US_IER
at91usart_writereg(sc, US_IER, sc->sc_ier); // re-enable interrupts
at91usart_writereg(sc, US_IER, sc->sc_ier);
at91usart_writereg(sc, US_IER, sc->sc_ier);
USARTREG(US_IER) = USART_INT_RXRDY;
US_IER, US_CSR_ENDTX);