Buzzer Beep Using PWM on Raspberry Pi Pico with Embedded Rust

In this exercise, we will generate a beep sound using a buzzer. The idea is similar to blinking an LED, but instead of toggling a GPIO HIGH and LOW, we control the PWM duty cycle.

We will repeatedly switch the PWM duty cycle between 50 percent and 0 percent, with a delay in between. When the duty cycle is 50 percent, the buzzer produces sound. When it is 0 percent, the sound stops. Repeating this creates a clear beep.

You can try this without changing the PWM frequency. In this example, we set the PWM frequency to 440.0 Hz, which corresponds to the A4 musical note. You do not need to know anything about musical notes for this. The important point is that we generate a fixed-frequency tone and turn the sound on and off by changing the duty cycle.

Create Project from template

We will start by creating a new project using the Embassy framework. As usual, generate the project from the template with cargo-generate:

cargo generate --git https://github.com/ImplFerris/pico2-template.git --tag v0.3.1

When prompted, give your project a name like “buzzer-beep” and choose “embassy” as the HAL. Enable defmt logging, if you have a debug probe so you can view logs also.

Additional imports

As we have done before, we import the SetDutyCycle trait and the Pwm and Config types for PWM configuration.

#![allow(unused)]
fn main() {
use embassy_rp::pwm::{Config as PwmConfig, Pwm, SetDutyCycle};
}

Calculate TOP

You can either calculate the TOP value manually or use the calculator form shown in the previous chapter. This time, we will take a different approach.

We will keep the divider fixed at 64 and use a const fn to calculate the TOP value. Since we are not using phase-correct mode or the fractional divider, the calculation is simple.

#![allow(unused)]
fn main() {
const fn get_top(freq: f64, div_int: u8) -> u16 {
    assert!(div_int != 0, "Divider must not be 0");

    let result = 150_000_000. / (freq * div_int as f64);

    assert!(result >= 1.0, "Frequency too high");
    assert!(
        result <= 65535.0,
        "Frequency too low: TOP exceeds 65534 max"
    );

    result as u16 - 1
}

const PWM_DIV_INT: u8 = 64;
const PWM_TOP: u16 = get_top(440., PWM_DIV_INT);
}

Main logic

First, we configure the PWM with the calculated TOP value and the fixed divider. Then we create a PWM output for the buzzer. Inside the loop, we switch the duty cycle between 50 percent and 0 percent with a delay in between, which produces a repeating beep sound.

#![allow(unused)]
fn main() {
let mut pwm_config = PwmConfig::default();
pwm_config.top = PWM_TOP;
pwm_config.divider = PWM_DIV_INT.into();

let mut buzzer = Pwm::new_output_b(p.PWM_SLICE7, p.PIN_15, pwm_config);

loop {
    buzzer
        .set_duty_cycle_percent(50)
        .expect("50 is valid duty percentage");
    Timer::after_millis(1000).await;

    buzzer
        .set_duty_cycle_percent(0)
        .expect("0 is valid duty percentage");
    Timer::after_millis(1000).await;
}
}

If you want the beep to be shorter or faster, you can adjust the delay values.

Clone the existing project

You can clone (or refer to) the project I created and navigate to the buzzer-beep folder.

git clone https://github.com/ImplFerris/pico2-embassy-projects
cd pico2-embassy-projects/buzzer-beep

rp-hal version

If you want to see the same example implemented using rp-hal, you can find it here.

git clone https://github.com/ImplFerris/pico2-rp-projects
cd pico2-rp-projects/buzzer-beep

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