Embedded Basics

10 examples introducing no_std, cross targets, and the embedded Rust toolchain.

Prerequisites

• Rust 1.97.0 with embedded targets
• cargo install probe-rs-tools for flashing and debug

rustup target add thumbv7em-none-eabihf

Basic Examples

1. Embedded Target Triple

rustup target add thumbv7em-none-eabihf   # Cortex-M4F
rustup target add thumbv6m-none-eabi      # Cortex-M0+

• Triple encodes CPU, OS (none), and ABI
• Match triple to your microcontroller datasheet
• eabihf means hardware floating point
• Set target in .cargo/config.toml

2. .cargo/config.toml

[build]
target = "thumbv7em-none-eabihf"
 
[target.thumbv7em-none-eabihf]
runner = "probe-rs run --chip STM32F429ZITx"

• Default target avoids passing --target every build
• runner auto-flashes on cargo run
• Chip name must match probe-rs database
• Commit config per board crate

3. Memory Layout (memory.x)

MEMORY
{
  FLASH : ORIGIN = 0x08000000, LENGTH = 2048K
  RAM   : ORIGIN = 0x20000000, LENGTH = 192K
}

• Linker script defines flash and RAM regions
• cortex-m-rt crate expects memory.x
• Wrong lengths cause link failures or silent corruption
• Copy from chip HAL examples

4. Minimal no_std Crate

#![no_std]
#![no_main]
 
use panic_halt as _;
 
#[no_mangle]
pub extern "C" fn main() -> ! {
    loop {}
}

• #![no_std] removes the standard library
• #![no_main] replaces Rust's main with embedded entry
• panic_halt stops on panic (replace with better handler in prod)
• Real apps use HAL init from chip crate

5. defmt Logging

use defmt::info;
 
info!("system started");
info!("tick {}", 42);

• Structured logging over RTT or serial
• Lower overhead than println! on embedded
• Requires defmt feature on dependencies
• Decode logs with defmt-print on host

Related: Debugging Firmware

6. HAL Initialization Pattern

let dp = pac::Peripherals::take().unwrap();
let rcc = dp.RCC.constrain();
let clocks = rcc.cfgr.freeze();
let gpioa = dp.GPIOA.split();

• pac = peripheral access crate (regs)
• hal = safe wrappers over pac
• take() enforces singleton peripherals
• Pattern varies by stm32* / rp2040 / nrf HAL

7. Cargo Features for Boards

[features]
default = ["stm32f429"]
stm32f429 = ["stm32f4xx-hal/stm32f429"]

• One crate can support multiple boards via features
• HAL crates gate chip support behind features
• CI should build all supported features
• Document feature matrix in README

Intermediate Examples

8. probe-rs Flash and Debug

probe-rs list
probe-rs run --chip STM32F429ZITx target/thumbv7em-none-eabihf/debug/app

• Replaces OpenOCD + GDB for many workflows
• cargo embed config uses probe-rs under the hood
• Supports RTT and breakpoints
• Works with CMSIS-DAP and ST-Link probes

9. Embassy Async Firmware

#[embassy_executor::main]
async fn main(_spawner: Spawner) {
    let p = embassy_stm32::init(Default::default());
    // async tasks...
}

• Embassy brings async/await to embedded without an OS
• Executor drives tasks cooperatively
• Requires embassy ecosystem crates
• See async embedded page for full patterns

Related: Async on Embedded (Embassy)

10. Project Layout

firmware/
  .cargo/config.toml
  memory.x
  build.rs           # copy memory.x
  src/
    main.rs
  Cargo.toml         # hal + panic + defmt

• build.rs links memory.x via cortex-m-rt
• Keep board support in a separate crate for reuse
• Pin dependency versions for reproducible firmware
• Use cargo binutils for size analysis (cargo size)

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

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