ATMEGA640 datasheet(207/436 Pages) MICROCHIP

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ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]

DS40002211A

22.6 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 func-

tion 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.

22.6.1

Sending Frames with 5 to 8 Data Bit

A data transmission is initiated by loading the transmit buffer with the data to be transmitted. 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 previous 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

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 initialized 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 10.

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.

Assembly Code Example(1)

USART_Transmit:

; Wait for empty transmit buffer

lds

r17, UCSRnA

sbrs

r17, UDREn

rjmp

USART_Transmit

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

sts

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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