
174
7530J–AVR–03/12
Atmel ATmega48/88/168 Automotive
17.5.2
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 UCS-
RnB before the low byte of the character is written to UDRn. 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.
Notes:
1. These transmit functions are written to be general functions. They can be optimized if the con-
tents of the UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used
after initialization.
2. The example code assumes that the part specific header file is included.
For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”
instructions must be replaced with instructions that allow access to extended I/O. Typically
“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
The ninth bit can be used for indicating an address frame when using multi processor communi-
cation mode or for other protocol handling as for example synchronization.
USART_Transmit:
; Wait for empty transmit buffer
sbis
UCSRnA,UDREn
rjmp
USART_Transmit
; Copy 9th bit from r17 to TXB8
cbi
UCSRnB,TXB8
sbrc
r17,0
sbi
UCSRnB,TXB8
; Put LSB data (r16) into buffer, sends the data
out
UDRn,r16
ret
void
USART_Transmit( unsigned int data )
{
/* Wait for empty transmit buffer */
while
( !( UCSRnA & (1<<UDREn))) )
;
/* Copy 9th bit to TXB8 */
UCSRnB &= ~(1<<TXB8);
if ( data & 0x0100 )
UCSRnB |= (1<<TXB8);
/* Put data into buffer, sends the data */
UDRn = data;
}