DOC7707 datasheet(154/307 Pages) ETC1

154 / 307 page

154

7707F–AVR–11/10

AT90USB82/162

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 TXB8 bit of the UCSRnB Register is used

after initialization.

2. See “About Code Examples” on page 7.

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.

17.5.3

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 compat-

ibility 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 Register Empty Interrupt will be executed as long as UDREn is set (provided that

global interrupts are enabled). UDREn is cleared by writing UDRn. When interrupt-driven data

transmission is used, the Data Register Empty interrupt routine must either write new data to

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

out

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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