The client data register I/O location (DATA.DATA) provides access to the
host transmit and receive data buffers. Reading valid data or writing data to be
transmitted can be successfully done only when SCL is held low by the client
(STATUS.CLKHOLD is set). An exception occurs when reading the last data byte after
the stop condition has been received.
Accessing DATA.DATA auto-triggers I2C
bus operations. The operation performed depends on the state of CTRLB.ACKACT,
CTRLB.SMEN and the type of access (read/write).
