Introduction to PWM

Pulse Width Modulation (PWM) is one of the most useful and versatile features of microcontrollers. It allows you to control the power delivered to a device by rapidly switching between ON and OFF states. This simple concept enables loads of interesting applications: dimming LEDs, controlling motor speeds, generating analog signals from digital outputs, and even creating audio.

The important thing to remember is this: if you switch something on and off fast enough, the average power delivered depends on how long it stays ON versus OFF.

Why Not Just Use GPIO?

You might wonder: “Why can’t I just toggle a GPIO pin on and off to control an LED’s brightness?” Technically, you could, but it comes with serious problems:

// Manual "bit-banging" approach (NOT RECOMMENDED)
while (1) {
    HAL_GPIO_WritePin(GPIOA, GPIO_PIN_5, GPIO_PIN_SET);    // ON
    HAL_Delay(25);
    HAL_GPIO_WritePin(GPIOA, GPIO_PIN_5, GPIO_PIN_RESET);  // OFF
    HAL_Delay(75);
}

The problems are:

  • Your CPU is blocked during delays, preventing other tasks
  • Precision suffers because HAL_Delay() isn’t exact as it can be affected by interrupts and other code
  • No responsiveness to events like button presses while timing
  • Inflexible frequencies – changing speed requires code changes
  • Power inefficient – the CPU is active the whole time, consuming more power

The Hardware PWM Solution

This is exactly why hardware PWM exists. Just like hardware timers run independently in the background, PWM uses timers to generate the switching signal automatically. You configure it once, and the hardware does the rest while your CPU continues running other code.

Here’s what a PWM signal looks like:

Period
|<---------->|
         ___        ___        ___    
    ____|   |______|   |______|   |___
Duty   ^    ^
Cycle  |    |

A PWM signal is characterized by two parameters:

  1. Frequency (Period): How fast the signal switches on and off
  2. Duty Cycle: The percentage of time the signal stays HIGH

For example:

  • 50% duty cycle = signal is ON half the time, OFF half the time = 50% average power
  • 25% duty cycle = signal is ON for 1/4 of the period = 25% average power
  • 100% duty cycle = signal is always ON = full power

Practical Applications

PWM is used everywhere in embedded systems:

LED Brightness Control

Instead of just ON/OFF, you can smoothly dim an LED by adjusting the duty cycle. 10% brightness = 10% duty cycle, 50% brightness = 50% duty cycle, etc.

Motor Speed Control

Motors respond to average voltage. By controlling the duty cycle of a PWM signal, you can make a motor run at any speed from stopped to full speed.

Servo Control

Servo motors interpret PWM pulse widths as position commands. Different pulse widths move the servo to different angles.

Audio and Signal Generation

PWM signals can be filtered with capacitors and resistors to generate analog voltage levels, allowing you to create audio or control analog circuits from a digital microcontroller.

How PWM Uses Hardware Timers

Remember from the timers introduction that a hardware timer counts from 0 to its period value, then resets and triggers an interrupt. PWM extends this idea:

The timer can automatically toggle a GPIO pin when the counter reaches a specific value (the compare value). This happens without any CPU involvement, or without the need for an ISR. The timer handles the switching, and you just set the parameters:

  1. CPU configures: “Toggle pin when counter reaches 25% of the period”
  2. Timer counts from 0 to 100% of period, toggling the pin at 25%
  3. Timer resets and repeats
  4. CPU is completely free to do other things

The duty cycle is simply the ratio of when the pin gets toggled:

\[\text{Duty Cycle (\%)} = \frac{\text{Compare Value}}{\text{Period}} \times 100\]

Key PWM Parameters

When you configure PWM on an STM32 microcontroller, you’ll set these parameters:

Parameter Meaning Example
Frequency How fast PWM switches (Hz or kHz) 1000 Hz = 1 millisecond period
Period Timer counts from 0 to this value 80 (with 1 MHz timer = 80 µs period)
Prescaler Clock divider (from timer setup) 80 (divides 80 MHz clock by 80)
Pulse/Compare When to toggle during the period 20 (means 20/80 = 25% duty cycle)

What’s Next?

Now that you understand what PWM does and why it’s useful, the next steps are:

  1. Learn how to configure PWM on STM32 microcontrollers using HAL
  2. Understand how to change duty cycle and frequency in code
  3. Explore practical applications like LED dimming and motor control
  4. Discover specialized PWM modes for specific applications

PWM transforms what would be a tedious, CPU-intensive task into something the hardware handles automatically. Let’s get started implementing it!

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