24.6.1 Sending Frames with 5 to 8 Data Bits

A data transmission is initiated by loading the transmit buffer with the data to be transmitted. The CPU can load the transmit buffer by writing to the UDR I/O location. The buffered data in the transmit buffer will be moved to the Shift Register when the Shift Register is ready to send a new frame. The Shift Register is loaded with new data if it is in idle state (no ongoing transmission) or immediately after the last stop bit of the previous frame is transmitted. When the Shift Register is loaded with new data, it will transfer one complete frame at the rate given by the Baud Register, U2X bit or by XCK depending on mode of operation.

The following code examples show a simple USART transmit function based on polling of the Data Register Empty (UDRE) Flag. When using frames with less than eight bits, the most significant bits written to the UDR are ignored. 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.

USART_Transmit:
   ; Wait for empty transmit buffer
   sbis    UCSRA,UDRE
   rjmp    USART_Transmit
   ; Put data (r16) into buffer, sends the data
   out    UDR,r16
   ret

void USART_Transmit( unsigned char data )
{
   /* Wait for empty transmit buffer */
   while ( !( UCSRA & (1<<UDRE)) )
      ;
   /* Put data into buffer, sends the data */
   UDR = data;
}