ATmega16 datasheet(145/315 Pages) ATMEL

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145

ATmega16(L)

2466E–AVR–10/02

Sending Frames with 9 Data

Bit

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

UCSRB before the low byte of the character is written to UDR. 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.

Note:

1. These transmit functions are written to be general functions. They can be optimized if

the contents of the UCSRB is static. (i.e., only the TXB8 bit of the UCSRB Register is

used after initialization).

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.

Transmitter Flags and

Interrupts

The USART transmitter has two flags that indicate its state: USART Data Register

Empty (UDRE) and Transmit Complete (TXC). Both flags can be used for generating

interrupts.

The Data Register Empty (UDRE) 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

the UCSRA register.

When the Data Register empty Interrupt Enable (UDRIE) bit in UCSRB is written to one,

the USART Data Register Empty Interrupt will be executed as long as UDRE is set (pro-

vided that global interrupts are enabled). UDRE is cleared by writing UDR. When

interrupt-driven data transmission is used, the data register empty Interrupt routine must

Assembly Code Example(1)

USART_Transmit:

; Wait for empty transmit buffer

sbis

UCSRA,UDRE

rjmp

USART_Transmit

; Copy 9th bit from r17 to TXB8

cbi

UCSRB,TXB8

sbrc

r17,0

sbi

UCSRB,TXB8

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

out

UDR,r16

ret

C Code Example(1)

void

USART_Transmit( unsigned int data )

{

/* Wait for empty transmit buffer */

while

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

;

/* Copy 9th bit to TXB8 */

UCSRB &= ~(1<<TXB8);

if ( data & 0x0100 )

UCSRB |= (1<<TXB8);

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

UDR = data;

}

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