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.
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; }
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.