ATA6614Q_14 Datasheet, PDF (176/311 Pages) ATMEL Corporation – 32K Flash Microcontroller with LIN Transceiver, 5V Regulator and Watchdog

5.19.7.3 Receive Compete Flag and Interrupt
The 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 receive buffer 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 receive complete interrupt will be
executed as long as the RXCn flag is set (provided that global interrupts are enabled). When interrupt-driven data reception
is used, the receive complete routine must read the received data from UDRn in order to clear the RXCn Flag, otherwise a
new interrupt will occur once the interrupt routine terminates.
5.19.7.4 Receiver Error Flags
The USART receiver has three error flags: frame error (FEn), data OverRun (DORn) and parity error (UPEn). All can be
accessed by reading UCSRnA. Common for the error flags is that they are located in the receive buffer together with the
frame for which they indicate the error status. Due to the buffering of the error flags, the UCSRnA must be read before the
receive 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 to the flag location. However, all flags must be set to zero when
the UCSRnA is written for upward compatibility 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 frame stored 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 for detecting out-of-sync conditions, detecting break conditions and protocol handling. The FEn
flag 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 zero when writing to UCSRnA.
The data OverRun (DORn) flag indicates data loss due to a receiver buffer full condition. A data OverRun occurs when the
receive buffer is full (two characters), it is a new character waiting in the receive shift register, and a new start bit is detected.
If the DORn flag is set there was one or more serial frame lost between the frame last read from UDRn, and the next frame
read from UDRn. For compatibility with future devices, always write this bit to zero when writing to UCSRnA. The DORn flag
is cleared when the frame received was successfully moved from the shift register to the receive buffer.
The parity error (UPEn) flag indicates that the next frame in the receive buffer had a parity error when received. If parity
check is not enabled the UPEn bit will always be read zero. For compatibility with future devices, always set this bit to zero
when writing to UCSRnA. For more details see Section 5.19.4.1 “Parity Bit Calculation” on page 170 and Section 5.19.7.5
“Parity Checker” on page 176.
5.19.7.5 Parity Checker
The parity checker is active when the high USART parity mode (UPMn1) bit is set. Type of parity check to be performed (odd
or even) is selected by the UPMn0 bit. When enabled, the parity checker calculates the parity of the data bits in incoming
frames and compares the result with the parity bit from the serial frame. The result of the check is stored in the receive buffer
together with the received data and stop bits. The parity error (UPEn) flag can then be read by software to check if the frame
had a parity error.
The UPEn bit is set if the next character that can be read from the receive buffer had a parity error when received and the
parity checking was enabled at that point (UPMn1 = 1). This bit is valid until the receive buffer (UDRn) is read.
5.19.7.6 Disabling the Receiver
In contrast to the transmitter, disabling of the receiver will be immediate. Data from ongoing receptions will therefore be lost.
When disabled (i.e., the RXENn is set to zero) the receiver will no longer override the normal function of the RxDn port pin.
The receiver buffer FIFO will be flushed when the receiver is disabled. Remaining data in the buffer will be lost.
176 ATA6614Q [DATASHEET]
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