24.7.1 Receiving Frames with 5 to 8 Data Bits

The Receiver starts data reception when it detects a valid start bit. Each bit that follows the start bit will be sampled at the baud rate or XCK clock, and shifted into the Receive Shift Register until the first stop bit of a frame is received. A second stop bit will be ignored by the Receiver. When the first stop bit is received (i.e., a complete serial frame is present in the Receive Shift Register), the contents of the Shift Register will be moved into the receive buffer. The receive buffer can then be read by reading the UDR I/O location.

The following code example shows a simple USART receive function based on polling of the Receive Complete (RXC) Flag. When using frames with less than eight bits the most significant bits of the data read from the UDR will be masked to zero. The USART has to be initialized before the function can be used.

USART_Receive:
   ; Wait for data to be received
   sbis  UCSRA, RXC
   rjmp  USART_Receive
   ; Get and return received data from buffer
   in    r16, UDR
   ret

unsigned char USART_Receive( void )
{
   /* Wait for data to be received */
   while ( !(UCSRA & (1<<RXC)) )
      ;
   /* Get and return received data from buffer */
   return UDR;
}