First embedded program (LED blink) in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output behavior of this LED blink code?

Consider this embedded C code snippet for blinking an LED connected to PORTB pin 0. What will the LED do when this code runs?

Embedded C

#include <avr/io.h>
#include <util/delay.h>

int main(void) {
    DDRB |= (1 << 0); // Set PORTB0 as output
    while(1) {
        PORTB ^= (1 << 0); // Toggle PORTB0
        _delay_ms(500);
    }
    return 0;
}

A The LED will stay constantly on without blinking.

B The LED will stay constantly off without blinking.

C The LED connected to PORTB0 will blink on and off every 500 milliseconds.

D The code will cause a compile error due to missing header files.

💡 Hint

Look at how PORTB0 is toggled inside the infinite loop with a delay.

The code sets PORTB0 as output and toggles its state every 500 ms, causing the LED to blink on and off repeatedly.

❓ Predict Output

intermediate

What error does this LED blink code produce?

Examine this code snippet intended to blink an LED on PORTD pin 2. What error will occur when compiling?

Embedded C

#include <avr/io.h>

int main(void) {
    DDRD = 1 << 2; // Set PORTD2 as output
    while(1) {
        PORTD = 1 << 2; // Turn LED on
        _delay_ms(1000);
        PORTD = 0 << 2; // Turn LED off
        _delay_ms(1000);
    }
    return 0;
}

A No error; the LED will blink every 1 second.

B SyntaxError: _delay_ms is not declared or included.

C Runtime error: PORTD assignment causes undefined behavior.

D Compilation error: DDRD cannot be assigned with bit shift.

💡 Hint

Check if all required headers are included for delay functions.

The code uses _delay_ms but does not include <util/delay.h>, so the compiler will complain about an undeclared function.

🔧 Debug

advanced

Why does this LED blink code fail to toggle the LED?

This code is supposed to blink an LED on PORTC pin 5, but the LED stays off. What is the bug?

Embedded C

#include <avr/io.h>
#include <util/delay.h>

int main(void) {
    DDRC |= (1 << 5); // Set PORTC5 as output
    while(1) {
        PORTC = (1 << 5); // Turn LED on
        _delay_ms(300);
        PORTC = 0; // Turn LED off
        _delay_ms(300);
    }
    return 0;
}

A PORTC assignment overwrites other pins; should use PORTC ^= (1 << 5) to toggle.

B DDRC is not set correctly; should use DDRC = (1 << 5) instead of |=.

C Delay time is too short to see blinking.

D Missing return statement in main function.

💡 Hint

Think about how PORTC is assigned inside the loop and what happens to other pins.

Assigning PORTC directly overwrites all pins, so toggling does not happen properly. Using XOR toggle (^) preserves other pins and toggles the LED.

📝 Syntax

advanced

Which option fixes the syntax error in this LED blink code?

Identify the option that corrects the syntax error in this code snippet:

#include <avr/io.h>
#include <util/delay.h>

int main(void) {
    DDRB |= (1 << 3) // Set PORTB3 as output
    while(1) {
        PORTB ^= (1 << 3);
        _delay_ms(250);
    }
    return 0;
}

A Remove the while loop braces {}

B Replace _delay_ms(250) with delay(250)

C Change DDRB |= (1 << 3) to DDRB = (1 << 3)

D Add a semicolon after DDRB |= (1 << 3);

💡 Hint

Look carefully at the line before the while loop.

The missing semicolon after DDRB |= (1 << 3) causes a syntax error. Adding it fixes the code.

🚀 Application

expert

How many times will the LED blink in 10 seconds?

This code blinks an LED connected to PORTD pin 1. How many times will the LED turn on and off in 10 seconds?

Embedded C

#include <avr/io.h>
#include <util/delay.h>

int main(void) {
    DDRD |= (1 << 1); // Set PORTD1 as output
    for (int i = 0; i < 20; i++) {
        PORTD ^= (1 << 1); // Toggle LED
        _delay_ms(250);
    }
    return 0;
}

A 10 times

B 20 times

C 40 times

D 5 times

💡 Hint

Each toggle changes LED state. Count full on-off cycles in 10 seconds.

The loop toggles LED 20 times with 250 ms delay each toggle. One full blink cycle (on + off) takes 2 toggles = 500 ms. So 20 toggles = 5 seconds, which equals 10 full blinks.