After reset, the receiver is disabled. The user must enable the receiver by setting the RXEN bit in the Control register (US_CR). However, the receiver registers can be programmed before the receiver clock is enabled.
After reset, the transmitter is disabled. The user must enable it by writing a ‘1’ to US_CR.TXEN. However, the transmitter registers can be programmed before being enabled.
The receiver and the transmitter can be enabled together or independently.
At any time, the software can perform a reset on the receiver or the transmitter of the USART by writing a ‘1’ to the corresponding bit US_CR.RSTRX and US_CR.RSTTX respectively. The software resets clear the status flag and reset internal state machines but the user interface configuration registers hold the value configured prior to software reset. Regardless of what the receiver or the transmitter is performing, the communication is immediately stopped.
The user can also independently disable the receiver or the transmitter by writing a ‘1’ to US_CR.RXDIS and US_CR.TXDIS, respectively. If the receiver is disabled during a character reception, the USART waits until the end of reception of the current character, then the reception is stopped. If the transmitter is disabled while it is operating, the USART waits the end of transmission of both the current character and character being stored in the Transmit Holding register (US_THR). If a timeguard is programmed, it is handled normally.