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2.9 KiB

use panic_halt as _;
use stm32f0xx_hal as hal;
use crate::hal::{gpio::*, pac, prelude::*};
use cortex_m::{interrupt::Mutex, peripheral::syst::SystClkSource::Core, Peripherals};
use cortex_m_rt::{entry, exception};
use core::cell::RefCell;
use core::mem::swap;
// A type definition for the GPIO pin to be used for our LED
type LEDPIN = gpiob::PB3<Output<PushPull>>;
// Mutex protected structure for our shared GPIO pin
static GPIO: Mutex<RefCell<Option<LEDPIN>>> = Mutex::new(RefCell::new(None));
fn main() -> ! {
if let (Some(mut p), Some(cp)) = (pac::Peripherals::take(), Peripherals::take()) {
cortex_m::interrupt::free(move |cs| {
let mut rcc = p.RCC.configure().sysclk(48.mhz()).freeze(&mut p.FLASH);
// Get access to individual pins in the GPIO port
let gpioa = p.GPIOB.split(&mut rcc);
// (Re-)configure the pin connected to our LED as output
let led = gpioa.pb3.into_push_pull_output(cs);
// Transfer GPIO into a shared structure
swap(&mut Some(led), &mut GPIO.borrow(cs).borrow_mut());
let mut syst = cp.SYST;
// Initialise SysTick counter with a defined value
unsafe { syst.cvr.write(1) };
// Set source for SysTick counter, here full operating frequency (== 48MHz)
// Set reload value, i.e. timer delay 48 MHz/4 Mcounts == 12Hz or 83ms
syst.set_reload(4_000_000 - 1);
// Start counting
// Enable interrupt generation
loop {
// Define an exception handler, i.e. function to call when exception occurs. Here, if our SysTick
// timer generates an exception the following handler will be called
fn SysTick() {
// Our moved LED pin
static mut LED: Option<LEDPIN> = None;
// Exception handler state variable
static mut STATE: u8 = 1;
// If LED pin was moved into the exception handler, just use it
if let Some(led) = LED {
// Check state variable, keep LED off most of the time and turn it on every 10th tick
if *STATE < 10 {
// Turn off the LED
// And now increment state variable
*STATE += 1;
} else {
// Turn on the LED
// And set new state variable back to 0
*STATE = 0;
// Otherwise move it out of the Mutex protected shared region into our exception handler
else {
// Enter critical section
cortex_m::interrupt::free(|cs| {
// Swap globally stored data with SysTick private data
swap(LED, &mut GPIO.borrow(cs).borrow_mut());