Table of Contents
13.3.2 Operation in Asynchronous Mode
www.DataSheet4U.com
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 13-2 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), 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.
1
(LSB)
Serial data
0
D0
Start
bit
1 bit
Figure 13-2 Data Format in Asynchronous Communication (Example: 8-Bit Data with
D1
D2
D3
D4
Transmit or receive data
7 bits or 8 bits
One unit of data (character or frame)
Parity and 2 Stop Bits)
466
(MSB)
D5
D6
D7
0/1
Parity
bit
1 bit or
no bit
Idle (mark) state
1
1
1
Stop
bit
1 bit or
2 bits
Table of Contents
