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Using a Hardware Abstraction Layer
In this section we'll start using the hardware features of the nRF52840 and the board.
Open the src/bin/led.rs file.
The dk crate / library is a Hardware Abstraction Layer (HAL) over the nRF52840 Development Kit. 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.
The dk::init function we have been calling in all programs initializes a few of the nRF52840 peripherals and returns a Board structure that provides access to those peripherals. We'll first look at the Leds API. Open the documentation for the dk crate running the following command from the beginner/apps folder:
$ cargo doc -p dk --open
Check the API docs of the Led abstraction then run the led program. Two of the green LEDs on the board should turn on; the other two 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.
Now, uncomment the log::set_max_level line. This will make the logs more verbose; they will now include logs from the board initialization function (dk::init) and from the Led API.
Among the logs you'll find the line "I/O pins have been configured for digital output". At this point the electrical pins of the nRF52840 microcontroller has been configured to drive the 4 LEDs on the board.
After the dk::init logs you'll find logs about the Led API. As the logs indicate an LED becomes active when the output of the pin is a logical zero, which is also referred as the "low" state. This "active low" configuration does not apply to all boards: it depends on how the pins have been wired to the LEDs. You should refer to the board documentation to find out which pins are connected to LEDs and whether "active low" or "active high" applies to it.