ATMEGA640 datasheet(208/436 Pages) MICROCHIP

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

DS40002211A

22.6.2

Sending Frames with 9 Data Bit

If 9-bit characters are used (UCSZn = 7), the ninth bit must be written to the TXB8 bit in UCSRnB before the low

byte of the character is written to UDRn. The following code examples show a transmit function that handles 9-bit

characters. For the assembly code, the data to be sent is assumed to be stored in registers R17:R16.

Notes: 1. These transmit functions are written to be general functions. They can be optimized if the contents of the UCSRnB

is static. For example, only the TXB8 bit of the UCSRnB Register is used after initialization.

2. See “About Code Examples” on page 10.

The ninth bit can be used for indicating an address frame when using multi processor communication mode or for

other protocol handling as for example synchronization.

22.6.3

Transmitter Flags and Interrupts

The USART Transmitter has two flags that indicate its state: USART Data Register Empty (UDREn) and Transmit

Complete (TXCn). Both flags can be used for generating interrupts.

The Data Register Empty (UDREn) Flag indicates whether the transmit buffer is ready to receive new data. This bit

is set when the transmit buffer is empty, and cleared when the transmit buffer contains data to be transmitted that

has not yet been moved into the Shift Register. For compatibility with future devices, always write this bit to zero

when writing the UCSRnA Register.

When the Data Register Empty Interrupt Enable (UDRIEn) bit in UCSRnB is written to one, the USART Data Reg-

ister Empty Interrupt will be executed as long as UDREn is set (provided that global interrupts are enabled).

Assembly Code Example(1)(2)

USART_Transmit:

; Wait for empty transmit buffer

sbis

UCSRnA,UDREn

rjmp

USART_Transmit

; Copy 9th bit from r17 to TXB8

cbi

UCSRnB,TXB8

sbrc

r17,0

sbi

UCSRnB,TXB8

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

sts

UDRn,r16

ret

C Code Example(1)(2)

void

USART_Transmit( unsigned int data )

{

/* Wait for empty transmit buffer */

while

( !( UCSRnA & (1<<UDREn))) )

;

/* Copy 9th bit to TXB8 */

UCSRnB &= ~(1<<TXB8);

if ( data & 0x0100 )

UCSRnB |= (1<<TXB8);

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

UDRn = data;

}

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