PWM generation using timers in Embedded C - Mini Project: Build & Apply

PWM generation using timers

📖 Scenario: You are working on a small embedded system that controls the brightness of an LED using PWM (Pulse Width Modulation). PWM is generated using a timer peripheral in the microcontroller.This project will guide you through setting up the timer registers to generate a PWM signal with a specific duty cycle.

🎯 Goal: Build a simple embedded C program that configures a timer to generate a PWM signal with a 50% duty cycle on a specific output pin.

📋 What You'll Learn

Create variables to represent timer registers

Set the timer period to a fixed value

Set the duty cycle to 50%

Configure the timer control register to enable PWM mode

Print the configured register values

💡 Why This Matters

🌍 Real World

PWM signals are used to control LED brightness, motor speed, and other devices by adjusting the power delivered.

💼 Career

Understanding timer-based PWM generation is essential for embedded systems engineers working on hardware control and device drivers.

1

Create timer register variables

Create three unsigned int variables called TIMER_PERIOD, TIMER_DUTY, and TIMER_CONTROL and initialize them all to 0.

Embedded C

// Create three unsigned int variables TIMER_PERIOD, TIMER_DUTY, and TIMER_CONTROL initialized to 0
// Your code here

Hint

Use unsigned int type and set each variable to 0.

2

Set timer period

Set the variable TIMER_PERIOD to 1000 to represent the timer period for PWM.

Embedded C

unsigned int TIMER_PERIOD = 0;
unsigned int TIMER_DUTY = 0;
unsigned int TIMER_CONTROL = 0;

// Set TIMER_PERIOD to 1000
// Your code here

Hint

Assign 1000 to TIMER_PERIOD.

3

Set duty cycle to 50%

Set the variable TIMER_DUTY to half of TIMER_PERIOD using the expression TIMER_PERIOD / 2.

Embedded C

unsigned int TIMER_PERIOD = 1000;
unsigned int TIMER_DUTY = 0;
unsigned int TIMER_CONTROL = 0;

// Set TIMER_DUTY to half of TIMER_PERIOD
// Your code here

Hint

Use TIMER_PERIOD / 2 to set TIMER_DUTY.

4

Enable PWM mode and print registers

Set TIMER_CONTROL to 0x01 to enable PWM mode. Then write printf statements to print TIMER_PERIOD, TIMER_DUTY, and TIMER_CONTROL values.

Embedded C

unsigned int TIMER_PERIOD = 1000;
unsigned int TIMER_DUTY = TIMER_PERIOD / 2;
unsigned int TIMER_CONTROL = 0;

// Enable PWM mode and print register values
// Your code here

Hint

Set TIMER_CONTROL = 0x01; and use printf to display all three variables.

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