Using a Hardware Abstraction Layer

✅ Open the stm32-code/standalone-app/src/bin/standalone-led.rs file.

You’ll see that it initializes your board using the nucleo_u5a5zj_bsp crate:

let board = nucleo_u5a5zj_bsp::NonSecureBoard::new();

This grants you access to the board’s peripherals, like its LEDs.

The nucleo-u5a5zj-bsp library is a Board Support Package (BSP) tailored to this training to make accessing the peripherals used in this exercise as seamless as possible. You can find its source code at stm32-code/nucleo-u5a5zj-bsp/.

nucleo-u5a5zj-bsp is based on a custom Hardware Abstraction Layer we wrote for the STM32U5A5, called stm32u5-tiny-hal. The purpose of a HAL is to abstract away the device-specific details of the hardware, for example registers, and instead expose a higher level API more suitable for application development. Again, this one was designed to be small and easily understood and provide just enough support for these exercises. You can find its source code at stm32-code/stm32u5-tiny-hal/.

The nucleo_u5a5zj_bsp::NonSecureBoard::new function we have been calling in all programs initializes a few of the STM32U5A5’s peripherals and returns a NonSecureBoard structure that provides access to those peripherals. We’ll first look at the Leds API.

✅ Run the standalone-led program. Two of the LEDs on the board should turn on; the other one should stay off.

NOTE this program will not terminate itself. Within VS code you need to click “Kill terminal” (garbage bin icon) in the bottom panel to terminate it.

✅ Open the documentation for the nucleo-u5a5zj-bsp crate by running the following command from the stm32-code folder:

cargo doc -p nucleo-u5a5zj-bsp --open

✅ Check the API docs of the Led abstraction, and look at its source code. Change the standalone-led program, so that the Red and Blue LEDs are turned on, and the Green LED is turned off.

Note that on this board, turning an LED “on” means setting its corresponding GPIO pin “high”. On other boards you might set the GPIO pin “low” to turn the LED “on”. You should refer to the board documentation to find out how your board is configured.

🔎 When writing your own embedded project, you can implement your own BSP similar to nucleo-u5a5zj-bsp, or use the matching HAL crate for your chip directly. Check out awesome-embedded-rust if there’s a BSP for the board you want to use, or a HAL crate for the chip you’d like to use. Or you can even write a HAL from scratch, like we did!