Reading digital input pin state in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of reading a digital input pin?

Consider the following embedded C code snippet that reads a digital input pin connected to a button. What value will be printed if the button is pressed (assuming active HIGH)?

Embedded C

#include <stdio.h>
#define BUTTON_PIN 2

int digitalRead(int pin) {
    // Simulated hardware input: returns 1 if pressed, 0 if not
    if (pin == BUTTON_PIN) return 1;
    return 0;
}

int main() {
    int buttonState = digitalRead(BUTTON_PIN);
    printf("Button state: %d\n", buttonState);
    return 0;
}

A Button state: 1

B Button state: 0

C Button state: -1

D Compilation error

💡 Hint

Think about what digitalRead returns when the button is pressed.

The digitalRead function returns 1 when the button is pressed (active HIGH). So the printed output is 'Button state: 1'.

❓ Predict Output

intermediate

What happens if you read an unconfigured input pin?

Given this code snippet, what output will it produce if the input pin is not configured and floating?

Embedded C

#include <stdio.h>
#define INPUT_PIN 5

int digitalRead(int pin) {
    // Simulate floating pin returns random 0 or 1
    if (pin == INPUT_PIN) return 0; // Simulated as LOW
    return 1;
}

int main() {
    int state = digitalRead(INPUT_PIN);
    printf("Pin state: %d\n", state);
    return 0;
}

A Pin state: 1

B Runtime error

C Pin state: 0

D Pin state: -1

💡 Hint

Floating pins often read as LOW or unpredictable, here simulated as LOW.

The digitalRead function simulates the floating pin as LOW (0), so the output is 'Pin state: 0'.

🔧 Debug

advanced

Why does this code always print 0 for the input pin?

Find the bug in this code that causes the input pin state to always print 0, even when the button is pressed.

Embedded C

#include <stdio.h>
#define BUTTON_PIN 3

int digitalRead(int pin) {
    if (pin == BUTTON_PIN) return 1;
    return 0;
}

int main() {
    int buttonState = 0;
    if (BUTTON_PIN = 3) {
        buttonState = digitalRead(BUTTON_PIN);
    }
    printf("Button state: %d\n", buttonState);
    return 0;
}

A The condition uses assignment '=' instead of comparison '==', so it always evaluates true but BUTTON_PIN is changed.

B digitalRead always returns 0 for BUTTON_PIN 3.

C buttonState is never assigned any value.

D Missing semicolon after digitalRead call.

💡 Hint

Check the if condition syntax carefully.

The if condition uses '=' which assigns 3 to BUTTON_PIN (which is a macro, so this causes a compile error or unexpected behavior). It should use '==' to compare. This causes the code to behave incorrectly.

🧠 Conceptual

advanced

What is the effect of enabling pull-up resistor on a digital input pin?

In embedded systems, enabling an internal pull-up resistor on a digital input pin means:

A The pin is connected internally to ground, so it reads LOW when not pressed.

B The pin is connected internally to a high voltage, so it reads HIGH when not pressed.

C The pin is disconnected internally, so it floats and reads random values.

D The pin output is driven HIGH constantly.

💡 Hint

Pull-up resistors pull the pin voltage up to a high level when no external signal is applied.

Enabling a pull-up resistor connects the pin internally to a high voltage through a resistor, so the pin reads HIGH when the button is not pressed (open). Pressing the button connects it to ground, reading LOW.

❓ Predict Output

expert

What is the output of this code reading multiple digital input pins?

Given the following code that reads three digital input pins and prints their states, what is the output?

Embedded C

#include <stdio.h>

int digitalRead(int pin) {
    switch(pin) {
        case 1: return 0;
        case 2: return 1;
        case 3: return 1;
        default: return -1;
    }
}

int main() {
    int states[3];
    for (int i = 0; i < 3; i++) {
        states[i] = digitalRead(i + 1);
    }
    printf("States: %d %d %d\n", states[0], states[1], states[2]);
    return 0;
}

A States: 1 0 1

B States: -1 -1 -1

C States: 1 1 0

D States: 0 1 1

💡 Hint

Check the digitalRead return values for each pin number.

digitalRead returns 0 for pin 1, 1 for pin 2, and 1 for pin 3. So the printed states are '0 1 1'.

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