Build it

The first step is to build our "binary" crate. Because the microcontroller has a different architecture than your computer we'll have to cross compile. Cross compiling in Rust land is as simple as passing an extra --target flag to rustcor Cargo. The complicated part is figuring out the argument of that flag: the name of the target.

The microcontroller in the F3 has a Cortex-M4F processor in it. rustc knows how to cross compile to the Cortex-M architecture and provides 4 different targets that cover the different processor families within that architecture:

  • thumbv6m-none-eabi, for the Cortex-M0 and Cortex-M1 processors
  • thumbv7m-none-eabi, for the Cortex-M3 processor
  • thumbv7em-none-eabi, for the Cortex-M4 and Cortex-M7 processors
  • thumbv7em-none-eabihf, for the Cortex-M4F and Cortex-M7F processors

For the F3, we'll use the thumbv7em-none-eabihf target. Before cross compiling you have to download a pre-compiled version of the standard library (a reduced version of it actually) for your target. That's done using rustup:

rustup target add thumbv7em-none-eabihf

You only need to do the above step once; rustup will re-install a new standard library (rust-std component) whenever you update your toolchain.

With the rust-std component in place you can now cross compile the program using Cargo.

NOTE Make sure you are in the src/05-led-roulette directory and run cargo build command below to create the executable:

cargo build --target thumbv7em-none-eabihf

On your console you should see something like:

$ cargo build --target thumbv7em-none-eabihf
   Compiling typenum v1.12.0
   Compiling semver-parser v0.7.0
   Compiling version_check v0.9.2
   Compiling nb v1.0.0
   Compiling void v1.0.2
   Compiling autocfg v1.0.1
   Compiling cortex-m v0.7.1
   Compiling proc-macro2 v1.0.24
   Compiling vcell v0.1.3
   Compiling unicode-xid v0.2.1
   Compiling stable_deref_trait v1.2.0
   Compiling syn v1.0.60
   Compiling bitfield v0.13.2
   Compiling cortex-m v0.6.7
   Compiling cortex-m-rt v0.6.13
   Compiling r0 v0.2.2
   Compiling stm32-usbd v0.5.1
   Compiling stm32f3 v0.12.1
   Compiling usb-device v0.2.7
   Compiling cfg-if v1.0.0
   Compiling paste v1.0.4
   Compiling stm32f3-discovery v0.6.0
   Compiling embedded-dma v0.1.2
   Compiling volatile-register v0.2.0
   Compiling nb v0.1.3
   Compiling embedded-hal v0.2.4
   Compiling semver v0.9.0
   Compiling generic-array v0.14.4
   Compiling switch-hal v0.3.2
   Compiling num-traits v0.2.14
   Compiling num-integer v0.1.44
   Compiling rustc_version v0.2.3
   Compiling bare-metal v0.2.5
   Compiling cast v0.2.3
   Compiling quote v1.0.9
   Compiling generic-array v0.13.2
   Compiling generic-array v0.12.3
   Compiling generic-array v0.11.1
   Compiling panic-itm v0.4.2
   Compiling lsm303dlhc v0.2.0
   Compiling as-slice v0.1.4
   Compiling micromath v1.1.0
   Compiling accelerometer v0.12.0
   Compiling chrono v0.4.19
   Compiling aligned v0.3.4
   Compiling rtcc v0.2.0
   Compiling cortex-m-rt-macros v0.1.8
   Compiling stm32f3xx-hal v0.6.1
   Compiling aux5 v0.2.0 (~/embedded-discovery/src/05-led-roulette/auxiliary)
   Compiling led-roulette v0.2.0 (~/embedded-discovery/src/05-led-roulette)
    Finished dev [unoptimized + debuginfo] target(s) in 17.91s

NOTE Be sure to compile this crate without optimizations. The provided Cargo.toml file and build command above will ensure optimizations are off.

OK, now we have produced an executable. This executable won't blink any LEDs, it's just a simplified version that we will build upon later in the chapter. As a sanity check, let's verify that the produced executable is actually an ARM binary:

cargo readobj --target thumbv7em-none-eabihf --bin led-roulette -- --file-header

The cargo readobj .. above is equivalent to readelf -h target/thumbv7em-none-eabihf/debug/led-roulette and should produce something similar to:

$ cargo readobj --target thumbv7em-none-eabihf --bin led-roulette -- --file-header
    Finished dev [unoptimized + debuginfo] target(s) in 0.02s
ELF Header:
  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00
  Class:                             ELF32
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           ARM
  Version:                           0x1
  Entry point address:               0x8000195
  Start of program headers:          52 (bytes into file)
  Start of section headers:          818328 (bytes into file)
  Flags:                             0x5000400
  Size of this header:               52 (bytes)
  Size of program headers:           32 (bytes)
  Number of program headers:         4
  Size of section headers:           40 (bytes)
  Number of section headers:         22
  Section header string table index: 20

Next, we'll flash the program into our microcontroller.