Operation In Asynchronous Mode - Hitachi H8S/2338 Series Hardware Manual

11.3.2

Operation in Asynchronous Mode

In asynchronous mode, characters are sent or received, each preceded by a start bit indicating the
start of communication and one or two stop bits indicating the end of communication. Serial
communication is thus carried out with synchronization established on a character-by-character
basis.
Inside the SCI, the transmitter and receiver are independent units, enabling full-duplex
communication. Both the transmitter and the receiver also have a double-buffered structure, so
that data can be read or written during transmission or reception, enabling continuous data
transfer.
Figure 11-2 shows the general format for asynchronous serial communication.
In asynchronous serial communication, the communication line is usually held in the mark state
(high level). The SCI monitors the communication line, and when it goes to the space state (low
level), recognizes a start bit and starts serial communication.
One serial communication character consists of a start bit (low level), followed by data (in LSB-
first order), a parity bit (high or low level), and finally one or two stop bits (high level).
In asynchronous mode, the SCI performs synchronization at the falling edge of the start bit in
reception. The SCI samples the data on the 8th pulse of a clock with a frequency of 16 times the
length of one bit, so that the transfer data is latched at the center of each bit.
1 LSB
Serial
0 D0
data
Start
bit
1 bit
Figure 11-2 Data Format in Asynchronous Communication
Data Transfer Format: Table 11-10 shows the data transfer formats that can be used in
asynchronous mode. Any of 12 transfer formats can be selected according to the SMR setting.
D1 D2 D3 D4
Transmit/receive data
7 or 8 bits
One unit of transfer data (character or frame)
(Example with 8-Bit Data, Parity, Two Stop Bits)
MSB
D5 D6 D7 0/1
Parity
bit
1 bit,
or none
Idle state
(mark state)
1
1 1
Stop bit(s)
1 or
2 bits
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