> <@firefrommoonlight:matrix.org> 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);
>     });
> ```

firefrommoonlight: I'm following up on your example from before. Somehow I'm not allowed to write to the `rqr` register:

```rust
1. the method `modify` exists for struct `Reg<u32, hal::pac::usart1::_RQR>`, but its trait bounds were not satisfied
the following trait bounds were not satisfied:  `Reg<u32, hal::pac::usart1::_RQR>: Readable`
```

This is my code:
```rust
    unsafe { (*pac::USART1::ptr()).rqr.modify(|_, w| w.rxfrq().set_bit()) }

```