ATmega16 datasheet(154/357 Pages) ATMEL

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154

2466S–AVR–05/09

ATmega16(L)

Data Reception –

The USART

Receiver

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

ter 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 oper-

ation 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. See “About Code Examples” on page 7.

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

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;

}

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