```rust

static SPI_INSTANCE: Mutex<
    CriticalSectionRawMutex,
    RefCell<Option<&'static mut (dyn SpiDevice<u16, Error = Infallible> + Send)>>,
> = Mutex::new(RefCell::new(None));

fn foo(buffer: &[u16]) {
    SPI_INSTANCE.lock(|refcell| {
        let mut spi_ref = refcell.borrow_mut();
        let spi = spi_ref.as_mut().unwrap();
        spi.write(buffer).expect("SPI transfer failed");
    });
}