Receiving frames with 9 databits, Atmega32(l) – Rainbow Electronics ATmega32L User Manual
Page 149
149
ATmega32(L)
2503C–AVR–10/02
Receiving Frames with 9
Databits
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
9-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 9th 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 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 */
if
( status & (1<<FE)|(1<<DOR)|(1<<PE) )
return
-1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return
((resh << 8) | resl);
}