USART.TXDATAH holds the ninth data bit in
the character to be transmitted when operating with serial frames with nine data bits.
When used this bit must be written before writing to USART.TXDATAL except if CHSIZE in
USART.CTRLC is set to 9BIT Low byte first where USART.TXDATAL should be written first.
This bit is unused in Master SPI mode of
operation.
Transmit Data
Register
This bit is used
when CHSIZE=9BIT in USART.CTRLC.