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'.