Add a nonblocking LED display driver

[mattheww]

This is the core of the code from https://github.com/mattheww/microbit-blinkenlights, as discussed in #13.

I've left out the font and scrolling support to start with.

Notes:

• I haven't done anything with the existing 'blocking' led.rs module.

• The rustdoc is using RFC1946-style links, which aren't stable but do work on docs.rs . I can remove those if you don't want to require nightly rustdoc.

[therealprof]

This looks good. Two things I'm not too happy about is funny formatting in some places and the custom timer implementation, it would be great if we could work that into the regular timer code.

[mattheww]

Formatting: would it make sense for me to run the whole thing through rustfmt? With the default settings?

Can you say more about where the timer code could move to?

[mattheww]

For what it's worth, I've pushed a much simpler version of microbit_timer.rs, now I've looked a bit harder about what svd2rust is actually generating.

[therealprof]

Formatting: would it make sense for me to run the whole thing through rustfmt? With the default settings?

Yes, please.

[therealprof]

Can you say more about where the timer code could move to?

The timer code lives mostly in the underlying nrf51-hal crate: https://github.com/nrf-rs/nrf51-hal/blob/master/src/timer.rs

There're a number of issues and PRs floating around here and in the nrf51-hal crate which were never finished.

[mattheww]

I've run rustfmt.

[mattheww]

I've looked at the embedded-hal timers.

The current timer-related traits aren't suitable:

• they don't provide a way to request an interrupt, only an nb-style wait() method
• they only support programming a single capture/compare register, but the display code needs two.

Supporting interrupts looks like the more serious limitation.

I could propose something for inclusion in nrf51-hal based on what's currently in microbit_timer.rs (but a bit more general), but it wouldn't be related to any currently-defined embedded-hal interfaces.

(I've read rust-embedded/embedded-hal#57.)

[therealprof]

@mattheww Providing platform specific interfaces on top of the embedded-hal traits is expected and desired. The advantage of putting that code into nrf51-hal is that boards other than the micro:bit can also use that functionality, possibly for completely unrelated hardware.

Of course if there was a way to create a universal abstraction over that would be excellent but that is entirely optional...

[mattheww]

Closing in favour of draft PR #16