Rev. 5.00, 09/03, page 455 of 760
14.3.2 Operation in Asynchronous Mode
In asynchronous m ode, each transmitted or received character beg i ns with a start bit and end s with
a stop bit. Serial communication is synchronized one character at a time.
The transmitting and receiving sections of th e SCI are independent, so full duplex com munication
is possible. The transmitter and receiver are both double buffered, so data can be written and r ead
while transmitting and receivi ng 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), p arity bit (high or low), and stop bi t (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
0D1D2D3D4D5D6D7
11
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 Tw o Stop Bits)