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ATmega128
USART
Initialization
The USART has to be initialized before any communication can take place. The initialization pro-
cess 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 TXC 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 TXC flag must be cleared
before each transmission (before UDR 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.
Assembly Code Example (1)
USART_Init:
; Set baud rate
out
out
UBRRH, r17
UBRRL, r16
; Enable receiver and transmitter
ldi
out
r16, (1<<RXEN)|(1<<TXEN)
UCSRB,r16
; Set frame format: 8data, 2stop bit
ldi
out
r16, (1<<USBS)|(3<<UCSZ0)
UCSRC,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 );
...
}
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);
}
Note:
1. See “About Code Examples” on page 8.
More advanced initialization routines can be made that include frame format as parameters,
disable 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.
176
2467X–AVR–06/11
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