A data transmission is initiated by loading the Transmit buffer (DATA in USART.TXDATA) with the data to be sent. The data in the transmit buffer are moved to the Shift Register when the Shift Register is empty and ready to send a new frame. The Shift Register is loaded if it is in idle state (no ongoing transmission) or immediately after the last stop bit of the previous frame is transmitted. When the Shift Register is loaded with data, it will transfer one complete frame.
When the entire frame in the Shift Register has been shifted out and there are no new data present in the transmit buffer, the Transmit Complete Interrupt Flag (TXCIF in USART.STATUS) is set and the optional interrupt is generated.
TXDATA can only be written when the Data Register Empty Flag (DREIF in USART.STATUS) is set, indicating that the register is empty and ready for new data.
When using frames with fewer than eight bits, the most-significant bits written to TXDATA are ignored. If 9-bit characters are used, DATA[8] in USART.TXDATAH has to be written before DATA[7:0] in USART.TXDATAL.