Crate cortex_m_rt [−] [src]

Minimal startup / runtime for Cortex-M microcontrollers

Features

This crate provides

  • Before main initialization of the .bss and .data sections.

  • Before main initialization of the FPU (for targets that have a FPU).

  • A panic_fmt implementation that just calls abort that you can opt into through the "abort-on-panic" Cargo feature. If you don't use this feature you'll have to provide the panic_fmt lang item yourself. Documentation here

  • A minimal start lang item to support the standard fn main() interface. (The processor goes to sleep (loop { asm!("wfi") }) after returning from main)

  • A linker script that encodes the memory layout of a generic Cortex-M microcontroller. This linker script is missing some information that must be supplied through a memory.x file (see example below).

  • A default exception handler tailored for debugging that lets you inspect what was the state of the processor at the time of the exception. By default, all exceptions are serviced by this handler but each exception can be individually overridden using the exception! macro. The default exception handler itself can also be overridden using the default_handler! macro.

  • A _sheap symbol at whose address you can locate a heap.

Example

Creating a new bare metal project. (I recommend you use the cortex-m-quickstart template as it takes of all the boilerplate shown here)

$ cargo new --bin app && cd $_

$ # add this crate as a dependency
$ edit Cargo.toml && cat $_
[dependencies.cortex-m-rt]
features = ["abort-on-panic"]
version = "0.3.0"

$ # tell Xargo which standard crates to build
$ edit Xargo.toml && cat $_
[dependencies.core]
stage = 0

[dependencies.compiler_builtins]
features = ["mem"]
stage = 1

$ # memory layout of the device
$ edit memory.x && cat $_
MEMORY
{
  /* NOTE K = KiBi = 1024 bytes */
  FLASH : ORIGIN = 0x08000000, LENGTH = 128K
  RAM : ORIGIN = 0x20000000, LENGTH = 8K
}

$ edit src/main.rs && cat $_

#![feature(used)]
#![no_std]

extern crate cortex_m_rt;

fn main() {
    // do something here
}

// As we are not using interrupts, we just register a dummy catch all
// handler
#[link_section = ".vector_table.interrupts"]
#[used]
static INTERRUPTS: [extern "C" fn(); 240] = [default_handler; 240];

extern "C" fn default_handler() {
    loop {}
}
$ cargo install xargo

$ xargo rustc --target thumbv7m-none-eabi -- \
      -C link-arg=-Tlink.x -C linker=arm-none-eabi-ld -Z linker-flavor=ld

$ arm-none-eabi-objdump -Cd $(find target -name app) | head

Disassembly of section .text:

08000400 <cortex_m_rt::reset_handler>:
 8000400:       b580            push    {r7, lr}
 8000402:       466f            mov     r7, sp
 8000404:       b084            sub     sp, #8

Symbol interfaces

This crate makes heavy use of symbols, linker sections and linker scripts to provide most of its functionality. Below are described the main symbol interfaces.

DEFAULT_HANDLER

This weak symbol can be overridden to override the default exception handler that this crate provides. It's recommended that you use the default_handler! to do the override, but below is shown how to manually override the symbol:

#[no_mangle]
pub extern "C" fn DEFAULT_HANDLER() {
    // do something here
}

.vector_table.interrupts

This linker section is used to register interrupt handlers in the vector table. The recommended way to use this section is to populate it, once, with an array of weak functions that just call the DEFAULT_HANDLER symbol. Then the user can override them by name.

Example

Populating the vector table

// Number of interrupts the device has
const N: usize = 60;

// Default interrupt handler that just calls the `DEFAULT_HANDLER`
#[linkage = "weak"]
#[naked]
#[no_mangle]
extern "C" fn WWDG() {
    unsafe {
        asm!("b DEFAULT_HANDLER" :::: "volatile");
        core::intrinsics::unreachable();
    }
}

// You need one function per interrupt handler
#[linkage = "weak"]
#[naked]
#[no_mangle]
extern "C" fn WWDG() {
    unsafe {
        asm!("b DEFAULT_HANDLER" :::: "volatile");
        core::intrinsics::unreachable();
    }
}

// ..

// Use `None` for reserved spots in the vector table
#[link_section = ".vector_table.interrupts"]
#[no_mangle]
#[used]
static INTERRUPTS: [Option<extern "C" fn()>; N] = [
    Some(WWDG),
    Some(PVD),
    // ..
];

Overriding an interrupt (this can be in a different crate)

// the name must match the name of one of the weak functions used to
// populate the vector table.
#[no_mangle]
pub extern "C" fn WWDG() {
    // do something here
}

memory.x

This file supplies the information about the device to the linker.

MEMORY

The main information that this file must provide is the memory layout of the device in the form of the MEMORY command. The command is documented here, but at a minimum you'll want to create two memory regions: one for Flash memory and another for RAM.

The program instructions (the .text section) will be stored in the memory region named FLASH, and the program static variables (the sections .bss and .data) will be allocated in the memory region named RAM.

_stack_start

This symbol provides the address at which the call stack will be allocated. The call stack grows downwards so this address is usually set to the highest valid RAM address plus one (this is an invalid address but the processor will decrement the stack pointer before using its value as an address).

If omitted this symbol value will default to ORIGIN(RAM) + LENGTH(RAM).

Example

Allocating the call stack on a different RAM region.

MEMORY
{
  /* call stack will go here */
  CCRAM : ORIGIN = 0x10000000, LENGTH = 8K
  FLASH : ORIGIN = 0x08000000, LENGTH = 256K
  /* static variables will go here */
  RAM : ORIGIN = 0x20000000, LENGTH = 40K
}

_stack_start = ORIGIN(CCRAM) + LENGTH(CCRAM);

_stext

This symbol indicates where the .text section will be located. If not specified in the memory.x file it will default to right after the vector table -- the vector table is always located at the start of the FLASH region.

The main use of this symbol is leaving some space between the vector table and the .text section unused. This is required on some microcontrollers that store some configuration information right after the vector table.

Example

Locate the .text section 1024 bytes after the start of the FLASH region.

_stext = ORIGIN(FLASH) + 0x400;

_sheap

This symbol is located in RAM right after the .bss and .data sections. You can use the address of this symbol as the start address of a heap region. This symbol is 4 byte aligned so that address will be a multiple of 4.

Example

extern crate some_allocator;

// Size of the heap in bytes
const SIZE: usize = 1024;

extern "C" {
    static mut _sheap: u8;
}

fn main() {
    unsafe {
        let start_address = &mut _sheap as *mut u8;
        some_allocator::initialize(start_address, SIZE);
    }
}

Macros

default_handler

This macro lets you override the default exception handler
exception

Assigns a handler to an exception