The setup looks like this:
```rust
    unsafe { NVIC::unmask(interrupt::USART1) }

    unsafe {

        (*pac::USART1::ptr()).cr1.modify(|_, w| w.ue().clear_bit());
        while (*pac::USART1::ptr()).cr1.read().ue().bit_is_set() {}

        (*pac::USART1::ptr()).cr1.modify(|_, w| w.cmie().set_bit());

        // Allow an 8-bit address to be set in `add`.
        (*pac::USART1::ptr()).cr2.modify(|_, w| {
            w.addm7().set_bit();
            w.add().bits(MSG_START_BYTES[0])
        });

        (*pac::USART1::ptr()).cr1.modify(|_, w| w.ue().set_bit());
    }
```