ATmega88 datasheet(173/340 Pages) ATMEL

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7530H–AVR–02/09

ATmega48/88/168 Automotive

17.5.1

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 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 sig-

nificant 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. The example code assumes that the part specific header file is included.

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”.

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

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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