LED control patterns in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output pattern of this LED control code?

Consider an 8-LED array controlled by this code snippet. What pattern will the LEDs show after running the loop?

Embedded C

for (int i = 0; i < 8; i++) {
    PORT = (1 << i);
    _delay_ms(100);
}

A LEDs light up one by one from right to left, only one LED on at a time.

B All LEDs turn on simultaneously and stay on.

C LEDs light up one by one from left to right, only one LED on at a time.

D LEDs blink all together on and off repeatedly.

💡 Hint

Look at the bit shift operation and how it affects the PORT value.

The code shifts a single bit from the least significant bit to the most significant bit, lighting one LED at a time from left to right.

❓ Predict Output

intermediate

What will be the final PORT value after this LED pattern code?

This code runs a pattern on an 8-LED array. What is the final value of PORT after the loop finishes?

Embedded C

PORT = 0;
for (int i = 0; i < 8; i++) {
    PORT |= (1 << i);
    _delay_ms(50);
}

A 1 (only the first LED on)

B 128 (only the last LED on)

C 0 (all LEDs off)

D 255 (all LEDs on)

💡 Hint

Think about how the OR operation accumulates bits.

The OR operation adds each bit one by one, so after 8 iterations all bits are set, making PORT 255.

🔧 Debug

advanced

Why does this LED blinking code fail to light LEDs correctly?

Identify the error in this LED blinking code that causes no LEDs to light up.

Embedded C

for (int i = 0; i < 8; i++) {
    PORT = (1 << i);
    PORT = 0;
    _delay_ms(100);
}

A The PORT is cleared immediately after setting, so LEDs turn off too fast to see.

B The bit shift operator is used incorrectly; it should be << instead of >>.

C The loop counter i is not initialized properly.

D The delay function is missing, so LEDs never light.

💡 Hint

Look at the order of setting and clearing PORT and the delay timing.

PORT is set then immediately cleared before the delay, so LEDs turn off instantly and appear not to light.

📝 Syntax

advanced

Which option contains a syntax error in this LED pattern code?

Find the option that will cause a syntax error when compiling this LED control snippet.

Embedded C

for (int i = 0; i < 8; i++) {
    PORT = (1 << i);
    _delay_ms(100);
}

A for (int i = 0; i < 8; i++) { PORT = (1 << i); _delay_ms(100); }

B for (int i = 0; i < 8 i++) { PORT = (1 << i); _delay_ms(100); }

C } ;)001(sm_yaled_ ;)i << 1( = TROP { )++i ;8 < i ;0 = i tni( rof

D or (int i = 0; i < 8; i++) { PORT = (1 << i); _delay_ms(100); }

💡 Hint

Check the for loop syntax carefully.

Option B misses a semicolon between the loop conditions, causing a syntax error.

🚀 Application

expert

How many unique LED patterns are generated by this code?

This code cycles through LED patterns on an 8-LED array. How many unique patterns does it produce?

Embedded C

for (int i = 0; i < 8; i++) {
    PORT = (0xFF >> i) << i;
    _delay_ms(100);
}

A 8 unique patterns

B 7 unique patterns

C 9 unique patterns

D 16 unique patterns

💡 Hint

Analyze how the bit shifts affect the LED pattern each iteration.

The loop runs 8 times, each time shifting the pattern, producing 8 unique LED patterns.