H8SLASH3003 datasheet(470/715 Pages) RENESAS

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13.3.2 Operation in Asynchronous Mode

In asynchronous mode each transmitted or received character begins with a start bit and ends with

a stop bit. Serial communication is synchronized one character at a time.

The transmitting and receiving sections of the SCI are independent, so full duplex communication

is possible. The transmitter and receiver are both double buffered, so data can be written and read

while transmitting and receiving are in progress, enabling continuous transmitting and receiving.

Figure 13-2 shows the general format of asynchronous serial communication. In asynchronous

serial communication the communication line is normally held in the mark (high) state. The SCI

monitors the line and starts serial communication when the line goes to the space (low) state,

indicating a start bit. One serial character consists of a start bit (low), data (LSB first), parity bit

(high or low), and stop bit (high), in that order.

When receiving in asynchronous mode, the SCI synchronizes at the falling edge of the start bit.

The SCI samples each data bit on the eighth pulse of a clock with a frequency 16 times the bit

rate. Receive data is latched at the center of each bit.

Figure 13-2 Data Format in Asynchronous Communication (Example: 8-Bit Data with

Parity and 2 Stop Bits)

Serial data

Idle (mark) state

0/1

(LSB)

(MSB)

Start

bit

Transmit or receive data

Parity

bit

Stop

bit

One unit of data (character or frame)

1 bit

7 bits or 8 bits

1 bit or

no bit

1 bit or

2 bits

452

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