Datasheet

ATmega48PA/88PA/168PA [DATASHEET]
9223F–AVR–04/14
160
20.6.2 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 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.
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
(1)(2)
USART_Transmit:
; Wait for empty transmit buffer
in r16, UCSRnA
sbrs r16, 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
C Code Example
(1)(2)
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;
}
Notes: 1. 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.
2. See Section 6. “About Code Examples” on page 7.