25.7.1 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 XCKn 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 UDRn I/O location.

Assembly Code Example

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 UDR0 will be masked to zero. The USART 0 has to be initialized before the function can be used. For the assembly code, the received data will be stored in R16 after the code completes.

USART_Receive:
   ; Wait for data to be received
   in    r17, UCSR0A
   sbrs  r17, RXC
   rjmp  USART_Receive
   ; Get and return received data from buffer
   in    r16, UDR0
   ret
unsigned char USART_Receive( void )
{
   /* Wait for data to be received */
   while ( !(UCSR0A & (1<<RXC)) )
      ;
   /* Get and return received data from buffer */
   return UDR0;
}


For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with instructions that allow access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.


The function simply waits for data to be present in the receive buffer by checking the RXC Flag, before reading the buffer and returning the value.