Hello World

In the last section, you wrote a sort of "Hello World" program. But for embedded programmers, the "real Hello World" is to blink an LED — any LED — on and off once per second. A program that does this is commonly known as a "blinky".

Why blinky? Because this shows that you have enough control of the board you're working with to perform this simple task. You can get a program loaded onto the machine and running, you can find and turn on the appropriate pin on the MCU, you can delay for a fixed amount of time. Once you have this much control, other tasks become much more straightforward.

In previous chapters, you found out several ways to load a program onto your MB2. Now it's just a question of which pin you turn on and off, and how you delay between these actions.

Let's start by finding out how to work with the needed pins. There's a path you can follow for this if you know how to read electronic circuit "schematic" diagrams. You can find the MB2 schematic, find an LED on that schematic that you want to turn on and off, and find what GPIO pins on the nRF52833 are attached to that LED. (The MB2 is a bit unusual in this regard: usually an LED is attached to just one pin that turns it on or off. The LED "display" on the MB2 is hooked up in a more complicated way to allow turning on and off combinations of LEDs at once: a feature that we will be using shortly.)

We will work with the LED in the upper-left corner of the MB2 display. Tracing the ROW1 and COL1 wires this LED is connected to, we can see that they go to pins on the nRF52833 labeled AC17/P0.21 and B11/AIN4/P0.28. Digging further through the documentation we find that AC17 and B11 are the row and column indices of the physical pins (solder balls, really) on the bottom of the chip — useless to us. AIN4 just means that this pin can act as an "Analog Input", which is also currently useless to us. (It will come into play later.)

This leaves P0.21 and P0.28. These labels correspond to bits in the memory of the nRF52833 that can be turned on and off to get the LED to light up. Because electronics reasons, if pin P0.21 is turned on (thus outputting 3.3V) and pin P0.28 is turned off (thus accepting voltage) the LED will light up.

But what do we do in software to cause this to occur? We will work at the level of the nrf52833-hal crate. The Hardware Abstraction Layer (HAL) is a chunk of software designed to make a particular microcontroller easier to work with. As can be seen from the name, we have one for the microcontroller on the MB2. It happens to contain everything needed to turn our target LED on.

Take a look at examples/light-up.rs in this chapter's directory, and then try running it. You could use something fancy like before, but we have it set up so that

cargo run --example light-up

will load and run your program. That one LED should now be brightly lit!

#![no_main]
#![no_std]

use cortex_m_rt::entry;
use nrf52833_hal::{gpio, pac};
use panic_halt as _;

#[entry]
fn main() -> ! {
    let peripherals = pac::Peripherals::take().unwrap();
    let p0 = gpio::p0::Parts::new(peripherals.P0);
    let _row1 = p0.p0_21.into_push_pull_output(gpio::Level::High);
    let _col1 = p0.p0_28.into_push_pull_output(gpio::Level::Low);

    #[allow(clippy::empty_loop)]
    loop {}
}

Note that we access the Peripheral Access Crate (PAC) for this chip through our HAL crate. There's a complicated dance needed to get access to our pins. Finally, since we can just initialize the pins to the right levels, we don't need to set them. Wiggling the pins is a topic for the next section.

Discovery

Hello World