ATmega48/88/168
Data Reception – The
USART Receiver
The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the
UCSRnB Register to one. When the Receiver is enabled, the normal pin operation of
the RxDn 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
XCKn 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 XCKn 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 UDRn I/O
location.
The following code example shows a simple USART receive function based on polling
of the Receive Complete (RXCn) Flag. When using frames with less than eight bits the
most significant bits of the data read from the UDRn 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 UCSRnA, RXCn
rjmp USART_Receive
; Get and return received data from buffer
in
r16, UDRn
ret
C Code Example(1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRnA & (1<<RXCn)) )
;
/* Get and return received data from buffer */
return UDRn;
}
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 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.
171
2545D–AVR–07/04
ATMEL [ ATMEL ]
