be placed directly in the main routine, or be combined with initialization code for other
I/O modules.
Data Transmission – The The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the
UCSRnB Register. When the Transmitter is enabled, the normal port operation of the
USART Transmitter
TxDn pin is overridden by the USART and given the function as the Transmitter’s serial
output. The baud rate, mode of operation and frame format must be set up once before
doing any transmissions. If synchronous operation is used, the clock on the XCKn pin
will be overridden and used as transmission clock.
Sending Frames with 5 to 8
Data Bit
A data transmission is initiated by loading the transmit buffer with the data to be trans-
mitted. The CPU can load the transmit buffer by writing to the UDRn I/O location. The
buffered data in the transmit buffer will be moved to the Shift Register when the Shift
Register is ready to send a new frame. The Shift Register is loaded with new data if it is
in idle state (no ongoing transmission) or immediately after the last stop bit of the previ-
ous frame is transmitted. When the Shift Register is loaded with new data, it will transfer
one complete frame at the rate given by the Baud Register, U2Xn bit or by XCKn
depending on mode of operation.
The following code examples show a simple USART transmit function based on polling
of the Data Register Empty (UDREn) Flag. When using frames with less than eight bits,
the most significant bits written to the UDRn are ignored. The USART has to be initial-
ized before the function can be used. For the assembly code, the data to be sent is
assumed to be stored in Register R16
Assembly Code Example(1)
USART_Transmit:
; Wait for empty transmit buffer
sbis UCSRnA,UDREn
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out
ret
UDRn,r16
C Code Example(1)
void USART_Transmit( unsigned char data )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) )
;
/* Put data into buffer, sends the data */
UDRn = data;
}
Note:
1. See “About Code Examples” on page 8.
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 inter-
rupt is utilized, the interrupt routine writes the data into the buffer.
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
212
ATmega640/1280/1281/2560/2561
2549A–AVR–03/05
ATMEL [ ATMEL ]
