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SERIAL COMMUNICATIONS UNIT
1
2
3
4
5
6
7
8
9
10
Parity
Stop
Start
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
Bit 6
or
Bit
Bit
Bit 7
A1274-0A
Figure 11-1. Typical 10-Bit Asynchronous Data Frame
When discussing asynchronous communications, it makes sense to talk about the receive ma-
chine (RX machine) and the transmit machine (TX machine) separately. Each is completely in-
dependent. Transmission and reception can occur simultaneously, making the asynchronous
modes full-duplex.
11.1.1.1
RX Machine
The RX machine (Figure 11-2) shifts the received serial data into the receive shift register. When
the reception has completed, the data is then moved into the Serial Receive Buffer (SxRBUF)
Register. From there, the user can read the received data byte.
The RX machine samples the RXD pin, looking for a logical low (start bit) signifying the begin-
ning of a reception. Once the logical low has been detected, the RX machine begins the receive
process. Each expected bit-time is divided into eight samples by the 8X baud clock. The RX ma-
chine takes the three middle samples and, based on a two-out-of-three majority, determines the
data bit value. This oversampling is common for asynchronous serial ports and improves noise
immunity. This majority value is then shifted into the receive shift register.
Using this method, the RX machine can tolerate incoming baud rates that differ from its own in-
ternal baud rates by 2.5% overspeed and 5.5% underspeed. These limits exceed the CCITT ex-
tended signaling rate specifications.
A stop bit is expected by the RX machine after the proper number of data bits. When the stop bit
has been validated, the data from the shift register is copied into SxRBUF and the Receive Inter-
rupt (RI) bit is set. Note that the stop bit is actually validated right after its middle three samples
are taken. Therefore, the data is moved into SxRBUF and the RI bit is set approximately in the
middle of the stop bit time.
11-2
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