The host data register I/O location (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
host (STATUS.CLKHOLD is set). An exception to this is reading the last data byte
after the stop condition has been sent.
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).
When CTRLC.DATA32B=1, read and write transactions from/to the DATA
register are 32 bit in size. Otherwise, reads and writes are 8 bit.
Writing or reading DATA.DATA when not in smart mode does
not require synchronization.