Rust is a systems programming language that has gained popularity in recent years for its focus on safety, speed, and concurrency. One area where Rust shows great promise is in embedded development, particularly on Cortex-M microcontrollers. Cortex-M chips are ARM-based MCUs that are designed for low-power embedded applications. Using Rust for Cortex-M development offers some compelling advantages.
Why Use Rust for Cortex-M Development?
Here are some of the main reasons why Rust is well-suited for Cortex-M development:
- Memory safety without garbage collection – Rust’s ownership and borrowing system eliminates entire classes of memory bugs without imposing runtime overhead like garbage collection.
- Concurrency support – Rust’s thread safety makes it much easier to write concurrent firmware code.
- Excellent performance – Rust code compiles to efficient native machine code that is competitive with C/C++.
- Abstraction capabilities – Rust provides modern programming language features like enums, structs, traits, generics, etc. This makes firmware code easier to write, read and maintain.
- Large and growing ecosystem – The Rust ecosystem has crates for embedded development like embedded-hal, cortex-m-rtic, nb, etc.
- Portability – Rust’s compilation model makes it easy to cross-compile Rust code for deployment on embedded devices.
- Safety and correctness – Rust’s strong type system catches bugs at compile-time. The borrow checker prevents entire classes of bugs.
- No runtime or ISR costs – The Rust language has no runtime, so all code compiles to pure machine instructions with zero overhead.
For resource-constrained Cortex-M chips, Rust provides memory safety guarantees and excellent performance without imposing runtime costs like garbage collection. Concurrency support also makes Rust well-suited for more advanced Cortex-M SOCs.
Getting Started with Cortex-M Rust
Here are the basic steps to set up a Rust development environment for Cortex-M:
- Install the Rust compiler and toolchain from rustup.rs
- Get a cross-compiler toolchain for ARM Cortex-M. The easiest way is to use the arm-none-eabi-gcc toolchain.
- Install cargo-binutils for objdump, nm, etc. for embedded debugging.
- Add support for your Cortex-M MCU. Many chips already have Rust crates like stm32f1xx-hal, nrf52832-hal, etc.
- Install a debugger like OpenOCD, J-Link, etc. Rust has good GDB support.
- Create a Cargo project and configure the build settings for your Cortex-M chip.
- Start coding! The cortex-m-quickstart repo has template code to get started.
The cortex-m-rt crate provides runtime support for Rust on Cortex-M chips. Important things like the vector table, boot sequence, interrupt handling, etc. are all handled for you. The hal crate for your specific Cortex-M chip provides peripherals and registers mappings.
Unique Benefits of Rust for Cortex-M
Here are some ways that Rust provides unique benefits for Cortex-M development beyond what C provides:
Memory Safety
Rust’s ownership and borrowing system eliminates entire classes of memory bugs like use-after-free, double-frees, buffer overflows, etc. This is enforced at compile-time so you don’t pay any runtime costs.
Fearless Concurrency
Rust’s concurrency support through threads, channels, atomics, Mutexes, etc. makes concurrent firmware much easier to write. The compiler enforces thread safety.
Higher-Level Abstractions
Rust provides high-level features like enums, structs, traits, generics, operator overloading, etc. This enables code reuse and reduces boilerplate.
Easy Cross-Compilation
The Rust toolchain makes it easy to cross-compile your Rust crate for deployment on a Cortex-M target. Just run cargo build –target with the target spec.
Functionality Without Bloat
Rust provides a lot of functionality like package management, testing, documentation, etc. without imposing runtime bloat like garbage collection.
Platform-Specific Crates
Rust makes it easy to develop portable crates through abstractions like embedded-hal. But you can also access platform-specific registers and peripherals through device crates.
Macro Meta-Programming
Rust macros allow generating boilerplate code safely and hygienically. This reduces overhead for common routines.
FFI Interoperability
Rust provides ways to safely call C code using FFI. This allows gradually porting existing firmware code.
Cortex-M Development Techniques in Rust
Here are some useful techniques for developing firmware on Cortex-M chips with Rust:
Managing the Vector Table
The cortex-m-rt crate handles vector table management. You can override default handlers by defining your own.
Setting Up Memory Regions
You can configure RAM, flash, and stack memory regions using the memory.x file or linker scripts.
Configuring Peripherals
Device crates like stm32f30x-hal provide type-safe access to peripheral registers and functionality.
Starting Up Devices
Cortex-m-rt handles starting up devices. You can add chip-specific logic inside rust_begin_unwind.
Handling Interrupts
The interrupt attribute makes a function execute in interrupt context. Use Mutexes and atomics for data sharing.
Using DMA Transfers
DMA helps offload data transfers without CPU involvement. Abstract DMA behind a thread-safe API.
Interfacing External Hardware
Use embedded-hal traits to generically interface with external I2C, SPI, etc. devices.
Writing Reusable Libraries
Leverage traits and generics to write reusable crates for different Cortex-M chips.
Debugging Tips
Use panic messages, debug macros, and tools like GDB, JLink, OpenOCD, etc. for debugging.
Code Optimizations
Use optimizations in Cargo.toml and profile-guided optimization to improve performance.
Real-World Example Cortex-M Rust Projects
Here are some real-world Rust projects that target Cortex-M chips:
- Tock OS – An embedded operating system designed for running multiple concurrent applications on Cortex-M and RISC-V chips.
- MicroPython – An implementation of MicroPython that runs on Cortex-M chips with just 256KB of RAM.
- mynewt-rust – A Rust implementation of the Apache Mynewt IoT OS framework.
- Audio Player – An embedded MP3 audio player running on the STM32F4.
- Midi Controller – A DIY MIDI controller using a STM32 blue pill and Rust.
- Servo Control – Controlling hobby servos with an STM32F3 board.
- Stepper Driver – Driving stepper motors using a Cortex-M development board.
These projects demonstrate how Rust allows developers to build highly capable and safe embedded systems software on top of Cortex-M chips.
Conclusion
Rust’s focus on safety, speed and concurrency makes it an excellent choice for developing embedded software on ARM’s Cortex-M series microcontrollers. Rust provides memory safety guarantees to eliminate bugs and overheads at compile-time. It makes writing complex concurrent firmware much easier and safer. And the availability of easy to use device crates and extensive tooling ensures a productive development workflow. With its growing community and ecosystem, Rust on Cortex-M has the potential to become the default choice for embedded programming beyond C and C++.
