SH7018 Datasheet, PDF (243/431 Pages) Renesas Technology Corp – SuperH™ RISC engine

12.3.2 Operation in Asynchronous Mode
In the 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 SCI1 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 12.2 shows the general format of asynchronous serial communication. In asynchronous
serial communication, the communication line is normally held in the marking (high) state. The
SCI1 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 the asynchronous mode, the SCI1 synchronizes on the falling edge of the start
bit. The SCI1 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.
1
(LSB)
Idling (marking state)
(MSB)
1
Serial _{0 D0}
data
Start
bit
1 bit
D1 D2 D3 D4 D5 D6
Transmit/receive data
7 or 8 bits
D7 0/1 1 1
Parity Stop
bit
bit
1 or 1 or
no bit 2 bits
One unit of communication data (characters or frames)
Figure 12.2 Data Format in Asynchronous Communication (Example: 8-bit Data with
Parity and Two Stop Bits)
227