ATmega64A datasheet(181/392 Pages) ATMEL

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181

8160C–AVR–07/09

ATmega64A

More advanced initialization routines can be made that include frame format as parameters, dis-

able 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.

20.6

Data Transmission – The USART Transmitter

The USART Transmitter is enabled by setting the Transmit Enable (TXENn) bit in the UCSRnB

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.

20.6.1

Sending Frames with 5 to 8 Data Bits

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 register,

U2Xn 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 (UDREn) flag. When using frames with less than eight bits, the most signif-

icant 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 8.

For I/O Registers located in extended I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI”

instructions must be replaced with instructions that allow access to extended I/O. Typically

“LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.

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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