The USART will not release the TXD pin override if:
- The USART transmitter is
disabled by writing the TXEN bit in USART.CTRLB to ‘0’ while the
USART receiver is disabled (RXEN in USART.CTRLB is ‘0’)
- Both the USART transmitter
and receiver are disabled at the same time by writing the TXEN and RXEN bits
in USART.CTRLB to ‘0’