Atmega128rfa1 – Rainbow Electronics ATmega128RFA1 User Manual
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8266A-MCU Wireless-12/09
ATmega128RFA1
UDRn is moved from the transmit buffer 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 buffer. 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 1.
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.
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
in r16, UDRn
ret
C Code Example
(1)
unsigned char USART_Receive( void )
{
/* Wait for empty transmit buffer */
while ( !( UCSRnA & (1< /* Put data into buffer, sends the data */ UDRn = data; /* Wait for data to be received */ while ( !(UCSRnA & (1< /* Get and return received data from buffer */ return UDRn; } Notes: