Rainbow-electronics ATmega8515L Manual do Utilizador descarregar pdf (Página 139)

139

ATmega8515(L)

2512A–AVR–04/02

Sending Frames with 9 Data

Bits

If 9-bit characters areused(UCSZ=7),theninthbit must be writtentotheTXB8 bit in

UCSRBbeforethe lowbyte of the character iswritten to UDR. The following code

examplesshow atransmit function that handles9-bit characters. For theassembly

code, the data to be sent is assumed to be stored in Registers R17:R16.

Note: 1. Thesetransmit functions are written to be generalfunctions.Theycan beoptimized if

the contents of the UCSRB isstatic. For example, only theTXB8 bit of the UCSRB

Theninthbit can beusedfor indicatinganaddress frame when using multi processor

communication modeorfor otherprotocolhandling asfor example synchronization.

Transmitter Flags and

Interrupts

The USART Transmitterhas two flags that indicate itsstate:USART Data Register

Empty(UDRE) and Transmit Complete (TXC). Bothflagscan beusedfor generating

interrupts.

The Data RegisterEmpty(UDRE) flag indicateswhether thetransmit buffer isready to

receive newdata. Thisbit isset when thetransmit buffer is empty, andclearedwhen the

transmit buffercontainsdata to betransmitted that has not yet been moved into the Shift

the UCSRA Register.

When the Data RegisterEmptyInterrupt Enable (UDRIE) bit in UCSRB iswrittentoone,

the USART Data RegisterEmptyInterrupt will beexecuted aslong asUDRE isset (pro-

vided that global interrupts areenabled). UDRE isclearedbywriting UDR. When

interrupt-driven data transmission is used, the Data RegisterEmptyInterrupt routine

musteitherwrite newdata to UDRinorder to clearUDRE ordisablethe data register

Assembly Code Example

(1)

USART_Transmit:

;

Wait for empty transmit buffer

sbis UCSRA,UDRE

rjmp USART_Transmit

;

Copy ninth 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

CCode Example

(1)

void USART_Transmit( unsigned int data )

{

Wait for empty transmit buffer

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

;

Copy ninth 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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