However, the receive buffering has two improvements that will affect the compatibility in
some special cases:
• A second Buffer Register has been added. The two Buffer Registers operate as a
circular FIFO buffer. Therefore the UDR must only be read once for each incoming
data! More important is the fact that the Error Flags (FE and DOR) and the ninth
data bit (RXB
are buffered with the data in the receive buffer. Therefore the status
bits must always be read before the UDR Register is read. Otherwise the error
status will be lost since the buffer state is lost.
• The Receiver Shift Register can now act as a third buffer level. This is done by
allowing the received data to remain in the serial Shift Register (see Figure 61) if the
Buffer Registers are full, until a new start bit is detected. The USART is therefore
more resistant to Data OverRun (DOR) error conditions.