I looked at the datasheet and the registers for the Sercom in serial mode.
on the second page, i find:
Up to six Serial Communication Interfaces (SERCOM), each configurable to operate as either:
• USART with full-duplex and single-wire half-duplex configuration
but i find no other references to achieving half duplex configuration or operation.
If i disable the transmitter, with CTRLB.TXEN, does the TX pin mode (normally output) change?
Looking for a nudge in the right direction.