
int state;
state = digitalRead();
if(state == HIGH) {
    // do something
}

medium

A. Pin should be set as OUTPUT before reading.

B. The if condition syntax is incorrect.

C. digitalRead returns a float, not int.

D. Missing pin number argument in digitalRead function.

Solution

1. Step 1: Check digitalRead usage

   digitalRead requires the pin number as an argument, but here it is called without any argument.

2. Step 2: Verify other parts of code

   Pin mode setting is missing but not the main error asked; the if condition syntax is correct.

3. Final Answer:

   Missing pin number argument in digitalRead function. -> Option D

4. Quick Check:

   digitalRead needs pin argument = B [OK]

Hint: digitalRead always needs pin number inside parentheses [OK]

Common Mistakes:

• Calling digitalRead without pin argument
• Setting pin as OUTPUT before reading input
• Misunderstanding return type of digitalRead

5. You want to detect if a switch connected to pin 3 is ON and turn on an LED connected to pin 13 accordingly. Which code snippet correctly reads the switch state and controls the LED?

hard

A. pinMode(3, INPUT);
pinMode(13, OUTPUT);
if (digitalRead(3) == HIGH) {
digitalWrite(13, HIGH);
} else {
digitalWrite(13, LOW);
}

B. pinMode(3, OUTPUT);
pinMode(13, INPUT);
if (digitalRead(13) == HIGH) {
digitalWrite(3, HIGH);
}

C. pinMode(3, INPUT);
if (digitalRead(13) == HIGH) {
digitalWrite(3, HIGH);
}

D. pinMode(13, OUTPUT);
digitalWrite(3, HIGH);
if (digitalRead(3) == HIGH) {
digitalWrite(13, HIGH);
}

Solution

1. Step 1: Set correct pin modes

   Pin 3 is input for the switch, so pinMode(3, INPUT). Pin 13 is output for the LED, so pinMode(13, OUTPUT).

2. Step 2: Read switch and control LED

   If digitalRead(3) is HIGH (switch ON), turn LED on with digitalWrite(13, HIGH), else turn it off with digitalWrite(13, LOW).

3. Final Answer:

   pinMode(3, INPUT); pinMode(13, OUTPUT); if (digitalRead(3) == HIGH) { digitalWrite(13, HIGH); } else { digitalWrite(13, LOW); } -> Option A

4. Quick Check:

   Input pin read controls output pin write = A [OK]

Hint: Input pin set INPUT, output pin set OUTPUT, read then write [OK]

Common Mistakes:

• Setting input pin as OUTPUT or vice versa
• Reading wrong pin for switch state
• Not controlling LED pin properly