25.6.2 Sending Frames with 9 Data Bit
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 multi processor communication mode or for other protocol handling as for example synchronization.
Assembly Code Example
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
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
ret
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.