PWM

In this chapter, we will learn how to digitally control the brightness of the LED using Pulse-width modulation. We will continue using the same hello_world project from the previous workshops.

esp-hal crate provides a PWM driver in mcpwm module. Let's start by importing the necessary modules and types:

#![allow(unused)]
fn main() {
use esp_hal::mcpwm::operator::PwmPinConfig;
use esp_hal::mcpwm::timer::PwmWorkingMode;
use esp_hal::mcpwm::{McPwm, PeripheralClockConfig};
use esp_hal::time::Rate;
}

Next, let set up the PWM peripheral:

#![allow(unused)]
fn main() {
let clock_cfg = PeripheralClockConfig::with_frequency(Rate::from_mhz(40)).unwrap();
let mut mcpwm = McPwm::new(peripherals.MCPWM0, clock_cfg);
let timer_clock_cfg = clock_cfg
    .timer_clock_with_frequency(255, PwmWorkingMode::Increase, Rate::from_khz(20))
    .expect("could not determine parameters for the requested frequency");

mcpwm.operator0.set_timer(&mcpwm.timer0);
mcpwm.timer0.start(timer_clock_cfg);
}

💡 Note the use of expect. expect performs the same function as unwrap, but allows us to write a helpful error message. In general, use of unwrap is discouraged outside of "example" code. Ideally, the errors should be handled gracefully or at least, provide an explanation of what has happened.

We'll change the LED pin to GPIO16, so we can have active-high configuration:

#![allow(unused)]
fn main() {
let led = Output::new(
    peripherals.GPIO16,
    esp_hal::gpio::Level::Low,
    OutputConfig::default(),
);
}

Then, we will transfer GPIO to the PWM instance:

#![allow(unused)]
fn main() {
let mut led = mcpwm.operator0.with_pin_a(led, PwmPinConfig::UP_ACTIVE_HIGH);
}

💡 Note how we're re-using the name led. This is called rebinding and allows us to write cleaner code.

Finally, let's modify our application to cycle through different brightness levels in a loop:

#![allow(unused)]
fn main() {
let mut sequence = [32, 128, 255].iter().cycle();
loop {
    led.set_timestamp(*sequence.next().unwrap());
    delay.delay_millis(1000);
}
}