24.8.4 Receiver Error Flags

The USART receiver has three error flags: Frame Error (FE), Data OverRun (DOR) and Parity Error (UPE). 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 cannot 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 FE flag indicates the state of the first stop bit of the next readable frame stored in the receive buffer. The FE flag is zero when the stop bit was correctly read as '1', and the FE 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 FE flag is not affected by the setting of the USBS 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 DOR flag indicates data loss due to a receiver buffer full condition. A DOR occurs when the receive buffer is full (two characters), a new character is waiting in the Receive Shift register, and a new start bit is detected. If the DOR flag is set, one or more serial frames were lost between the last frame read from UDR, and the next frame read from UDR. For compatibility with future devices, always write this bit to zero when writing to UCSRnA. The DOR flag is cleared when the frame received was successfully moved from the Shift register to the receive buffer.

The Parity Error (UPE) flag indicates that the next frame in the receive buffer had a UPE when received. If Parity Check is not enabled the UPE bit will always read '0'. For compatibility with future devices, always set this bit to zero when writing to UCSRnA. For more details see Parity Bit Calculation and 'Parity Checker' below.