Section 21.5 and 21.6 of the '328 data sheet show sample Initialization and transfer codes as functions.
"The following simple USART initialization code examples show one assembly and one C function that are equal in
functionality. The examples assume polling (no interrupts enabled). 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."
void USART_Init( unsigned int baud )
{
UBRRn = 0;
/* Setting the XCKn port pin as output, enables master mode. */
XCKn_DDR |= (1<<XCKn);
/* Set MSPI mode of operation and SPI data mode 0. */
UCSRnC = (1<<UMSELn1)|(1<<UMSELn0)|(0<<UCPHAn)|(0<<UCPOLn);
/* Enable receiver and transmitter. */
UCSRnB = (1<<RXENn)|(1<<TXENn);
/* Set baud rate. */
/* IMPORTANT: The Baud Rate must be set after the transmitter is enabled
*/
UBRRn = baud;
}
"The following code examples show a simple USART in MSPIM mode transfer function based on polling of the Data
Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The USART has to be initialized before the
function can be used. For the assembly code, the data to be sent is assumed to be stored in Register R16 and the
data received will be available in the same register (R16) after the function returns.
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag, before loading it with
new data to be transmitted. The function then waits for data to be present in the receive buffer by checking the
RXCn Flag, before reading the buffer and returning the value."
unsigned char USART_Receive( void )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) );
/* Put data into buffer, sends the data */
UDRn = data;
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) );
/* Get and return received data from buffer */
return UDRn;
}