Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

Compiling for Microcontroller

Now let's talk about embedded systems. When it comes to compiling Rust code for a microcontroller, things work a little differently from normal desktop systems. Microcontrollers don’t usually run a full operating system like Linux or Windows. Instead, they run in a minimal environment, often with no OS at all. This is called a bare-metal environment.

Rust supports this kind of setup through its no_std mode. In normal Rust programs, the standard library (std) handles things like file systems, threads, heap allocation, and I/O. But none of those exist on a bare-metal microcontroller. So instead of std, we use a much smaller core library, which provides only the essential building blocks.

The Target Triple for Pico 2

The Raspberry Pi Pico 2 (RP2350 chip), as you already know that it is unique; it contains selectable ARM Cortex-M33 and Hazard3 RISC-V cores . You can choose which processor architecture to use.

ARM Cortex-M33 Target

For ARM mode, we have to use the target [thumbv8m.main-none-eabi](https://doc.rust-lang.org/nightly/rustc/platform-support/thumbv8m.main-none-eabi.html):

Let's break this down:

  • Architecture (thumbv8m.main): The Cortex-M33 uses the ARM Thumb-2 instruction set for ARMv8-M architecture.
  • Vendor (none): No specific vendor designation.
  • OS (none): No operating system - it's bare-metal.
  • ABI (eabi): Embedded Application Binary Interface, the standard calling convention for embedded ARM systems.

To install and use this target:

rustup target add thumbv8m.main-none-eabi
cargo build --target thumbv8m.main-none-eabi

RISC-V Hazard3 Target

For RISC-V mode, use the target [riscv32imac-unknown-none-elf](https://doc.rust-lang.org/nightly/rustc/platform-support/riscv32-unknown-none-elf.html):

riscv32imac-unknown-none-elf

Let's break this down:

  • Architecture (riscv32imac): 32-bit RISC-V with I (integer), M (multiply/divide), A (atomic), and C (compressed) instruction sets.
  • Vendor (unknown): No specific vendor.
  • OS (none): No operating system - it's bare-metal.
  • Format (elf): ELF (Executable and Linkable Format), the object file format commonly used in embedded systems.

To install and use this target:

rustup target add riscv32imac-unknown-none-elf
cargo build --target riscv32imac-unknown-none-elf

In our exercises, we'll mostly use the ARM mode. Some crates like panic-probe don't work in RISC-V mode.

Cargo Config

In the quick start, you might have noticed that we never manually passed the --target flag when running the cargo command. So how did it know which target to build for? That's because the target was already configured in the .cargo/config.toml file.

This file lets you store cargo-related settings, including which target to use by default. To set it up for Pico 2 in ARM mode, create a .cargo folder in your project root and add a config.toml file with the following content:

[build]
target = "thumbv8m.main-none-eabihf"

Now you don't have to pass --target every time. Cargo will use this automatically.