ATmega325P datasheet(172/364 Pages) ATMEL

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8023F–AVR–07/09

ATmega325P/3250P

18.6.2

Sending Frames with 9 Data Bit

If 9-bit characters are used (UCSZ = 7), the ninth bit must be written to the TXB8n 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 con-

tents of the UCSRnB is static. For example, only the TXB80 bit of the UCSRnB Register is

used after initialization.

2. See Section “5.” on page 10.

The ninth bit can be used for indicating an address frame when using multi processor communi-

cation mode or for other protocol handling as for example synchronization.

18.6.3

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

Assembly Code Example

(1)(2)

USART_Transmit:

; Wait for empty transmit buffer

sbis

UCSR0A,UDRE0

rjmp

USART_Transmit

; Copy 9th bit from r17 to TXB80

cbi

UCSR0B,TXB80

sbrc

r17,0

sbi

UCSR0B,TXB80

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

out

UDR0,r16

ret

C Code Example

(1)(2)

void

USART_Transmit( unsigned int data )

{

/* Wait for empty transmit buffer */

while

( !( UCSR0A & (1<<UDRE0))) )

;

/* Copy 9th bit to TXB80 */

UCSR0B &= ~(1<<TXB80);

if ( data & 0x0100 )

UCSR0B |= (1<<TXB80);

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

UDR0 = data;

}

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