* The ISR should look something like this:
```rust
static READ_BUF: Mutex<RefCell<[u8;10]>> = Mutex::new(RefCell::new([0;10]));
static READ_I: Mutex<Cell<usize>> = Mutex::new(Cell::new(0));
// ..

#[interrupt]
/// Async USART read interrupt handler
fn USART1() {
    unsafe{ (*pac::USART1::ptr()).rqr.modify(|_, w| w.rxfrq().set_bit() }

    free(|cs| {
        let mut buf = READ_BUF.borrow(cs).borrow_mut();

        let i = READ_I.borrow(cs);

        // Could put USART struct in a mutex too.
        buf[i.get()] = unsafe{ (*pac::USART1::ptr()).dr.read().dr().bits() as u8; }
        i.set(i.get() + 1);
    });
```