PWM generation using timers in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the timer prescaler for PWM generation.

Embedded C

TIM_HandleTypeDef htim1;

void PWM_Init() {
    htim1.Init.Prescaler = [1];
    // Other initialization code
}

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

A65535

B79

C1000

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as prescaler causes no division, leading to very fast timer overflow.

The prescaler divides the timer clock. Setting it to 79 divides the clock by 80, which is common for PWM timing.

2fill in blank

medium

Complete the code to set the PWM period by configuring the timer's auto-reload register.

Embedded C

void PWM_Init() {
    htim1.Init.Period = [1];
    // Other initialization code
}

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

A999

B79

C10000

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting period to 0 disables counting, no PWM generated.

The period value sets the timer overflow count. 999 means the timer counts from 0 to 999, giving 1000 counts per PWM cycle.

3fill in blank

hard

Fix the error in setting the PWM pulse width for 50% duty cycle.

Embedded C

TIM_OC_InitTypeDef sConfigOC = {0};
sConfigOC.Pulse = [1];
// Other PWM channel configuration

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Ahtim1.Init.Period / 2

Bhtim1.Init.Period

Chtim1.Init.Prescaler

Dhtim1.Init.Period * 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using full period as pulse causes 100% duty cycle, no off time.

Pulse value sets the duty cycle. Half of the period gives 50% duty cycle.

4fill in blank

hard

Fill both blanks to configure the timer for PWM mode and enable the preload feature.

Embedded C

sConfigOC.OCMode = [1];
sConfigOC.OCPreload = [2];

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ATIM_OCMODE_PWM1

BTIM_OCMODE_TIMING

CTIM_OCNORELOAD_ENABLE

DTIM_OCPRELOAD_ENABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using timing mode disables PWM output.

OCMode must be set to PWM1 for PWM output. Preload enable allows updating pulse value safely.

5fill in blank

hard

Fill all three blanks to start the PWM signal on channel 1 and enable the timer.

Embedded C

HAL_TIM_PWM_Start(&htim1, [1]);
__HAL_TIM_ENABLE(&htim1);
__HAL_TIM_SET_COUNTER(&htim1, [2]);
htim1.Instance->CR1 |= [3];

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

ATIM_CHANNEL_1

B0

CTIM_CR1_CEN

DTIM_CHANNEL_2

Attempts:

3 left

💡 Hint

Common Mistakes

Starting PWM on wrong channel disables output.

Start PWM on channel 1, reset counter to 0, and enable timer counter by setting CEN bit.

Practice

1. What does PWM stand for in embedded C programming when using timers?

easy

A. Pulse Width Modulation

B. Power Wave Management

C. Programmed Write Mode

D. Peripheral Watchdog Monitor

Solution

1. Step 1: Understand the acronym PWM

   PWM is a common term in embedded systems related to signal control.

2. Step 2: Match PWM with its full form

   PWM stands for Pulse Width Modulation, which controls signal timing using timers.

3. Final Answer:

   Pulse Width Modulation -> Option A

4. Quick Check:

   PWM = Pulse Width Modulation [OK]

Hint: Remember PWM controls signal timing by pulse width [OK]

Common Mistakes:

• Confusing PWM with power management terms
• Thinking PWM is a mode for writing data
• Mixing PWM with watchdog timer functions

2. Which of the following is the correct way to start a timer for PWM in embedded C?

easy

A. TIM_StartPWM();

B. HAL_TIM_PWM_Start(&htim);

C. StartTimerPWM();

D. timer_pwm_start();

Solution

1. Step 1: Identify standard embedded C HAL function

   HAL libraries use the prefix HAL_TIM_PWM_Start for starting PWM timers.

2. Step 2: Check syntax correctness

   HAL_TIM_PWM_Start(&htim); uses correct syntax with a timer handle pointer, matching HAL style.

3. Final Answer:

   HAL_TIM_PWM_Start(&htim); -> Option B

4. Quick Check:

   HAL_TIM_PWM_Start(&htim) is correct syntax [OK]

Hint: HAL functions start with HAL_ prefix and use handles [OK]

Common Mistakes:

• Using incorrect function names without HAL prefix
• Missing the timer handle argument
• Using lowercase or camelCase incorrectly

3. Given the following code snippet, what is the duty cycle percentage of the PWM signal?

uint32_t period = 1000;
uint32_t pulse = 250;
// Timer configured with period and pulse
// PWM output uses pulse width = pulse

medium

A. 100%

B. 75%

C. 50%

D. 25%

Solution

1. Step 1: Understand duty cycle formula

   Duty cycle = (pulse / period) * 100%

2. Step 2: Calculate duty cycle with given values

   Duty cycle = (250 / 1000) * 100% = 25%

3. Final Answer:

   25% -> Option D

4. Quick Check:

   Duty cycle = 25% [OK]

Hint: Duty cycle = pulse divided by period times 100% [OK]

Common Mistakes:

• Confusing pulse with period
• Calculating 1000/250 instead of 250/1000
• Forgetting to multiply by 100 for percentage

4. Identify the error in this PWM timer initialization code snippet:

TIM_HandleTypeDef htim;
htim.Init.Period = 1000;
htim.Init.Pulse = 500;
HAL_TIM_PWM_Start(&htim);

medium

A. Missing call to HAL_TIM_PWM_ConfigChannel before starting PWM

B. The 'Pulse' field requires pointer syntax like htim->Init.Pulse

C. Period value should be set after starting PWM

D. Timer handle 'htim' must be a pointer, not a variable

Solution

1. Step 1: Check PWM initialization sequence

   Before starting PWM, the channel must be configured with pulse value using HAL_TIM_PWM_ConfigChannel.

2. Step 2: Identify missing configuration call

   The code sets pulse in Init (which is incorrect) and skips HAL_TIM_PWM_ConfigChannel call.

3. Final Answer:

   Missing call to HAL_TIM_PWM_ConfigChannel before starting PWM -> Option A

4. Quick Check:

   ConfigChannel call needed before HAL_TIM_PWM_Start [OK]

Hint: Always configure PWM channel before starting timer [OK]

Common Mistakes:

• Setting pulse in Init structure instead of channel config
• Starting PWM without configuring channel
• Assuming timer handle must be pointer variable

5. You want to generate a PWM signal with a 60% duty cycle and a frequency of 1 kHz using a timer with a 72 MHz clock. Which timer period and pulse values should you set? Assume timer prescaler is set to 71 (divides clock by 72).

hard

A. Period = 1000, Pulse = 600

B. Period = 72000, Pulse = 43200

C. Period = 999, Pulse = 600

D. Period = 71, Pulse = 43

Solution

1. Step 1: Calculate timer frequency after prescaler

   Timer clock = 72 MHz / (71 + 1) = 1 MHz (1 microsecond per tick)

2. Step 2: Calculate period for 1 kHz PWM frequency

   Period = (Timer frequency / PWM frequency) - 1 = (1,000,000 / 1,000) - 1 = 999

3. Step 3: Calculate pulse for 60% duty cycle

   Pulse = Period * 0.6 = 999 * 0.6 = 599.4 ≈ 600

4. Final Answer:

   Period = 999, Pulse = 600 -> Option C

5. Quick Check:

   Period 999 and pulse 600 give 60% duty cycle at 1 kHz [OK]

Hint: Divide clock by prescaler, then calculate period and pulse [OK]

Common Mistakes:

• Not adding 1 to prescaler or period calculations
• Using full clock without prescaler division
• Rounding pulse incorrectly