How to Use defmt for Efficient Logging on Embedded Devices

Use the defmt crate to replace standard logging with a binary, zero-cost logging system that reduces flash usage and improves performance on embedded devices. Add the defmt and defmt-rtt dependencies to your Cargo.toml, initialize the logger in your entry point, and use defmt::info! macros instead of println!.

[dependencies]
defmt = "0.3"
defmt-rtt = "0.4"

[profile.release]
opt-level = "s"

#![no_std]
#![no_main]

use defmt_rtt::Config;
use cortex_m_rt::entry;
use panic_probe as _;

#[entry]
fn main() -> ! {
    defmt_rtt::init(Config::default());
    defmt::info!("Hello, embedded!");
    loop {}
}

// In your application logic
defmt::info!("Sensor value: {}", value);

Connect to the device using defmt decode to view the logs in real-time:

defmt decode -f /dev/ttyUSB0

defmt is a logging tool for tiny computers that sends messages as compact binary codes instead of text. This saves memory and makes your device run faster because it doesn't waste time formatting strings. Think of it like sending a secret code instead of writing a full sentence to save paper.