Datasheet

/* IMPORTANT: The Baud Rate must be set after the transmitter is enabled */
UBRRn = baud;
}
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About Code Examples on page 18
22.6. Data Transfer
Using the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit in the
UCSRnB register is set to one. When the Transmitter is enabled, the normal port operation of the TxDn
pin is overridden and given the function as the Transmitter's serial output. Enabling the receiver is
optional and is done by setting the RXENn bit in the UCSRnB register to one. When the receiver is
enabled, the normal pin operation of the RxDn pin is overridden and given the function as the Receiver's
serial input. The XCKn will in both cases be used as the transfer clock.
After initialization the USART is ready for doing data transfers. A data transfer is initiated by writing to the
UDRn I/O location. This is the case for both sending and receiving data since the transmitter controls the
transfer clock. The data written to 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
USART_MSPIM_Transfer:
; Wait for empty transmit buffer
in r16, UCSRnA
sbrs r16, 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:
in r16, UCSRnA
sbrs r16, RXCn
rjmp USART_MSPIM_Wait_RXCn
; Get and return received data from buffer
in r16, UDRn
ret
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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