LED roulette

Alright, let's start by building the following application:

I'm going to give you a high level API to implement this app but don't worry we'll do low level stuff later on. The main goal of this chapter is to get familiar with the flashing and debugging process.

The starter code is in the src directory of the book repository. Inside that directory there are more directories named after each chapter of this book. Most of those directories are starter Cargo projects.

Now, jump into the src/05-led-roulette directory. Check the src/main.rs file:


use cortex_m_rt::entry;
use panic_halt as _;
use microbit as _;

fn main() -> ! {
    let _y;
    let x = 42;
    _y = x;

    // infinite loop; just so we don't leave this stack frame
    loop {}

Microcontroller programs are different from standard programs in two aspects: #![no_std] and #![no_main].

The no_std attribute says that this program won't use the std crate, which assumes an underlying OS; the program will instead use the core crate, a subset of std that can run on bare metal systems (i.e., systems without OS abstractions like files and sockets).

The no_main attribute says that this program won't use the standard main interface, which is tailored for command line applications that receive arguments. Instead of the standard main we'll use the entry attribute from the cortex-m-rt crate to define a custom entry point. In this program we have named the entry point "main", but any other name could have been used. The entry point function must have signature fn() -> !; this type indicates that the function can't return -- this means that the program never terminates.

If you are a careful observer, you'll also notice there is a .cargo directory in the Cargo project as well. This directory contains a Cargo configuration file (.cargo/config) that tweaks the linking process to tailor the memory layout of the program to the requirements of the target device. This modified linking process is a requirement of the cortex-m-rt crate.

Furthermore, there is also an Embed.toml file

# chip = "nrf52833_xxAA" # uncomment this line for micro:bit V2
# chip = "nrf51822_xxAA" # uncomment this line for micro:bit V1

halt_afterwards = true

enabled = false

enabled = true

This file tells cargo-embed that:

  • we are working with either a nrf52833 or nrf51822, you will again have to remove the comments from the chip you are using, just like you did in chapter 3.
  • we want to halt the chip after we flashed it so our program does not instantly jump to the loop
  • we want to disable RTT, RTT being a protocol that allows the chip to send text to a debugger. You have in fact already seen RTT in action, it was the protocol that sent "Hello World" in chapter 3.
  • we want to enable GDB, this will be required for the debugging procedure

Alright, let's start by building this program.