The challenge

You are now well armed to face a challenge! Your task will be to implement the application I showed you at the beginning of this chapter.

Here's the GIF again:

Also, this may help:

This is a timing diagram. It indicates which LED is on at any given instant of time and for how long each LED should be on. On the X axis we have the time in milliseconds. The timing diagram shows a single period. This pattern will repeat itself every 800 ms. The Y axis labels each LED with a cardinal point: North, East, etc. As part of the challenge you'll have to figure out how each element in the Leds array maps to these cardinal points (hint: cargo doc --open ;-)).

Before you attempt this challenge, let me give you one last tip. Our GDB sessions always involve entering the same commands at the beginning. We can use a .gdb file to execute some commands right after GDB is started. This way you can save yourself the effort of having to enter them manually on each GDB session.

Place this openocd.gdb file in the root of the Cargo project, right next to the Cargo.toml:

$ cat openocd.gdb
target remote :3333
break main

Then modify the second line of the .cargo/config file:

$ cat .cargo/config
runner = "arm-none-eabi-gdb -q -x openocd.gdb" # <-
rustflags = [
  "-C", "link-arg=-Tlink.x",

With that in place, you should now be able to start a gdb session that will automatically flash the program and jump to the beginning of main:

$ cargo run --target thumbv7em-none-eabihf
     Running `arm-none-eabi-gdb -q -x openocd.gdb target/thumbv7em-none-eabihf/debug/led-roulette`
Reading symbols from target/thumbv7em-none-eabihf/debug/led-roulette...done.
Loading section .vector_table, size 0x188 lma 0x8000000
Loading section .text, size 0x3b20 lma 0x8000188
Loading section .rodata, size 0xb0c lma 0x8003cc0
Start address 0x8003b1c, load size 18356
Transfer rate: 20 KB/sec, 6118 bytes/write.
Breakpoint 1 at 0x800018c: file src/05-led-roulette/src/, line 9.
Note: automatically using hardware breakpoints for read-only addresses.

Breakpoint 1, main () at src/05-led-roulette/src/
9           let (mut delay, mut leds): (Delay, Leds) = aux5::init();