USART Initialization

The USART has to be initialized before any communication can take place. The initialization 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 TXC flag (UCSRnA.TXC) 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. The UCSRnA.TXC 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 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.

USART_Init:
   ; Set baud rate to UBRR0
   out    UBRR0H, r17
   out    UBRR0L, r16
   ; Enable receiver and transmitter
   ldi    r16, (1<<RXEN0)|(1<<TXEN0)
   out    UCSR0B,r16
   ; Set frame format: 8data, 2stop bit
   ldi    r16, (1<<USBS0)|(3<<UCSZ00)
   out    UCSR0C,r16
   ret

#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 */
   UBRR0H = (unsigned char)(ubrr>>8);
   UBRR0L = (unsigned char)ubrr;
   Enable receiver and transmitter */
   UCSR0B = (1<<RXEN0)|(1<<TXEN0);
   /* Set frame format: 8data, 2stop bit */
   UCSR0C = (1<<USBS0)|(3<<UCSZ00);
}

More advanced initialization routines can be written to 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.