ATmega169P datasheet(180/393 Pages) ATMEL

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180

8018N–AVR–08/09

ATmega169P

19.7

Data Reception – The USART Receiver

The USART Receiver is enabled by writing the Receive Enable (RXENn) bit in the UCSRnB

ridden 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.

19.7.1

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 10.

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.

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

r16, UDR0

ret

C Code Example(1)

unsigned char

USART_Receive( void )

{

/* Wait for data to be received */

while

( !(UCSR0A & (1<<RXC0)) )

;

/* Get and return received data from buffer */

return

UDR0;

}

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