fifo check on interrupt

src/internal_state.rs

 // Tracks the state of our instance, *not* the state of the radio.
-pub(crate) enum InternalState<const PACKET_LEN: usize> {
+#[derive(Debug)]
+pub enum InternalState<const PACKET_LEN: usize> {
     Idle,
     Rx {
         data: [u8; PACKET_LEN],

src/lib.rs

@@ -171,6 +171,10 @@ where
     }
 
     pub fn interrupt(&mut self) -> Result<(), TransferError<Spi::Error>> {
+        if self.radio.fifo_underflow_pending().te()? {
+            return Err(TransferError::FifoOverflow);
+        }
+
         match &mut self.state {
             InternalState::Idle => {
                 // we were not expecting an interrupt
@@ -252,6 +256,7 @@ where
             InternalState::Tx { data, i, .. } => (data, i),
             _ => return Ok(()),
         };
+
         // only look at the slice we haven't sent yet
         let data = &mut data[*i..];