Rainbow Electronics ATmega128L - Manual (Page 171)

171
ATmega128(L)
2467B–09/01
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 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. The example code assumes that the part specific header file is included.
For I/O registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and
“SBI” instructions must be replaced with instructions that allow access to extended
I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
More advanced initialization routines can be made that include frame format as parame-
ters, 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.
Data Transmission – The
USART Transmitter The USART transmitter is enabled by setting the Transmit Enable (TXEN) bit in the
UCSRB register. When the transmitter is enabled, the normal port operation of the TxD
pin is overridden by the USART and given the function as the transmitter’s serial output.
The baud rate, mode of operation and frame format must be set up once before doing
any transmissions. If synchronous operation is used, the clock on the XCK pin will be
overridden and used as transmission clock.
Assembly Code Example (1)
USART_Init:
; Set baud rate
out UBRRH, r17
out UBRRL, r16
; Enable receiver and transmitter
ldi r16, (1<<RXEN)|(1<<TXEN)
out UCSRB,r16
; Set frame format: 8data, 2stop bit
ldi r16, (1<<USBS)|(3<<UCSZ0)
out UCSRC,r16
ret
C Code Example (1)
void USART_Init( unsigned int baud )
{
/* Set baud rate */
UBRRH = (unsigned char)(baud>>8);
UBRRL = (unsigned char)baud;
/* Enable receiver and transmitter */
UCSRB = (1<<RXEN)|(1<<TXEN);
/* Set frame format: 8data, 2stop bit */
UCSRC = (1<<USBS)|(3<<UCSZ0);
}