LED-based debugging patterns in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output pattern of the LED after running this code?

Consider an embedded system where an LED is toggled every 500ms inside a loop. What pattern will the LED show?

Embedded C

void main() {
    while(1) {
        toggleLED(); // toggles LED state
        delay(500); // delay 500 milliseconds
    }
}

A LED blinks ON and OFF every 500ms continuously

B LED stays ON permanently

C LED stays OFF permanently

D LED blinks ON and OFF every 1000ms continuously

💡 Hint

Think about what toggle means and the delay duration.

The toggleLED() function changes the LED state from ON to OFF or OFF to ON each time it is called. Since it is called every 500ms, the LED changes state every 500ms, resulting in a blink every 500ms.

🧠 Conceptual

intermediate

Why use LED blinking patterns for debugging in embedded systems?

Which of the following is the main reason developers use LED blinking patterns for debugging embedded systems?

A To save power by turning LEDs on and off

B To test the LED hardware for manufacturing defects

C To improve the speed of the embedded processor

D To visually indicate system states or errors without a display or debugger

💡 Hint

Think about what is available when debugging embedded devices without screens.

LED blinking patterns provide a simple way to show system status or error codes visually when no screen or debugger is available.

❓ Predict Output

advanced

What is the LED output after running this code snippet?

This code uses a pattern to blink an LED twice quickly, then pause. What is the LED behavior?

Embedded C

void main() {
    while(1) {
        turnLEDOn();
        delay(200);
        turnLEDOff();
        delay(200);
        turnLEDOn();
        delay(200);
        turnLEDOff();
        delay(1000);
    }
}

A LED stays ON continuously

B LED blinks twice quickly, then stays off for 1 second, repeating

C LED blinks continuously with 200ms ON and 200ms OFF

D LED blinks once every 1 second

💡 Hint

Count the ON and OFF durations and the pause.

The LED turns ON for 200ms, OFF for 200ms, ON for 200ms, then OFF for 1000ms, creating two quick blinks followed by a longer pause.

🔧 Debug

advanced

What error does this LED blinking code cause?

This code is intended to blink an LED every second, but it does not work as expected. What error does it cause?

Embedded C

void main() {
    while(1) {
        turnLEDOn();
        delay(1000);
        turnLEDOff();
        delay(1000);
    }
}

A SyntaxError due to missing semicolon after delay(1000)

B Runtime error because turnLEDOff() is undefined

C LED stays ON permanently

D No error, code runs correctly

💡 Hint

Check punctuation carefully in C code.

The missing semicolon after delay(1000) causes a syntax error, preventing compilation.

🚀 Application

expert

How many unique LED blink states are shown by this code?

This code uses a 3-LED array to indicate system status by turning LEDs ON or OFF in a pattern. How many unique LED ON/OFF states does it produce in one full cycle?

Embedded C

void main() {
    int states[4][3] = {
        {1,0,0},
        {0,1,0},
        {0,0,1},
        {1,1,1}
    };
    int i = 0;
    while(1) {
        setLEDs(states[i][0], states[i][1], states[i][2]);
        delay(500);
        i = (i + 1) % 4;
    }
}

A 7 unique LED states

B 3 unique LED states

C 4 unique LED states

D 8 unique LED states

💡 Hint

Count the number of different LED ON/OFF combinations used.

The code cycles through 4 different LED states defined in the states array, so there are 4 unique states.