3. Building a crate for AVR

After setting up the compiler, you may use it to generate assembly or machine code targeting a specific AVR microcontroller model.

Choosing a --target

The Rust nightly compiler includes a built-in target for ATmega328 named avr-unknown-gnu-atmega328

If you wish to target a microcontroller other than ATmega328, or you want to change any of the default builtin options like the linking parameters, then you will need to export the builtin avr-unknown-gnu-atmega328 target to a custom target specification JSON file and modify it to suit your needs.

This target can be adapted to other microcontrollers as per the instructions in 3.1. The built-in avr-unknown-gnu-atmega328 target.

In summary, there are two options:

  • Use rustc --target=avr-unknown-gnu-atmega328 to use the default, builtin GCC based target for ATmega328
  • Or use rustc --target=my-custom-avr-target.json with either a JSON file adapted from the builtin avr-unknown-gnu-atmega328 target above, or otherwise build the file manually you wish to avoiding the default path entirely.

Make sure you use the nightly version of Rust, not the default stable channel

The best way to ensure a crate is using the Nightly compiler is to run rustup override set nightly inside a terminal within the root directory of the crate. After this is done, cargo will by-default use the AVR-enabled Nightly compiler any time cargo is used within the directory tree of the crate.

Compiling a crate

To compile and link an executable crate for AVR, run the following:

Using the builtin avr-unknown-gnu-atmega328 target:


#![allow(unused)]
fn main() {
cargo build -Z build-std=core --target avr-unknown-gnu-atmega328 --release
}

Using a custom target specification JSON:


#![allow(unused)]
fn main() {
cargo build -Z build-std=core --target /path/to/my-custom-avr-target.json --release
}

Either or these generate an AVR ELF file that can be subsequently flashed to a real device or ran inside a simulator. The ELF file will be available at target/<TARGET NAME>/release/<CRATE NAME>.elf.

Notes:

  • -Z build-std=core is required whenever AVR is being targeted. See 3.1. A note about the required Rust -Z build-std=<CRATE,> flag for more details.
  • --release is not strictly required - debug mode should be as correct as release mode - however, debug mode generates SLOW CODE, especially on AVR. Release mode is much better.

Example: An in-context example of compiling a crate is given for the LED blinking example in 3.2. Example - Building the blink program for AVR.

Targeting a different microcontroller model

The recommended way to do this is with a custom target specification JSON file per the instructions in 3.1. The built-in avr-unknown-gnu-atmega328 target.