Operation In Asynchronous Mode - Hitachi F-ZTAT H8/3039 Series Hardware Manual

11.3.2 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 11-2 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
Serial data 0
Start
bit
1 bit
Figure 11-2 Data Format in Asynchronous Communication (Example: 8-Bit Data with
356
(LSB)
D0 D1 D2 D3
Transmit or receive data
7 bits or 8 bits
One unit of data (character or frame)
Parity and 2 Stop Bits)
(MSB)
D4 D5 D6 D7
Idle (mark) state
1
0/1 1 1
Parity Stop
bit bit
1 bit or 1 bit or
no bit 2 bits