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XMEGA A [MANUAL]
8077I–AVR–11/2012
Figure 21-1. USART block diagram.
The clock generator includes a fractional baud rate generator that is able to generate a wide range of USART baud rates
from any system clock frequencies. This removes the need to use an external crystal oscillator with a specific frequency
to achieve a required baud rate. It also supports external clock input in synchronous slave operation.
The transmitter consists of a single write buffer (DATA), a shift register, and a parity generator. The write buffer allows
continuous data transmission without any delay between frames.
The receiver consists of a two-level receive buffer (DATA) and a shift register. Data and clock recovery units ensure
robust synchronization and noise filtering during asynchronous data reception. It includes frame error, buffer overflow,
and parity error detection.
When the USART is set in master SPI mode, all USART-specific logic is disabled, leaving the transmit and receive
buffers, shift registers, and baud rate generator enabled. Pin control and interrupt generation are identical in both modes.
The registers are used in both modes, but their functionality differs for some control settings.
An IRCOM module can be enabled for one USART to support IrDA 1.4 physical compliant pulse modulation and
demodulation for baud rates up to 115.2kbps. For details, refer to “IRCOM – IR Communication Module” on page 252.
21.3
Clock Generation
The clock used for baud rate generation and for shifting and sampling data bits is generated internally by the fractional
baud rate generator or externally from the transfer clock (XCK) pin. Five modes of clock generation are supported:
normal and double-speed asynchronous mode, master and slave synchronous mode, and master SPI mode.
PARITY
GENERATOR
BSEL [H:L]
DATA
(Transmit)
CTRLA
CTRLB
CTRLC
BAUD RATE GENERATOR
FRACTIONAL DIVIDE
TRANSMIT SHIFT REGISTER
RECEIVE SHIFT REGISTER
RxD
TxD
PIN
CONTROL
DATA
(Receive)
PIN
CONTROL
XCK
DATA
RECOVERY
CLOCK
RECOVERY
PIN
CONTROL
TX
CONTROL
RX
CONTROL
PARITY
CHECKER
DA
T
A
BUS
OSC
SYNC LOGIC
Clock Generator
Transmitter
Receiver
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