ATMEGA640 datasheet(216/449 Pages) ATMEL

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ATmega640/1280/1281/2560/2561

2549K–AVR–01/07

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.

Data Transmission – The

USART Transmitter

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.

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

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:

1. See “About Code Examples” on page 9.

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

(1)

USART_Transmit:

; Wait for empty transmit buffer

sbis

UCSRnA,UDREn

rjmp

USART_Transmit

; Put data (r16) into buffer, sends the data

out

UDRn,r16

ret

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;

}

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