Using Rust crates for an STM32 microcontroller board

Question

I'm currently trying to learn Rust (for embedded specifically), coming from a background of C for embedded systems and Python. So far, I've been reading Rust Programming Language and Rust for Embedded, and read a few blog posts on the web.

I want my first project to be a simple "Blinky", where an LED blinks infinitely. I've got an STM32L152CDISCOVERY board with an STM32L152 chip in it (basically same as STM32L151), which is a Cortex M3.

Instead of implementing everything from scratch, I want to leverage existing crates and HALs. I've found two that seem promising: stm32l1 and stm32l1xx-hal. I've tried to read the documentation of each of them and also part of the source code, but I still can't figure out how to use them correctly.

Got a few questions about Rust and about the crates:

1. I see that stm32l1xx-hal has a dependency on stm32l1. Do I need to add both as a dependency in my Cargo.toml file? Or will that create problems related to ownership?

2. Is this the correct way to add them? Why is the second one added like that [dependencies.stm32l1]?

   [dependencies]
   cortex-m-rt = "0.6.10"
   cortex-m-semihosting = "0.3.3"
   panic-halt = "0.2.0"
   stm32l1xx-hal = "0.1.0"
   
   [dependencies.stm32l1]
   version = "0.13.0"
   features = ["stm32l151", "rt"]
   
   

3. To blink the LD4 (which is connected to PB4 PB6), I've got to enable the GPIOB in the RCC register and then configure the pin to push pull output. By inspecting the documentation of stm32l1xx-hal, I see that there is an RCC struct and a PB4 struct with the method into_push_pull_output. However, I still don't understand how to use these structs: how to import them or how to get an instance of them.

I've seen code examples for stm32l1 but not for stm32l1xx-hal. I know that I can do this:

use stm32l1::{stm32l151};
...
let p = stm32l151::Peripherals::take().unwrap();
p.RCC.ahbenr.modify(|_,w| w.gpiopben().set_bit());

But in the source code of stm32l1xx-hal I see that the RCC part is already done in impl GpioExt for $GPIOX, but I don't know how to get to this "Parts" functionality.

Any help that points me in the right direction is appreciated.

Answer 1

I got some help from a Discord community. The answers were (modified a bit by me):

1. stm32l1xx-hal already depends on stm32l1 as seen here. There's no need to import it twice. It is enough to add to Cargo.toml:

   [dependencies.stm32l1xx-hal]
   version = "0.1.0"
   default-features = false
   features = ["stm32l151", "rt"]
   
   

• Note that the default-features = false is optional, but without it the compiler was giving me an error.

2. The syntaxes are equivalent, but as I said above, I only need to add the HAL one. You can add curly braces {} in the first style to add the options, such as:

   stm32l1xx-hal = { version = "0.1.0", features = ["stm32l151", "rt"]}
   
   

3. The right code for doing the blinky (which was on PB6, not PB4, sigh) was:

   #![no_main]
   #![no_std]
   
   use panic_halt as _;
   
   use cortex_m_rt::entry;
   use stm32l1xx_hal::delay::Delay;
   use stm32l1xx_hal::gpio::GpioExt;
   use stm32l1xx_hal::hal::digital::v2::OutputPin;
   use stm32l1xx_hal::rcc::{Config, RccExt};
   use stm32l1xx_hal::stm32::Peripherals;
   use stm32l1xx_hal::stm32::CorePeripherals;
   use stm32l1xx_hal::time::MicroSeconds;
   
   
   #[entry]
   fn main() -> ! {
   
       let p = Peripherals::take().unwrap();
       let cp = CorePeripherals::take().unwrap();
   
       // Get LED pin PB6
       let gpiob = p.GPIOB.split();
       let mut led = gpiob.pb6.into_push_pull_output();
   
       // Set up a delay
       let rcc = p.RCC.freeze(Config::default());
       let mut delay = Delay::new(cp.SYST, rcc.clocks);
   
       loop {
   
           // Turn LED On
           led.set_high().unwrap();
   
           delay.delay(MicroSeconds(1_000_000_u32));
   
           // Turn LED Off
           led.set_low().unwrap();
   
           delay.delay(MicroSeconds(1_000_000_u32));
       }
   }
   
   

• For me, the key was to understand that the split method can be called from the Peripherals because stm32l1xx-hal implemented split for a struct defined in stm32l1. In other words, the HAL crate is not only defining new structs, but also extending functionality for existing structs. I need to wrap my head around trait design patterns.