Datasheet

174
ATtiny828 [DATASHEET]
8371A–AVR–08/12
17.7.2 Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZ = 7) the ninth bit must be read from the RXB8 bit before reading the low bits from
UDR. This rule applies to the FE, DOR and UPE status flags, as well. Status bits must be read before data from UDR,
since reading UDR will change the state of the receive buffer FIFO and, consequently, state of TXB8, FE, DOR and UPE
bits.
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
in r18, UCSRA
in r17, UCSRB
in 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