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Rev. 5.00, 09/03, page 455 of 760
14.3.2 Operation in Asynchronous Mode
In asynchronous mode, each transmitted or received character begins with a start bit and ends with
a stop bit. Serial communication is synchronized one character at a time.
The transmitting and receiving sections of the SCI are independent, so full duplex communication
is possible. The transmitter and receiver are both double buffered, so data can be written and read
while transmitting and receiving are in progress, enabling continuous transmitting and receiving.
Figure 14.5 shows the general format of asynchronous serial communication. In asynchronous
serial communication, the communication line is normally held in the mark (high) state. The SCI
monitors the line and starts serial communication when the line goes to the space (low) state,
indicating a start bit. One serial character consists of a start bit (low), data (LSB first; starting from
the lowerest bit), parity bit (high or low), and stop bit (high), in that order.
When receiving in asynchronous mode, the SCI synchronizes at the falling edge of the start bit.
The SCI samples each data bit on the eighth pulse of a clock with a frequency 16 times the bit rate.
Receive data is latched at the center of each bit.
0D
0
D
1
D
2
D
3
D
4
D
5
D
6
D
7
1
1
0/1 1 1
(LSB) (MSB)
Serial
data
Start
bit
1 bit
Transmit/receive data
7 or 8 bits
One unit of communication data (character or frame)
Idle (mark) state
Parity
bit
Stop
bit
1 or
no bit
1 or
2 bits
Figure 14.5 Example of Data Format in Asynchronous Communication
(8-Bit Data with Parity and Two Stop Bits)