Operation In Asynchronous Mode - Hitachi H8/3062 Hardware Manual

Single-chip microcomputer

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Operation in Asynchronous Mode

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

one or two stop bits. 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 the receiver are both double-buffered, so data can be written and

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

Figure 12.2 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 one or two stop bits (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 12.2 Data Format in Asynchronous Communication

Communication Formats: Table 12.10 shows the 12 communication formats that can be selected

in asynchronous mode. The format is selected by settings in SMR.

Transmit or receive data

One unit of data (character or frame)

(Example: 8-Bit Data with Parity and 2 Stop Bits)

Idle (mark) state