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10.3.4
Operation in Asynchronous Mode
In asynchronous communication mode, a start bit indicating the start of communication and a stop
bit indicating the end of communication are added to each character that is sent/received. In this
way synchronization is achieved for each character as a self-contained unit.
SCI3 consists of independent transmit and receive modules, giving it the capability of full duplex
communication. Both the transmit and receive modules have a double-buffer configuration,
allowing data to be read or written during communication operations so that data can be
transmitted and received continuously.
(1) Transmit/Receive Formats: Figure 10.5 shows the general format for asynchronous serial
communication.
The communication line in asynchronous communication mode normally stays at the high level, in
the “mark” state. SCI3 monitors the communication line, and begins serial data communication
when it detects a “space” (low-level signal), which is regarded as a start bit.
One character consists of a start bit (low level), transmit/receive data (in LSB-first order), a parity
bit (high or low level), and finally a stop bit (high level), in this order.
In asynchronous data receiving, synchronization is carried out at the falling edge of the start bit.
SCI3 samples data on the 8th pulse of a clock that has 16 times the frequency of one-bit interval,
so each bit of data is latched at its center.
Serial
data
One unit of data (character or frame)
Start
bit
Transmit or receive data
Parity
bit
(MSB)
Stop bit
1 bit
1 bit
or
none
1 or 2
bits
7 or 8 bits
Mark
state
(LSB)
1
Figure 10.5 Data Format in Asynchronous Serial Communication Mode