main.rs

static BUTTON: AtomicBool = AtomicBool::new(false);

#[entry]
fn main() {
    let mut cp = cortex_m::Peripherals::take().unwrap();
    let dp = stm32f30x::Peripherals::take().unwrap();

    let mut flash = dp.FLASH.constrain();
    let mut rcc = dp.RCC.constrain();

    let clocks = rcc.cfgr.freeze(&mut flash.acr);
    let delay = Delay::new(cp.SYST, clocks);

    let mut gpioa = dp.GPIOA.split(&mut rcc.ahb);
    let button = gpioa.pa0.into_floating_input(&mut gpioa.moder, &mut gpioa.pupdr);
    cp.NVIC.enable(Interrupt::EXTI0);
    unsafe {
        dp.SYSCFG.exticr1.write(|w| w.exti0().bits(0)); // EXTI0 source = PA0 (button)
        dp.EXTI.imr1.write(|w| w.mr0().set_bit());      // unmask EXTI0
        dp.EXTI.ftsr1.write(|w| w.tr0().set_bit());     // trigger on falling edge
        cp.NVIC.set_priority(Interrupt::EXTI0, 1);
        NVIC::unpend(Interrupt::EXTI0);
    }
  
    let mut itm = cp.ITM;
  
    loop {
        if BUTTON.load(Ordering::SeqCst) {
            iprintln!(&mut itm.stim[0], "Button pushed!");
            BUTTON.store(false, Ordering::SeqCst);
        }
    }
}

interrupt!(EXTI0, button_isr);
fn button_isr() {
    BUTTON.store(true, Ordering::SeqCst);
}