* The ISR should look something like this:
```rust
#[interrupt]
/// Timer interrupt handler
fn USART1() {
    unsafe{ (*pac::USART1::ptr()).rqr.modify(|_, w| w.rxfrq().set_bit() }

    free(|cs| {
        let mut b = READ_BUF.borrow(cs).borrow_mut();
        let mut b = u.as_mut();

        // Could put USART struct in a mutex too.
        buf[i] = unsafe{ (*pac::USART1::ptr()).dr.read().dr().bits() as u8; }
        // + some mechanism to incr buf.  Like a global `Mutex<Cell<usize>>`
    });
```