Sending Frames with 9 Data Bits

If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to the TXB8 bit in UCSRB before the Low byte of the character is written to UDR. 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(1)

USART_Transmit:
   ; Wait for empty transmit buffer
   sbis    UCSRA,UDRE
   rjmp    USART_Transmit
   ; Copy 9th bit from r17 to TXB8
   cbi     UCSRB,TXB8
   sbrc    r17,0
   sbi     UCSRB,TXB8
   ; Put LSB data (r16) into buffer, sends the data
   out    UDR,r16
   ret
void USART_Transmit( unsigned int data )
{
   /* Wait for empty transmit buffer */
   while ( !( UCSRA & (1<<UDRE))) )
      ;
   /* Copy 9th bit to TXB8 */
   UCSRB &= ~(1<<TXB8);
   if ( data & 0x0100 )
      UCSRB |= (1<<TXB8);
   /* Put data into buffer, sends the data */
   UDR = data;
}
Note: 1. These transmit functions are written to be general functions. They can be optimized if the contents of the UCSRB is static. For example, only the TXB8 bit of the UCSRB Register is used after initialization.

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