Rainbow-electronics Atmega169L Manuel d'utilisateur télécharger pdf (Page 160)

160

ATmega169V/L

2514A–AVR–08/02

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’sserial

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 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. 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

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

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;

}

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Rainbow-electronics Atmega169L Manuel d'utilisateur Page 160

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