dev_stories

🟠 How to handle pulse width modulation

PWM (pulse width modulation) is used to control the power supplied to the load.

🔸 Classic method - reducing the voltage through a rheostat or potentiometer.
🔻 Disadvantage: inefficient, since part of the energy is lost as heat.

🔸 PWM method - supplying voltage in short pulses with a fixed amplitude.
Instead of changing the voltage, the duty cycle changes:

More on time → more power
Less on time → less power

This control method is efficient and causes almost no losses.

Brief

mainly used to control the power supplied to the device.
in some cases it can be used to transmit information for configuring the device.
has a low power loss and the ability to digitally/software control.

It has 2 main parameters

Frequency (switching frequency, Hz),
Duty cycle (percentage of time in "HIGH" state, from 0% to 100%).

Duty cycle

Percentage of time during which the signal is active
- signal with 20% duty cycle is active 20% of the time
- signal with 50% duty cycle is active 50% of the time
- signal with 80% duty cycle is active 80% of the time

The duty cycle can be set or controlled in 3 main ways
- Manual method (Use various knobs or sliders)
- Automatic (for example, using feedback on the output voltage in a pulse power supply)
- Software (using some kind of intelligent system that changes the pulse width depending on external logic or parameters)

Average voltage level

PWM - often used to vary power by changing the average voltage level across a load.

average load = peak voltage(5V) * duty cycle(20%) = 1V

Switching Frequency

Switching frequency, which is defined as the reciprocal of the time between the leading edges of the voltage pulses.
The required switching frequency depends on the load or application area
- can be from 10Hz to several kHz
- for applications where faster or more precise control is required - higher switching frequencies are used
- but in most applications the switching frequency is fixed
Signals can have the same duty cycles, but different switching frequencies

Examples of PWM Systems

System	Description
1. Visual Devices	● Screen brightness control (high frequency to avoid flickering) ● RGB LED color control
2. Audio	● Digital to analog conversion via filtering ● Use efficient Class D amplifiers
3. Switching Power Supplies (SMPS)	● The basis of switching converters ● Use MOSFETs to switch pulses
4. DC Motors	● Fan or pump speed control ● Lower duty cycle → lower speed ● Inertia smooths out operation
5. Servos	● PWM sets position, not speed ● Pulse width corresponds to deviation from neutral ● Power is supplied via a separate channel
6. Inverters (DC→AC)	● Generating an alternating signal from PWM pulses
7. Chargers	● Charging batteries (including from solar panels) ● PWM regulates average voltage to prevent overcharging and reverse current

🧱 Embedded Rust approach:

HAL library (e.g. stm32f1xx-hal, rp2040-hal, esp32-hal) provides API to peripherals.
PWM is usually implemented via timers (TIM).
Each PWM channel is assigned a GPIO pin and a timer.

🔨 General flow of creating PWM in Rust

Access PWM slices (Slices::new)
Configure PWM7 (it controls GPIO15 pin)
Configure channel B of this PWM (channel connected to GPIO15)
Connect GPIO15 to PWM as output
In the loop, change the duty cycle value to get the breathing effect

💡 Implementation: RP2040

use rp2040_hal::{self as hal, pwm::Slices};

    // PWM Slices Init
    let pwm_slices = Slices::new(pac.PWM, &mut pac.RESETS);
    // Get 7th slice (controls GPIO 14 (A) and GPIO 15 (B))
    let mut pwm_slice = pwm_slices.pwm7;

    pwm_slice.set_ph_correct(); // Phase correct mode = smoother signal
    pwm_slice.set_top(255); // 8-bit duty cycle precision
    pwm_slice.enable(); // enable slice

    // Get channel_b from 7th slice
    let mut channel_b = pwm_slice.channel_b;
    // Make channel_b output to 15th pin
    let _pin = channel_b.output_to(pins.gpio15);

    let mut duty: i16 = 0;
    let mut step: i16 = 1;

    loop {
        let _ = channel_b.set_duty_cycle(duty as u16);
        timer.delay_us(1000);

        duty += step;
        if duty >= 255 || duty <= 0 {
            step = -step;
        }
    }

🧰 Tips for working with PWM

Do not exceed the frequency - some devices work up to 1-10 kHz.
PWM resolution depends on the timer (8/10/12/16 bit).
For multi-channel PWM use different channels of the same timer or timers.
Use a logic analyzer or oscilloscope for visualization.
Timers often work asynchronously - glitches at startup are possible.
For silent PWM (LED) use frequencies > 1 kHz.

PWM (Pulse Width Modulation)