ATmega48/88/168
The function simply waits for the transmit buffer to be empty by checking the UDREn Flag,
before loading it with new data to be transmitted. If the Data Register Empty interrupt is utilized,
the interrupt routine writes the data into the buffer.
19.6.2
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 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.
Assembly Code Example(1)(2)
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
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 con-
tents of the UCSRnB is static. For example, only the TXB8 bit of the UCSRnB Register is used
after initialization.
2. See ”About Code Examples” on page 9.
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.
19.6.3
Transmitter Flags and Interrupts
The USART Transmitter has two flags that indicate its state: USART Data Register Empty
(UDREn) and Transmit Complete (TXCn). Both flags can be used for generating interrupts.
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2545M–AVR–09/07
ATMEL [ ATMEL ]
