Datasheet

201
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
Figure 22-1. USART Block Diagram
(1)
Note: 1. See Figure 1-1 on page 2, Figure 1-3 on page 4, Table 13-12 on page 80, Table 13-15 on page 82, Table 13-24 on
page 88 and Table 13-27 on page 90 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 con-
sists of synchronization logic for external clock input used by synchronous slave operation, and the baud rate
generator. The XCKn (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 (UDRn). The Receiver supports the same frame formats as the
Transmitter, and can detect Frame Error, Data OverRun and Parity Errors.
22.3 Clock Generation
The Clock Generation logic generates the base clock for the Transmitter and Receiver. The USARTn supports four
modes of clock operation: Normal asynchronous, Double Speed asynchronous, Master synchronous and Slave
synchronous mode. The UMSELn bit in USART Control and Status Register C (UCSRnC) selects between asyn-
chronous and synchronous operation. Double Speed (asynchronous mode only) is controlled by the U2Xn found in
the UCSRnA Register. When using synchronous mode (UMSELn = 1), the Data Direction Register for the XCKn
pin (DDR_XCKn) controls whether the clock source is internal (Master mode) or external (Slave mode). The XCKn
pin is only active when using synchronous mode.
PARITY
GENERATOR
UBRR[H:L]
UDR (Transmit)
UCSRA UCSRB UCSRC
BAUD RATE GENERATOR
TRANSMIT SHIFT REGISTER
RECEIVE SHIFT REGISTER RxD
TxD
PIN
CONTROL
UDR (Receive)
PIN
CONTROL
XCK
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DATA BUS
OSC
SYNC LOGIC
Clock Generator
Transmitter
Receiver