Renesas Hitachi H8S/2191, Hardware Manual, Page: 495 / 1065

Rev. 2.0, 11/00, page 468 of 1037
23.3.2 Operation in Asynchronous Mode
In asynchronous mode, characters are sent or received, each preceded by a start bit indicating the
start of communication and followed by 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 SCI1, 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 23.2 shows the general format for asynchronous serial communication.
In asynchronous serial communication, the transmission line is usually held in the mark state
(high level). The SCI1 monitors the transmission 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 SCI1 performs synchronization at the falling edge of the start bit in
reception. The SCI1 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.
LSB
Start
bit
MSB
Idle state
(mark state)
Stop
bit(s)
0
Transmit/receive data
D0 D1 D2 D3 D4 D5 D6 D7 0/1 1 1
1 1
Serial
data
Parity
bit
1 bit 1 or 2 bits 7 or 8 bits 1 bit,
or none
One unit of transfer data (character or frame)
Figure 23.2 Data Format in Asynchronous Communication
(Example with 8-Bit Data, Parity, Two Stop Bits)