Section 12 Serial Communication Interface (SCI)
Rev. 3.00 Sep. 27, 2007 Page 444 of 758
REJ09B0243-0300
12.4.2
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. Both the transmitter and receiver have a double-buffered structure so that data can be
read or written during transmission or reception, enabling continuous data transfer.
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 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.
LSB
Start
bit
MSB
Idle state
(mark state)
Stop bit
0
Transmit/receive data
D0
D1
D2
D3
D4
D5
D6
D7
0/1
1
1
1
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 12.2 Example of Data Format in Asynchronous Communication
(8-Bit Data with Parity and Two Stop Bits)
Содержание SH7124 R5F7124
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