ATMEGA640 datasheet(215/449 Pages) ATMEL

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ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

USART Initialization

The USART has to be initialized before any communication can take place. The initial-

ization process normally consists of setting the baud rate, setting frame format and

enabling the Transmitter or the Receiver depending on the usage. For interrupt driven

USART operation, the Global Interrupt Flag should be cleared (and interrupts globally

disabled) when doing the initialization.

Before doing a re-initialization with changed baud rate or frame format, be sure that

there are no ongoing transmissions during the period the registers are changed. The

TXCn Flag can be used to check that the Transmitter has completed all transfers, and

the RXC Flag can be used to check that there are no unread data in the receive buffer.

Note that the TXCn Flag must be cleared before each transmission (before UDRn is

written) if it is used for this purpose.

The following simple USART initialization code examples show one assembly and one

C function that are equal in functionality. The examples assume asynchronous opera-

tion using polling (no interrupts enabled) and a fixed frame format. The baud rate is

given as a function parameter. For the assembly code, the baud rate parameter is

assumed to be stored in the r17:r16 Registers.

Note:

1. See “About Code Examples” on page 9.

Assembly Code Example

(1)

USART_Init:

; Set baud rate

out

UBRRnH, r17

out

UBRRnL, r16

; Enable receiver and transmitter

ldi

r16, (1<<RXENn)|(1<<TXENn)

out

UCSRnB,r16

; Set frame format: 8data, 2stop bit

ldi

r16, (1<<USBSn)|(3<<UCSZn0)

out

UCSRnC,r16

ret

C Code Example

(1)

#define FOSC 1843200// Clock Speed

#define BAUD 9600

#define (MYUBRR FOSC/16/BAUD-1)

void main( void )

{...

USART_Init ( MYUBRR );

...} // main

void USART_Init( unsigned int ubrr){

/* Set baud rate */

UBRRH = (unsigned char)(ubrr>>8);

UBRRL = (unsigned char)ubrr;

/* Enable receiver and transmitter */

UCSRB = (1<<RXEN)|(1<<TXEN);

/* Set frame format: 8data, 2stop bit */

UCSRC = (1<<USBS)|(3<<UCSZ0);

} // USART_Init

ATMEGA640 Datasheet(PDF) 215 Page - ATMEL Corporation

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