H8SSLASH2214 datasheet(550/936 Pages) RENESAS

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Section 12 Serial Communication Interface (SCI)

Rev.4.00 Sep. 18, 2008 Page 490 of 872

REJ09B0189-0400

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 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 12.5 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 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.

When the ABCS bit in SEMR0 is set to 1, SCI0 samples the data on the 4th pulse of a clock with a

frequency of 8 times the length of one 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 12.5 Data Format in Asynchronous Communication

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

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