Rainbow-electronics ATmega8515L Manual do Utilizador descarregar pdf (Página 142)

142

ATmega8515(L)

2512A–AVR–04/02

Receiving Frames with 9 Data

Bits

If 9-bit characters areused(UCSZ=7)theninthbit must be readfrom theRXB8 bit in

UCSRBbefore reading the lowbitsfrom the UDR. Thisruleapplies to the FE, DOR, and

PEstatusflags aswell.Readstatusfrom UCSRA, then data from UDR. Reading the

UDR I/Olocation will change the state of the receive bufferFIFOandconsequently the

TXB8,FE,DOR, and PEbits, which all are stored in the FIFO, will change.

The following codeexample shows a simple USART receive function that handlesboth

9-bit characters and the statusbits.

Note: 1. Theexample codeassumes that the part specificheaderfileis included.

Assembly Code Example

(1)

USART_Receive:

;

Wait for data to be received

sbis UCSRA, RXC

rjmp USART_Receive

;

Get status and ninth bit, then data from buffer

in r18, UCSRA

in r17, UCSRB

in r16, UDR

;

If error, return -1

andi r18,(1<<FE)|(1<<DOR)|(1<<PE)

breq USART_ReceiveNoError

ldi r17, HIGH(-1)

ldi r16, LOW(-1)

USART_ReceiveNoError:

;

Filter the ninth bit, then return

lsr r17

andi r17, 0x01

ret

CCode Example

(1)

unsigned int USART_Receive( void )

{

unsigned char status, resh, resl;

Wait for data to be received

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

;

Get status and ninth bit, then data

from buffer

status = UCSRA;

resh = UCSRB;

resl = UDR;

If error, return -1

if ( status & (1<<FE)|(1<<DOR)|(1<<PE) )

return -1;

Filter the ninth bit, then return

resh = (resh >> 1) & 0x01;

return ((resh << 8) | resl);

}

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