First embedded program (LED blink) in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to turn on the LED by setting the pin high.

Embedded C

PORTB |= (1 << [1]);

Drag options to blanks, or click blank then click option'

A5

B7

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong pin number.

Forgetting to use bit shift operator.

Pin 3 is used to control the LED, so setting bit 3 high turns it on.

2fill in blank

medium

Complete the code to configure pin 3 of PORTB as output.

Embedded C

DDRB |= (1 << [1]);

Drag options to blanks, or click blank then click option'

A4

B3

C2

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Configuring the wrong pin.

Using '=' instead of '|=' operator.

Setting bit 3 in DDRB configures pin 3 as an output.

3fill in blank

hard

Fix the error in the delay loop to create a visible LED blink.

Embedded C

for (int i = 0; i < [1]; i++) { __asm__("nop"); }

Drag options to blanks, or click blank then click option'

A1000

B100

C10

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using too small a delay value.

Forgetting to include the loop.

A delay of 1000 iterations creates a visible blink delay.

4fill in blank

hard

Fill both blanks to toggle the LED on pin 3 of PORTB.

Embedded C

PORTB [1]= (1 << [2]);

Drag options to blanks, or click blank then click option'

A^

B|

C3

D&

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR (|) instead of XOR (^).

Using wrong pin number.

Using XOR (^) with bit 3 toggles the LED state.

5fill in blank

hard

Fill all four blanks to create a complete LED blink cycle: turn on, delay, turn off.

Embedded C

PORTB [1]= (1 << [2]);
for (int i = 0; i < [3]; i++) { __asm__("nop"); }
PORTB [4]= ~(1 << [2]);

Drag options to blanks, or click blank then click option'

A|=

B3

C1000

D&=

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operators for turning off the LED.

Delay too short to see blink.

Use '|=' to turn on the LED, delay with 1000 loops, then '&=' with negated bit to turn off.

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