ATmega128 datasheet(178/386 Pages) ATMEL

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178

2467S–AVR–07/09

ATmega128

Data Transmission

– The USART

Transmitter

The USART Transmitter is enabled by setting the Transmit Enable (TXEN) bit in the UCSRB

den 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 synchro-

nous operation is used, the clock on the XCK 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 transmitted. The

CPU can load the transmit buffer by writing to the UDR 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 register,

U2X bit or by XCK depending on mode of operation.

The following code examples show a simple USART transmit function based on polling of the

Data Register Empty (UDRE) flag. When using frames with less than eight bits, the most signifi-

cant bits written to the UDR 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 9..

The function simply waits for the transmit buffer to be empty by checking the UDRE flag, before

loading it with new data to be transmitted. If the data register empty interrupt is utilized, the inter-

rupt routine writes the data into the buffer.

Assembly Code Example(1)

USART_Transmit:

; Wait for empty transmit buffer

sbis

UCSRA,UDRE

rjmp

USART_Transmit

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

out

UDR,r16

ret

C Code Example

(1)

void

USART_Transmit( unsigned char data )

{

/* Wait for empty transmit buffer */

while

( !( UCSRA & (1<<UDRE)) )

;

/* Put data into buffer, sends the data */

UDR = data;

}

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