Datasheet

343
32133D–11/2011
UC3D
Figure 20-4. Transmitter Status
20.6.3.2 Asynchronous Receiver
If the USART is configured in an asynchronous operating mode (MR.SYNC = 0), the receiver will
oversample the RXD input line by either 8 or 16 times the baud rate clock, as selected by the
Oversampling Mode bit (MR.OVER). If the line is zero for half a bit period (four or eight consecu-
tive samples, respectively), a start bit will be assumed, and the following 8th or 16th sample will
determine the logical value on the line, in effect resulting in bit values being determined at the
middle of the bit period.
The number of data bits, endianess, parity mode, and stop bits are selected by the same bits
and fields as for the transmitter (MR.CHRL, MODE9, MSBF, PAR, and NBSTOP). The synchro-
nization mechanism will only consider one stop bit, regardless of the used protocol, and when
the first stop bit has been sampled, the receiver will automatically begin looking for a new start
bit, enabling resynchronization even if there is a protocol miss-match. Figure 20-5 and Figure
20-6 illustrate start bit detection and character reception in asynchronous mode.
Figure 20-5. Asynchronous Start Bit Detection
D0 D1 D2 D3 D4 D5 D6 D7
TXD
Start
Bit
Parity
Bit
Stop
Bit
Baud Rate
Clock
Start
Bit
Write
THR
D0 D1 D2 D3 D4 D5 D6 D7
Parity
Bit
Stop
Bit
TXRDY
TXEMPTY
Sampling
Clock (x16)
RXD
Start
Detection
Sampling
Baud Rate
Clock
RXD
Start
Rejection
Sampling
12345678
123456701234
123456789 10111213141516
D0
Sampling