Rainbow Electronics ATmega3290P_V - Manual (Page 171)

171
ATmega329/3290/649/6490
2552H–AVR–11/06
Data Reception – The
USART Receiver The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the UCS-
RnB Register to one. 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 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.
Note: 1. See “About Code Examples” on page 8.
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.
Receiving Frames with 9 Data
Bits If 9-bit characters are used (UCSZ=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.
Assembly Code Example (1)
USART_Receive:
; Wait for data to be received
sbis UCSR0A, RXC0
rjmp USART_Receive
; Get and return received data from buffer
in r16, UDR0
ret
C Code Example (1)
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSR0A & (1<
;
/* Get and return received data from buffer */
return UDR0;
}