Datasheet

83
ATmega323(L)
1457E–11/01
Data Reception – The
USART Receiver
The USART receiver is enabled by setting the Receive Enable (RXEN) bit in the
UCSRB register. When the receiver is enabled, the normal pin operation of the RxD pin
is overridden by the USART and given the function as the receiver’s serial input. The
baud rate, mode of operation and frame format must be set up once before any serial
reception can be done. If synchronous operation is used, the clock on the XCK pin will
be used as transfer clock.
Receiving Frames with 5 to 8
Data Bits
The receiver starts data reception when it detects a valid start bit. Each bit that follows
the start bit will be sampled at the baud rate or XCK clock, and shifted into the receive
shift register until the first stop bit of a frame is received. A second stop bit will be
ignored by the receiver. When the first stop bit is received, i.e. a complete serial frame is
present in the receive shift register, the contents of the shift register will be moved into
the receive buffer. The receive buffer can then be read by reading the UDR I/O location.
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.
Note: 1. The example code assumes that the part specific header file is included.
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.
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
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRA & (1<<RXC)) ) {};
/* Get and return received data from buffer */
return UDR;
}