ATmega128L datasheet(182/386 Pages) ATMEL

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182

2467S–AVR–07/09

ATmega128

Receiving Frames with

9 Data Bits

If 9-bit characters are used (UCSZ=7) the ninth bit must be read from the RXB8 bit in UCSRB

before reading the low bits from the UDR. This rule applies to the FE, DOR and UPE status

flags as well. Read status from UCSRA, then data from UDR. Reading the UDR I/O location will

change the state of the receive buffer FIFO and consequently the TXB8, FE, DOR, and UPE

bits, which all are stored in the FIFO, will change.

The following code example shows a simple USART receive function that handles both nine bit

characters and the status bits.

Assembly Code Example

(1)

USART_Receive:

; Wait for data to be received

sbis

UCSRA, RXC

rjmp

USART_Receive

; Get status and 9th bit, then data from buffer

r18, UCSRA

r17, UCSRB

r16, UDR

; If error, return -1

andi

r18,(1<<FE)|(1<<DOR)|(1<<UPE)

breq

USART_ReceiveNoError

ldi

r17, HIGH(-1)

ldi

r16, LOW(-1)

USART_ReceiveNoError:

; Filter the 9th bit, then return

lsr

r17

andi

r17, 0x01

ret

C Code 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 9th bit, then data */

/* from buffer */

status = UCSRA;

resh = UCSRB;

resl = UDR;

/* If error, return -1 */

( status & (1<<FE)|(1<<DOR)|(1<<UPE) )

return

-1;

/* Filter the 9th bit, then return */

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

return

((resh << 8) | resl);

}

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