25.7.2 Receiving Frames with 9 Data Bits

If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8 bit in UCSRnB before reading the low bits from the UDRn. This rule applies to the FE, DOR and UPE Status Flags as well. Read status from UCSRnA, then data from UDRn. Reading the UDRn I/O location will change the state of the receive buffer FIFO and consequently the TXB8, FE, DOR and UPE bits, which all are stored in the FIFO, will change.

Assembly Code Example

The following code example shows a simple receive function for USART0 that handles both nine bit characters and the status bits. For the assembly code, the received data will be stored in R17:R16 after the code completes.

USART_Receive:
   ; Wait for data to be received
   in     r16, UCSR0A
   sbrs   r16, RXC
   rjmp   USART_Receive
   ; Get status and 9th bit, then data from buffer
   in     r18, UCSR0A
   in     r17, UCSR0B
   in     r16, UDR0
   ; If error, return -1
   andi   r18,(1<<FE)|(1<<DOR)|(1<<UPE)
   breq   USART_ReceiveNoError
   ldi    r17, HIGH(-1)
   ldi    r16, LOW(-1)
USART_ReceiveNoError:
   ; Filter the 9th bit, then return
   lsr    r17
   andi   r17, 0x01
   ret
unsigned int USART_Receive( void )
{
   unsigned char status, resh, resl;
   /* Wait for data to be received */
   while ( !(UCSR0A & (1<<RXC)) )
      ;
   /* Get status and 9th bit, then data */
   /* from buffer */
   status = UCSR0A;
   resh = UCSR0B;
   resl = UDR0;
   /* If error, return -1 */
   if ( status & (1<<FE)|(1<<DOR)|(1<<UPE) )
      return -1;
   /* Filter the 9th bit, then return */
   resh = (resh >> 1) & 0x01;
   return ((resh << 8) | resl);
}

The receive function example reads all the I/O Registers into the Register File before any computation is done. This gives an optimal receive buffer utilization since the buffer location read will be free to accept new data as early as possible.