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.

Practice

1. What is the main purpose of the first embedded program that blinks an LED?

easy

A. To read data from a sensor

B. To turn the LED on and off repeatedly

C. To send data over a network

D. To display text on a screen

Solution

1. Step 1: Understand the program's goal

   The program is designed to control an LED by switching it on and off.

2. Step 2: Identify the blinking action

   Blinking means turning the LED on, then off repeatedly to create a visible effect.

3. Final Answer:

   To turn the LED on and off repeatedly -> Option B

4. Quick Check:

   LED blinking = on/off repeatedly [OK]

Hint: Blinking means LED toggles on and off repeatedly [OK]

Common Mistakes:

• Confusing blinking with sensor reading
• Thinking it sends data over network
• Assuming it displays text

2. Which line correctly sets the pin connected to the LED as an output in embedded C?

easy

A. DDRB |= (1 << 5);

B. PORTB |= (1 << 5);

C. PINB |= (1 << 5);

D. DDRB &= ~(1 << 5);

Solution

1. Step 1: Understand DDR register usage

   DDRB controls the data direction of port B pins; setting a bit to 1 makes it output.

2. Step 2: Analyze each option

   DDRB |= (1 << 5); sets bit 5 of DDRB to 1, configuring pin 5 as output. PORTB |= (1 << 5); sets PORTB bit, which controls output level, not direction. PINB |= (1 << 5); sets PINB, which reads input pins. DDRB &= ~(1 << 5); clears bit 5 in DDRB, making it input.

3. Final Answer:

   DDRB |= (1 << 5); -> Option A

4. Quick Check:

   Set DDRB bit = output pin [OK]

Hint: Use DDRx |= (1 << pin) to set output pin [OK]

Common Mistakes:

• Using PORT register to set direction
• Using PIN register instead of DDR
• Clearing DDR bit instead of setting

3. Given the code snippet below, what will be the output behavior of the LED?

DDRB |= (1 << 5);
while(1) {
  PORTB |= (1 << 5);
  _delay_ms(500);
  PORTB &= ~(1 << 5);
  _delay_ms(500);
}

medium

A. LED stays ON permanently

B. LED blinks with 1 second ON and 1 second OFF

C. LED stays OFF permanently

D. LED blinks with 500ms ON and 500ms OFF

Solution

1. Step 1: Analyze pin direction and control

   DDRB |= (1 << 5) sets pin 5 as output. PORTB |= (1 << 5) turns LED ON. PORTB &= ~(1 << 5) turns LED OFF.

2. Step 2: Understand delay timing

   Each _delay_ms(500) pauses for 500 milliseconds, so LED is ON for 500ms, then OFF for 500ms, repeating.

3. Final Answer:

   LED blinks with 500ms ON and 500ms OFF -> Option D

4. Quick Check:

   Delay 500ms ON/OFF = blinking every 1 second [OK]

Hint: Delay 500ms twice means 1 second blink cycle [OK]

Common Mistakes:

• Confusing delay time units
• Assuming LED stays always ON or OFF
• Ignoring bitwise operations on PORTB

4. Identify the error in the following embedded C code for blinking an LED:

DDRB = (1 << 5);
while(1) {
  PORTB = (1 << 5);
  _delay_ms(500);
  PORTB = 0;
  _delay_ms(500);
}

medium

A. DDRB should use |= instead of = to avoid clearing other pins

B. PORTB should use &= ~ instead of = to turn off LED

C. _delay_ms function is missing a header file

D. The code has no errors and will blink LED correctly

Solution

1. Step 1: Check DDRB assignment

   Using DDRB = (1 << 5) overwrites all other bits, possibly affecting other pins. Using DDRB |= (1 << 5) sets only bit 5 without changing others.

2. Step 2: Verify PORTB usage

   PORTB = (1 << 5) and PORTB = 0 are valid to turn LED ON and OFF by setting and clearing all bits respectively.

3. Final Answer:

   DDRB should use |= instead of = to avoid clearing other pins -> Option A

4. Quick Check:

   Use |= to set bits without clearing others [OK]

Hint: Use |= to set bits, not = which clears others [OK]

Common Mistakes:

• Overwriting DDRB clears other pin directions
• Confusing PORTB clearing methods
• Assuming no header needed for delay

5. You want to modify the LED blink program to blink twice as fast without changing the delay function. Which change should you make?

hard

A. Change the LED pin to a different port

B. Reduce both delay calls to 250ms

C. Toggle the LED pin twice inside the same delay period

D. Increase the delay calls to 1000ms

Solution

1. Step 1: Understand blinking speed relation

   Blink speed depends on how often the LED state changes. Delays control timing between toggles.

2. Step 2: Analyze options

   Reduce both delay calls to 250ms changes delay times, but question says without changing delay function. Toggle the LED pin twice inside the same delay period toggles LED pin twice in same delay period, effectively doubling blink frequency. Change the LED pin to a different port changes port, unrelated to speed. Increase the delay calls to 1000ms increases delay, slowing blink.

3. Final Answer:

   Toggle the LED pin twice inside the same delay period -> Option C

4. Quick Check:

   More toggles per delay = faster blink [OK]

Hint: Toggle LED more often per delay to speed up blink [OK]

Common Mistakes:

• Changing delay values despite instructions
• Changing port does not affect blink speed
• Increasing delay slows blinking