Sending Frames with 9 Data Bits

If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before the low byte of the character is written to UDRn.

The ninth bit can be used for indicating an address frame when using multiprocessor communication mode or for another protocol handling as for example synchronization.

The following code examples show a transmit function that handles 9-bit characters. For the assembly code, the data to be sent is assumed to be stored in registers R17:R16.

Assembly Code Example

   ; Wait for empty transmit buffer
   in      r18, UCSR0A
   sbrs    r18, UDRE
   rjmp    USART_Transmit
   ; Copy 9th bit from r17 to TXB8
   cbi     UCSR0B,TXB8
   sbrc    r17,0
   sbi     UCSR0B,TXB8
   ; Put LSB data (r16) into buffer, sends the data
   out    UDR0,r16

C Code Example

void USART_Transmit( unsigned int data )
   /* Wait for empty transmit buffer */
   while ( !( UCSR0A & (1<<UDRE))) )
   /* Copy 9th bit to TXB8 */
   UCSR0B &= ~(1<<TXB8);
   if ( data & 0x0100 )
      UCSR0B |= (1<<TXB8);
   /* Put data into buffer, sends the data */
   UDR0 = data;
Note: These transmit functions are written to be general functions. They can be optimized if the contents of the UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used after initialization.