Duty cycle control for motor/LED in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the PWM duty cycle to 50%.

Embedded C

PWM_SetDutyCycle([1]);

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

A0

B255

C128

D64

Attempts:

3 left

💡 Hint

Common Mistakes

Using 255 sets the duty cycle to 100%, not 50%.

Using 0 turns the output off.

The PWM duty cycle value ranges from 0 to 255. 128 sets it to about 50%.

2fill in blank

medium

Complete the code to increase the duty cycle by 10%.

Embedded C

dutyCycle = dutyCycle + [1];

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

A26

B25

C10

D15

Attempts:

3 left

💡 Hint

Common Mistakes

Using 10 increases duty cycle by too little.

Using 25 is close but less accurate.

10% of 255 is about 25.5, rounded to 26 to increase duty cycle by 10%.

3fill in blank

hard

Fix the error in the code to prevent duty cycle overflow.

Embedded C

if (dutyCycle [1] 255) {
    dutyCycle = 255;
}

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

A>=

B<

C==

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>=' causes off-by-one error.

Using '<' or '==' does not prevent overflow.

The condition should check if dutyCycle is greater than 255 to cap it at max.

4fill in blank

hard

Fill both blanks to create a dictionary that maps LED names to their duty cycles if duty cycle is above 100.

Embedded C

ledDutyCycles = { [1]: [2] for [1] in leds if [2] > 100 }

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

Aled

BdutyCycle

Cleds[led]

DledDutyCycles

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'dutyCycle' as key or value is incorrect.

Using 'ledDutyCycles' inside comprehension causes error.

The dictionary comprehension uses 'led' as key and 'leds[led]' as value, filtering values above 100.

5fill in blank

hard

Fill all three blanks to create a dictionary of LEDs with duty cycles above 150, converting LED names to uppercase.

Embedded C

highDuty = { [1]: [2] for [3] in leds if [2] > 150 }

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

Aled.upper()

Bleds[led]

Cled

DdutyCycle

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'dutyCycle' as loop variable is wrong.

Not converting LED names to uppercase.

The dictionary keys are LED names in uppercase, values are duty cycles, iterating over 'led'.

Practice

1. What does the duty cycle control in a motor or LED?

easy

A. The percentage of time the device is ON during each cycle

B. The voltage supplied to the device

C. The frequency of the power supply

D. The temperature of the device

Solution

1. Step 1: Understand duty cycle meaning

   The duty cycle is the fraction of one period in which a signal or system is active.

2. Step 2: Relate to motor/LED control

   For motors or LEDs, it controls how long the device is ON in each cycle, affecting speed or brightness.

3. Final Answer:

   The percentage of time the device is ON during each cycle -> Option A

4. Quick Check:

   Duty cycle = ON time percentage [OK]

Hint: Duty cycle means ON time percentage per cycle [OK]

Common Mistakes:

• Confusing duty cycle with voltage
• Thinking it controls frequency
• Assuming it controls temperature

2. Which of the following is the correct way to set a 50% duty cycle using a timer compare register in embedded C?

easy

A. OCR0 = 0; // Full OFF

B. OCR0 = 255; // Full ON

C. OCR0 = 128; // Assuming 8-bit timer with max 255

D. OCR0 = 64; // 25% duty cycle

Solution

1. Step 1: Understand timer resolution

   An 8-bit timer counts from 0 to 255, so max value is 255.

2. Step 2: Calculate 50% duty cycle value

   50% of 255 is approximately 128, so setting OCR0 to 128 sets 50% duty cycle.

3. Final Answer:

   OCR0 = 128; // Assuming 8-bit timer with max 255 -> Option C

4. Quick Check:

   50% of 255 = 128 [OK]

Hint: Half of max timer value sets 50% duty cycle [OK]

Common Mistakes:

• Using max value for 50% duty cycle
• Using zero for 50% duty cycle
• Confusing 25% with 50%

3. Given the code snippet below, what will be the duty cycle percentage set for the LED?

unsigned char duty = 64;
unsigned char max = 255;
float dutyCycle = (duty / (float)max) * 100;

medium

A. 25%

B. 50%

C. 75%

D. 64%

Solution

1. Step 1: Calculate duty/max ratio

   Divide duty (64) by max (255): 64 / 255 ≈ 0.25098.

2. Step 2: Convert ratio to percentage

   Multiply by 100: 0.25098 * 100 ≈ 25.1%.

3. Final Answer:

   25% -> Option A

4. Quick Check:

   (64/255)*100 ≈ 25% [OK]

Hint: Divide duty by max, multiply by 100 for percentage [OK]

Common Mistakes:

• Using integer division without casting
• Confusing duty value with percentage
• Rounding incorrectly

4. Identify the error in the following code that attempts to set a 75% duty cycle on an 8-bit timer:

unsigned char duty = 75;
unsigned char max = 255;
OCR0 = duty / max * 255;

medium

A. OCR0 register cannot be assigned directly

B. Integer division causes duty/max to be zero

C. max value should be 100, not 255

D. Duty cycle calculation is correct

Solution

1. Step 1: Analyze division operation

   Both duty and max are unsigned char, so duty/max does integer division: 75/255 = 0 (integer).

2. Step 2: Effect on OCR0 assignment

   Multiplying 0 by 255 results in 0, so OCR0 is set to 0, not 75% duty cycle.

3. Final Answer:

   Integer division causes duty/max to be zero -> Option B

4. Quick Check:

   Integer division truncates to zero [OK]

Hint: Cast to float before division to avoid zero result [OK]

Common Mistakes:

• Ignoring integer division rules
• Assuming OCR0 cannot be assigned
• Using wrong max value

5. You want to smoothly increase the brightness of an LED from 0% to 100% duty cycle using PWM on an 8-bit timer. Which approach is best to implement this in embedded C?

hard

A. Use a fixed OCR0 value of 128 for half brightness

B. Set OCR0 to 255 once and leave it

C. Toggle the LED pin ON and OFF manually without PWM

D. Use a loop to increment OCR0 from 0 to 255 with a delay between steps

Solution

1. Step 1: Understand smooth brightness control

   Smooth brightness change requires gradually changing the duty cycle from 0 to max.

2. Step 2: Implement gradual PWM change

   Increment OCR0 from 0 to 255 in a loop with delays to create smooth brightness ramp.

3. Final Answer:

   Use a loop to increment OCR0 from 0 to 255 with a delay between steps -> Option D

4. Quick Check:

   Gradual OCR0 change = smooth brightness [OK]

Hint: Increment OCR0 gradually with delay for smooth fade [OK]

Common Mistakes:

• Setting OCR0 once for full brightness only
• Using fixed mid value for all brightness
• Not using PWM for brightness control