25.6.1 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 UDRn 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, U2Xn bit or by XCKn depending on mode of operation.

Assembly Code Example

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 UDR0 are ignored. The USART 0 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 R17.

USART_Transmit:
   ; Wait for empty transmit buffer
   in      r17, UCSR0A
   sbrs    r17, UDRE
   rjmp    USART_Transmit
   ; Put data (r16) into buffer, sends the data
   out    UDR0,r16
   ret
void USART_Transmit( unsigned char data )
{
   /* Wait for empty transmit buffer */
   while ( !( UCSR0A & (1<<UDRE)) )
      ;
   /* Put data into buffer, sends the data */
   UDR0 = data;
}

The function simply waits for the transmit buffer to be empty by checking the UDRE Flag, before loading it with new data to be transmitted. If the Data Register Empty interrupt is utilized, the interrupt routine writes the data into the buffer.