Datasheet

Figure 25-1 USART Block Diagram
(1)
PARITY
GENERATOR
UBRRn [H:L]
UDRn(Transmit)
UCSRnA UCSRnB UCSRnC
BAUD RATE GENERATOR
TRANSMIT SHIFT REGISTER
RECEIVE SHIFT REGISTER RxDn
TxDn
PIN
CONTROL
UDRn (Receive)
PIN
CONTROL
XCKn
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DATA BUS
OSC
SYNC LOGIC
Clock Generator
Transmitter
Receiver
Note: 1. Refer to Pin Configurations, table Port D Pins Alternate Functions in Alternate Functions of Port
D and table Port E Pins Alternate Functions in Alternate Functions of Port E for USART pin placement.
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.
Related Links
Pin Configurations on page 14
Alternate Functions of Port D on page 103
Alternate Functions of Port E on page 105
Atmel ATmega64A [DATASHEET]
Atmel-8160E-ATmega64A_Datasheet_Complete-09/2015
246