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25.7. Data Reception – The USART Receiver

The USART Receiver is enabled by writing the Receive Enable (RXEN) bit in the UCSRB 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.

25.7.1. 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 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.

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.