Datasheet

211
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
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 func-
tion that are equal in functionality. The examples assume asynchronous operation 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 11.
More advanced initialization routines can be made that include frame format as parameters, dis-
able interrupts and so on. However, many applications use a fixed setting of the baud and
control registers, and for these types of applications the initialization code can be placed directly
in the main routine, or be combined with initialization code for other I/O modules.
Assembly Code Example
(1)
USART_Init:
; Set baud rate
sts UBRRnH, r17
sts UBRRnL, r16
ldi r16, (1<<U2Xn)
sts UCRnA, r16
; Enable receiver and transmitter
ldi r16, (1<<RXENn)|(1<<TXENn)
sts UCSRnB,r16
; Set frame format: 8data, 1stop bit
ldi r16, (2<<UMSELn)|(3<<UCSZn0)
sts 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