ATMEGA323-8AI datasheet(85/247 Pages) ATMEL

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ATmega323(L)

1457G–AVR–09/03

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 PE 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 PE 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.

Note:

1. The example code assumes that the part specific header file is 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

r18, UCSRA

r17, UCSRB

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

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 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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