ATMEGA2561V ATMEL [ATMEL Corporation], ATMEGA2561V Datasheet - Page 216

• Manufacturer Part Number: ATMEGA2561V
• Description: 8-bit Microcontroller with 64K/128K/256K Bytes In-System Programmable Flash
• Manufacturer: ATMEL [ATMEL Corporation]

Data Transmission – The
USART Transmitter
Sending Frames with 5 to 8
Data Bit
216
ATmega640/1280/1281/2560/2561
More advanced initialization routines can be made that include frame format as parame-
ters, disable interrupts and so on. However, many applications use a fixed setting of the
baud and control registers, and for these types of applications the initialization code can
be placed directly in the main routine, or be combined with initialization code for other
I/O modules.
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
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.
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
Note:
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.
Assembly Code Example
C Code Example
USART_Transmit:
void USART_Transmit( unsigned char data )
{
}
; Wait for empty transmit buffer
sbis UCSRnA,UDREn
rjmp USART_Transmit
; Put data (r16) into buffer, sends the data
out
ret
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1<<UDREn)) )
/* Put data into buffer, sends the data */
UDRn = data;
1. See “About Code Examples” on page 9.
UDRn,r16
;
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(1)
2549K–AVR–01/07