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10. Serial Communication Interface
Transmit/Receive Formats
Figure 10.7 shows the general format for asynchronous serial communication.
Start
Serial
bit
data
1 bit
Figure 10.7 Data Format in Asynchronous Serial Communication Mode
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: starting
with the least significant bit), a parity bit (high or low level), and finally a stop bit (high level), in
this order.
In asynchronous data receiving, synchronization is with the falling edge of the start bit. SCI3
samples data on the 8th pulse of a clock that has 16 times the frequency of the bit rate, so each bit
of data is latched at its center.
Table 10.14 shows the 12 transmit/receive formats formats that can be selected in asynchronous
mode. The format is selected in the serial mode register (SMR).
Rev.3.00 Jul. 19, 2007 page 284 of 532
REJ09B0397-0300
(LSB)
Transmit or receive data
7 or 8 bits
One unit of data (character or frame)
(MSB)
Parity
Stop bit
bit
1 bit
or
1 or 2
none
bits
1
Mark
state
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