DOC7707 datasheet(178/307 Pages) ETC1

178 / 307 page

178

7707F–AVR–11/10

AT90USB82/162

transmitter controls the transfer clock. The data written to UDRn is moved from the transmit buf-

fer to the shift register when the shift register is ready to send a new frame.

Note:

To keep the input buffer in sync with the number of data bytes transmitted, the UDRn register must

be read once for each byte transmitted. The input buffer operation is identical to normal USART

mode, i.e. if an overflow occurs the character last received will be lost, not the first data in the buf-

fer. This means that if four bytes are transferred, byte 1 first, then byte 2, 3, and 4, and the UDRn

is not read before all transfers are completed, then byte 3 to be received will be lost, and not byte

The following code examples show a simple USART in MSPIM mode transfer function based on

polling of the Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. The

USART has to be initialized before the function can be used. For the assembly code, the data to

be sent is assumed to be stored in Register R16 and the data received will be available in the

same register (R16) after the function returns.

The function simply waits for the transmit buffer to be empty by checking the UDREn Flag,

before loading it with new data to be transmitted. The function then waits for data to be present

in the receive buffer by checking the RXCn Flag, before reading the buffer and returning the

value..

Note:

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

Assembly Code Example

(1)

USART_MSPIM_Transfer:

; Wait for empty transmit buffer

sbis

UCSRnA, UDREn

rjmp

USART_MSPIM_Transfer

; Put data (r16) into buffer, sends the data

out

UDRn,r16

; Wait for data to be received

USART_MSPIM_Wait_RXCn:

sbis

UCSRnA, RXCn

rjmp

USART_MSPIM_Wait_RXCn

; Get and return received data from buffer

r16, UDRn

ret

C Code Example

(1)

unsigned char

USART_Receive( void )

{

/* Wait for empty transmit buffer */

while

( !( UCSRnA & (1<<UDREn)) );

/* Put data into buffer, sends the data */

UDRn = data;

/* Wait for data to be received */

while

( !(UCSRnA & (1<<RXCn)) );

/* Get and return received data from buffer */

return

UDRn;

}

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