Datasheet

The following code example shows a simple USART receive function based on polling of the Receive
Complete (RXC) Flag. When using frames with less than eight bits the most significant bits of the data
read from the UDR will be masked to zero. The USART has to be initialized before the function can be
used.
Assembly Code Example
(1)
USART_Receive:
; Wait for data to be received
sbis UCSRA, RXC
rjmp USART_Receive
; Get and return received data from buffer
in r16, UDR
ret
C Code Example
(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRA & (1<<RXC)) )
;
/* Get and return received data from buffer */
return UDR;
}
Note: 1. See About Code Examples.
The function simply waits for data to be present in the receive buffer by checking the
RXC Flag, before reading the buffer and returning the value.
Related Links
About Code Examples on page 19
23.7.2. Receiving Frames with 9 Data Bits
If 9-bit characters are used (UCSZ=7) the ninth bit must be read from the RXB8 bit 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.
Assembly Code Example
(1)
USART_Receive:
; Wait for data to be received
sbis r16, 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
Atmel ATmega32A [DATASHEET]
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