Datasheet

155
ATmega48/88/168 Automotive [DATASHEET]
7530K–AVR–07/14
The function simply waits for data to be present in the receive buffer by checking the RXCn flag, before reading the buffer
and returning the value.
17.6.2 Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZn=7) the ninth bit must be read from the RXB8n bit in UCSRnB before reading the low bits
from the UDRn. This rule applies to the FEn, DORn and UPEn status flags as well. Read status from UCSRnA, then data
from UDRn. Reading the UDRn I/O location will change the state of the receive buffer FIFO and consequently the TXB8n,
FEn, DORn and UPEn 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. For I/O registers located in extended
I/O map, “IN”, “OUT”, “SBIS”, “SBIC”, “CBI”, and “SBI” instructions must be replaced with instructions that allow
access to extended I/O. Typically “LDS” and “STS” combined with “SBRS”, “SBRC”, “SBR”, and “CBR”.
The receive function example reads all the I/O registers into the register file before any computation is done. This gives an
optimal receive buffer utilization since the buffer location read will be free to accept new data as early as possible.
Assembly Code Example
(1)
USART_Receive:
; Wait for data to be received
sbis UCSRnA, RXCn
rjmp USART_Receive
; Get status and 9th bit, then data from buffer
in r18, UCSRnA
in r17, UCSRnB
in r16, UDRn
; If error, return -1
andi r18,(1<<FEn)|(1<<DORn)|(1<<UPEn)
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 ( !(UCSRnA & (1<<RXCn)) )
;
/* Get status and 9th bit, then data */
/* from buffer */
status = UCSRnA;
resh = UCSRnB;
resl = UDRn;
/* If error, return -1 */
if ( status & (1<<FEn)|(1<<DORn)|(1<<UPEn) )
return -1;
/* Filter the 9th bit, then return */
resh = (resh >> 1) & 0x01;
return ((resh << 8) | resl);
}