The dashed boxes in the block diagram separate the three main parts of the USART (listed from the top): Clock
generator, Transmitter and Receiver. Control Registers are shared by all units. The clock generation logic consists
of synchronization logic for external clock input used by synchronous slave operation, and the baud rate generator.
The XCK (transfer clock) pin is only used by synchronous transfer mode. The Transmitter consists of a single write
buffer, a serial Shift Register, Parity Generator and control logic for handling different serial frame formats. The
write buffer allows a continuous transfer of data without any delay between frames. The Receiver is the most complex
part of the USART module due to its clock and data recovery units. The recovery units are used for
asynchronous data reception. In addition to the recovery units, the Receiver includes a parity checker, control
logic, a Shift Register and a two level receive buffer (UDR). The Receiver supports the same frame formats as the
Transmitter, and can detect Frame Error, Data OverRun and Parity Errors.