Datasheet

127
8246A–AVR–11/09
ATtiny2313A/4313
14.6.2 Sending Frames with 9 Data Bit
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.
Note: 1. See “Code Examples” on page 6.
2. These transmit functions are written to be general functions. They can be optimized if the con-
tents 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 communi-
cation mode or for other protocol handling as for example synchronization.
14.6.3 Transmitter Flags and Interrupts
The USART Transmitter has two flags that indicate its state: USART Data Register Empty
(UDRE) and Transmit Complete (TXC). Both flags can be used for generating interrupts.
The Data Register Empty (UDRE) flag indicates whether the transmit buffer is ready to receive
new data. This bit is set when the transmit buffer is empty, and cleared when the transmit buffer
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
C Code Example
(1)
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;
}