PWM generation using timers in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

PWM Duty Cycle Calculation Output

What is the output of the following code that calculates the PWM duty cycle percentage?

Embedded C

unsigned int timer_period = 1000;
unsigned int pulse_width = 250;
float duty_cycle = (pulse_width / (float)timer_period) * 100;
printf("Duty Cycle: %.1f%%\n", duty_cycle);

A Duty Cycle: 2.5%

B Duty Cycle: 0.0%

C Duty Cycle: 25.0%

D Duty Cycle: 250.0%

💡 Hint

Remember to cast integers to float before division to get a decimal result.

The pulse width divided by the timer period gives the fraction of the period the signal is high. Multiplying by 100 converts it to a percentage. Casting to float ensures decimal division.

🧠 Conceptual

intermediate

Timer Configuration for PWM Frequency

Which timer configuration will produce a PWM signal with a frequency of 1 kHz if the timer clock is 1 MHz?

A Set timer period to 1000 and prescaler to 1

B Set timer period to 100 and prescaler to 11

C Set timer period to 2000 and prescaler to 1

D Set timer period to 500 and prescaler to 3

💡 Hint

Frequency = timer_clock / (prescaler * period)

With a 1 MHz clock, setting prescaler to 1 and period to 1000 gives 1 MHz / (1 * 1000) = 1 kHz.

🔧 Debug

advanced

Identify the Error in PWM Initialization Code

What error will occur when running this PWM initialization code?

Embedded C

TIM_HandleTypeDef htim;
htim.Init.Prescaler = 0;
htim.Init.Period = 0;
HAL_TIM_PWM_Init(&htim);
HAL_TIM_PWM_Start(&htim, TIM_CHANNEL_1);

A No error, PWM starts with 0 frequency

B Runtime error: Division by zero in timer period calculation

C Compilation error: Missing semicolon

D Runtime error: Null pointer dereference

💡 Hint

Check the timer period value before initialization.

Setting period to zero causes division by zero when calculating timer frequency, leading to runtime error.

📝 Syntax

advanced

Correct Syntax for PWM Duty Cycle Update

Which option correctly updates the PWM duty cycle to 50% for a timer with period 1000?

Embedded C

/* Assume htim and channel are properly initialized */
uint32_t new_pulse = 500; // 50% of 1000
// Update PWM pulse here

A htim.Instance->CCR1 = new_pulse;

B htim->Instance->CCR1 = new_pulse;

C htim.Instance.CCR1 = new_pulse;

D htim.Instance->CCR1 = &new_pulse;

💡 Hint

Check the structure pointer and member access syntax.

htim is a structure, so use dot operator. Instance is a pointer, so use '->' to access CCR1.

🚀 Application

expert

Calculate PWM Signal Frequency from Timer Settings

Given a timer clock of 8 MHz, prescaler set to 7, and period set to 999, what is the PWM signal frequency?

A 100 Hz

B 8000 Hz

C 10000 Hz

D 1000 Hz

💡 Hint

Frequency = timer_clock / ((prescaler + 1) * (period + 1))

Frequency = 8,000,000 / ((7 + 1) * (999 + 1)) = 8,000,000 / (8 * 1000) = 1000 Hz.