45.6.3.7 Receiver Operations
When a character reception is completed, it is transferred to the Receive Holding register (US_RHR) and US_CSR.RXRDY rises. If a character is completed while RXRDY is set, the OVRE (Overrun Error) bit is set. The last character is transferred into US_RHR and overwrites the previous one. The OVRE bit is cleared by writing a ‘1’ to US_CR.RSTSTA.