H8S2245 datasheet(476/818 Pages) RENESAS

476 / 818 page

Section 12 Serial Communication Interface (SCI)

Rev.3.00 Mar. 26, 2007 Page 434 of 772

REJ09B0355-0300

12.3.2

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 12.2 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 SCI 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 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.

LSB

Start

bit

MSB

Idle state

(mark state)

Stop bit

Transmit/receive data

0/1

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 12.2 Data Format in Asynchronous Communication

(Example with 8-Bit Data, Parity, Two Stop Bits)

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