ATMega chip full datasheet - UCSD Department of Physics

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ATmega48A/PA/88A/PA/168A/PA/328/PAssembly Code Example (1)USART_Receive:; Wait for data to be receivedin r16, UCSRnAsbrs r16, RXCnrjmp USART_Receive; Get status and 9th bit, then data from bufferin r18, UCSRnAin r17, UCSRnBin r16, UDRn; If error, return -1andi r18,(1
ATmega48A/PA/88A/PA/168A/PA/328/PThe receive function example reads all the I/O Registers into the Register File before any computationis done. This gives an optimal receive buffer utilization since the buffer location read willbe free to accept new data as early as possible.20.7.3 Receive Compete Flag and InterruptThe USART Receiver has one flag that indicates the Receiver state.The Receive Complete (RXCn) Flag indicates if there are unread data present in the receive buffer.This flag is one when unread data exist in the receive buffer, and zero when the receivebuffer is empty (i.e., does not contain any unread data). If the Receiver is disabled (RXENn = 0),the receive buffer will be flushed and consequently the RXCn bit will become zero.When the Receive Complete Interrupt Enable (RXCIEn) in UCSRnB is set, the USART ReceiveComplete interrupt will be executed as long as the RXCn Flag is set (provided that global interruptsare enabled). When interrupt-driven data reception is used, the receive complete routinemust read the received data from UDRn in order to clear the RXCn Flag, otherwise a new interruptwill occur once the interrupt routine terminates.20.7.4 Receiver Error FlagsThe USART Receiver has three Error Flags: Frame Error (FEn), Data OverRun (DORn) andParity Error (UPEn). All can be accessed by reading UCSRnA. Common for the Error Flags isthat they are located in the receive buffer together with the frame for which they indicate theerror status. Due to the buffering of the Error Flags, the UCSRnA must be read before thereceive buffer (UDRn), since reading the UDRn I/O location changes the buffer read location.Another equality for the Error Flags is that they can not be altered by software doing a write tothe flag location. However, all flags must be set to zero when the UCSRnA is written for upwardcompatibility of future USART implementations. None of the Error Flags can generate interrupts.The Frame Error (FEn) Flag indicates the state of the first stop bit of the next readable framestored in the receive buffer. The FEn Flag is zero when the stop bit was correctly read (as one),and the FEn Flag will be one when the stop bit was incorrect (zero). This flag can be used fordetecting out-of-sync conditions, detecting break conditions and protocol handling. The FEnFlag is not affected by the setting of the USBSn bit in UCSRnC since the Receiver ignores all,except for the first, stop bits. For compatibility with future devices, always set this bit to zerowhen writing to UCSRnA.The Data OverRun (DORn) Flag indicates data loss due to a receiver buffer full condition. AData OverRun occurs when the receive buffer is full (two characters), it is a new character waitingin the Receive Shift Register, and a new start bit is detected. If the DORn Flag is set therewas one or more serial frame lost between the frame last read from UDRn, and the next frameread from UDRn. For compatibility with future devices, always write this bit to zero when writingto UCSRnA. The DORn Flag is cleared when the frame received was successfully moved fromthe Shift Register to the receive buffer.The Parity Error (UPEn) Flag indicates that the next frame in the receive buffer had a ParityError when received. If Parity Check is not enabled the UPEn bit will always be read zero. Forcompatibility with future devices, always set this bit to zero when writing to UCSRnA. For moredetails see ”Parity Bit Calculation” on page 183 and ”Parity Checker” on page 191.20.7.5 Parity CheckerThe Parity Checker is active when the high USART Parity mode (UPMn1) bit is set. Type of ParityCheck to be performed (odd or even) is selected by the UPMn0 bit. When enabled, the Parity8271D–AVR–05/11191
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Atmel Corporation2325 Orchard Parkw
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ATMega chip full datasheet - UCSD Department of Physics

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