24.7.2 Receiving Frames with 9 Data Bits

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

The following code example shows a simple USART receive function that handles both 9-bit characters and the status bits.

Assembly Code Example(1)

USART_Receive:
   ; Wait for data to be received
   sbis   r16, RXC
   rjmp   USART_Receive
   ; Get status and 9th bit, then data from buffer
   in     r18, UCSRA
   in     r17, UCSRB
   in     r16, UDR
   ; If error, return -1
   andi   r18,(1<<FE)|(1<<DOR)|(1<<PE)
   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 ( !(UCSRA & (1<<RXC)) )
      ;
   /* Get status and 9th bit, then data */
   /* from buffer */
   status = UCSRA;
   resh = UCSRB;
   resl = UDR;
   /* If error, return -1 */
   if ( status & (1<<FE)|(1<<DOR)|(1<<PE) )
      return -1;
   /* Filter the 9th bit, then return */
   resh = (resh >> 1) & 0x01;
   return ((resh << 8) | resl);
}
Note: 1. See About Code Examples.

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.