HD6433644R datasheet(333/549 Pages) RENESAS

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Section 10 Serial Communication Interface

Rev. 6.00 Sep 12, 2006 page 313 of 526

REJ09B0326-0600

10.3.4

Operation in Asynchronous Mode

In asynchronous mode, serial communication is performed with synchronization provided

character by character. A start bit indicating the start of communication and one or two stop bits

indicating the end of communication are added to each character before it is sent.

SCI3 has separate transmission and reception units, allowing full-duplex communication. As the

transmission and reception units are both double-buffered, data can be written during transmission

and read during reception, making possible continuous transmission and reception.

Data Transfer Format: The general data transfer format in asynchronous communication is

shown in figure 10.8.

Serial

data

Start

bit

1 bit

Transmit/receive data

Parity

bit

Stop

bit(s)

7 or 8 bits

One transfer data unit (character or frame)

1 bit

or none

1 or 2 bits

Mark

state

(MSB)

(LSB)

Figure 10.8 Data Format in Asynchronous Communication

In asynchronous communication, the communication line is normally in the mark state (high

level). SCI3 monitors the communication line and when it detects a space (low level), identifies

this as a start bit and begins serial data communication.

One transfer data character consists of a start bit (low level), followed by transmit/receive data

(LSB-first format, starting from the least significant bit), a parity bit (high or low level), and

finally one or two stop bits (high level).

In asynchronous mode, synchronization is performed by the falling edge of the start bit during

reception. The data is sampled on the 8th pulse of a clock with a frequency 16 times the bit period,

so that the transfer data is latched at the center of each bit.

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