ATmega128 datasheet(173/386 Pages) ATMEL

173 / 386 page

173

2467S–AVR–07/09

ATmega128

AVR USART vs. AVR

UART – Compatibility

The USART is fully compatible with the AVR UART regarding:

•

Bit locations inside all USART registers

•

Baud Rate Generation

•

Transmitter Operation

•

Transmit Buffer Functionality

•

Receiver Operation

However, the receive buffering has two improvements that will affect the compatibility in some

special cases:

•

A second buffer register has been added. The two buffer registers operate as a circular FIFO

buffer. Therefore the UDR must only be read once for each incoming data! More important is

the fact that the error flags (FE and DOR) and the ninth data bit (RXB8) are buffered with the

data in the receive buffer. Therefore the status bits must always be read before the UDR

•

The receiver Shift Register can now act as a third buffer level. This is done by allowing the

received data to remain in the serial Shift Register (see Figure 79) if the buffer registers are

full, until a new start bit is detected. The USART is therefore more resistant to Data OverRun

(DOR) error conditions.

The following control bits have changed name, but have same functionality and register location:

•

CHR9 is changed to UCSZ2

•

OR is changed to DOR

Clock Generation

The clock generation logic generates the base clock for the transmitter and receiver. The

USART supports four modes of clock operation: Normal Asynchronous, Double Speed Asyn-

chronous, Master Synchronous, and Slave Synchronous mode. The UMSEL bit in USART

Control and Status Register C (UCSRC) selects between asynchronous and synchronous oper-

ation. Double speed (Asynchronous mode only) is controlled by the U2X found in the UCSRA

pin (DDR_XCK) controls whether the clock source is internal (Master mode) or external (Slave

mode). The XCK pin is only active when using Synchronous mode.

Figure 80 shows a block diagram of the clock generation logic.

Figure 80. Clock Generation Logic, Block Diagram

Signal description:

Prescaling

Down-Counter

/ 2

UBRR

/ 4

/ 2

fosc

UBRR+1

Sync

OSC

XCK

txclk

U2X

UMSEL

DDR_XCK

xcki

xcko

DDR_XCK

rxclk

Edge

Detector

UCPOL

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