Add hello world example

hello-embedded/hello-world/.cargo/config.toml

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+[build]
+target = "thumbv7em-none-eabihf"
+
+[target.thumbv7em-none-eabihf]
+rustflags = [
+  "-C", "link-arg=-Tlink.x"
+]

hello-embedded/hello-world/.embed.toml

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+[default.general]
+chip = "nRF52833_xxAA"
+
+[default.rtt]
+enabled = true

hello-embedded/hello-world/.vscode/settings.json

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+{
+    "rust-analyzer.check.targets": "thumbv7em-none-eabihf",
+    "rust-analyzer.cargo.allTargets": false,
+    "rust-analyzer.check.allTargets": false
+}

hello-embedded/hello-world/Cargo.toml

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+[package]
+name = "hello-world"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+cortex-m = { version = "0.7.7", features = ["critical-section-single-core"] }
+cortex-m-rt = "0.7.3"
+panic-halt = "1.0.0"
+rtt-target = "0.5.0"

hello-embedded/hello-world/memory.x

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+MEMORY
+{
+  FLASH : ORIGIN = 0x00000000, LENGTH = 512K
+  RAM : ORIGIN = 0x20000000, LENGTH = 128K
+}

hello-embedded/hello-world/src/main.rs

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+#![no_std]
+#![no_main]
+
+use core::ptr::write_volatile;
+
+use cortex_m::asm::nop;
+use cortex_m_rt::entry;
+use panic_halt as _;
+use rtt_target::{rprintln, rtt_init_print};
+
+#[entry]
+fn main() -> ! {
+    rtt_init_print!();
+    rprintln!("Hello world!");
+
+    // The LED matrix is organized into rows (ROW1 to ROW5) and columns (COL1 to COL5). Each row and column represents
+    // connections to LEDs in a grid formation, where each intersection can turn on or off an LED by controlling the flow of current.
+    // In a 5x5 matrix like this, there are 25 LEDs, arranged in rows and columns. Each LED is placed at an intersection of a row and a column.
+    // By setting a row to a high (positive) voltage and a column to low (grounded), current can flow through the corresponding LED 
+    // in that row and column, causing it to light up.
+    // (see also Microbit Schematic at https://tech.microbit.org/hardware/schematic/, page 2
+
+    // In our example, we will use ROW1 and COL1 to turn on an LED. From the schematics (page 3), we can see that 
+    // ROW1 is connected to Port 0/Pin 21 and COL1 is connected to Port 0/Pin 28.
+
+    // Configuration of GPIO happens in the PIN_CNF registers. The data sheet for nRF52833
+    // (https://docs-be.nordicsemi.com/bundle/ps_nrf52833/attach/nRF52833_PS_v1.7.pdf?_LANG=enus) tells us that the
+    // base address for PIN_CNF registers is 0x5000_0000 (page 231). The offset for GPIO21 is 0x754 and for GPIO28 it is 0x770
+    // (page 232). We control the value of the pins using the OUT register. To set pin 21 high, we need to 
+    // write 1 to bit 21 of the OUT register (page 232). The offset for the OUT register is 0x504 (page 231).
+
+    const GPIO0_PINCNF21_ROW1_ADDR: *mut u32 = 0x5000_0754 as *mut u32;
+    const GPIO0_PINCNF28_COL1_ADDR: *mut u32 = 0x5000_0770 as *mut u32;
+    const DIR_OUTPUT_POS: u32 = 0; // Bit in configuration register to choose between input and output (page 269)
+    const PINCNF_DRIVE_LED: u32 = 1 << DIR_OUTPUT_POS; // 1 means output, 0 means input (page 269) -> we need 1
+
+    // Configure the direction of the pins to output
+    unsafe {
+        // Set the direction of the pins 21 and 28 to output
+        write_volatile(GPIO0_PINCNF21_ROW1_ADDR, PINCNF_DRIVE_LED);
+        write_volatile(GPIO0_PINCNF28_COL1_ADDR, PINCNF_DRIVE_LED);
+    }
+
+    const GPIO0_OUT_ADDR: *mut u32 = 0x5000_0504 as *mut u32;
+    const GPIO0_OUT_ROW1_POS: u32 = 21; // Bit in the OUT register to control GPIO21
+    let mut is_on = false;
+    loop {
+        rprintln!("Echo...");
+        unsafe {
+            // Set the value of the OUT register to turn on or off the LED
+            // We do not set the value for GPIO28 because we want it to be off
+            write_volatile(GPIO0_OUT_ADDR, (is_on as u32) << GPIO0_OUT_ROW1_POS);
+        }
+
+        for _ in 0..100_000 {
+            nop();
+        }
+
+        is_on = !is_on;
+    }
+}