48.5.3.1 Transmitter Reset, Enable and Disable

After device reset, the UART transmitter is disabled and must be enabled before being used. The transmitter is enabled by writing UART_CR.TXEN to ‘1’. From this command, the transmitter waits for a character to be written in the Transmit Holding register (UART_THR) before actually starting the transmission.

The programmer can disable the transmitter by writing UART_CR.TXDIS to ‘1’. If the transmitter is not operating, it is immediately stopped. However, if a character is being processed into the internal shift register and/or a character has been written in the UART_THR, the characters are completed before the transmitter is actually stopped.

The programmer can also put the transmitter in its reset state by writing the UART_CR.RSTTX to ‘1’. This immediately stops the transmitter, whether or not it is processing characters.