ATMega chip full datasheet - UCSD Department of Physics

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ATmega48A/PA/88A/PA/168A/PA/328/P21.6 Data TransferUsing the USART in MSPI mode requires the Transmitter to be enabled, i.e. the TXENn bit inthe UCSRnB register is set to one. When the Transmitter is enabled, the normal port operationof the TxDn pin is overridden and given the function as the Transmitter's serial output. Enablingthe 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 giventhe function as the Receiver's serial input. The XCKn will in both cases be used as the transferclock.After initialization the USART is ready for doing data transfers. A data transfer is initiated by writingto the UDRn I/O location. This is the case for both sending and receiving data since thetransmitter controls the transfer clock. The data written to UDRn is moved from the transmit bufferto 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 mustbe read once for each byte transmitted. The input buffer operation is identical to normal USARTmode, 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 UDRnis not read before all transfers are completed, then byte 3 to be received will be lost, and not byte1.The following code examples show a simple USART in MSPIM mode transfer function based onpolling of the Data Register Empty (UDREn) Flag and the Receive Complete (RXCn) Flag. TheUSART has to be initialized before the function can be used. For the assembly code, the data tobe sent is assumed to be stored in Register R16 and the data received will be available in thesame 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 presentin the receive buffer by checking the RXCn Flag, before reading the buffer and returning thevalue.8271D–AVR–05/11210
ATmega48A/PA/88A/PA/168A/PA/328/PAssembly Code Example (1)USART_MSPIM_Transfer:; Wait for empty transmit bufferin r16, UCSRnAsbrs r16, UDREnrjmp USART_MSPIM_Transfer; Put data (r16) into buffer, sends the dataout UDRn,r16; Wait for data to be receivedUSART_MSPIM_Wait_RXCn:in r16, UCSRnAsbrs r16, RXCnrjmp USART_MSPIM_Wait_RXCn; Get and return received data from bufferin r16, UDRnretC Code Example (1)unsigned char USART_Receive( void ){/* Wait for empty transmit buffer */while ( !( UCSRnA & (1
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Atmel Corporation2325 Orchard Parkw
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ATMega chip full datasheet - UCSD Department of Physics

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