Renesas SH7781 - Manual (Page 1104)

21. Serial Communication Interface with FIFO (SCIF)
Rev.1.00 Jan. 10, 2008 Page 1074 of 1658
REJ09B0261-0100
21.4.2 Operation in Asynchronous Mode
In asynchronous mode, a character that consists of data with a start bit indicating the start of
communication and a stop bit indicating the end of communication is transmitted or received. In
this mode, serial communication is performed with synchronization achieved character by
character.
Inside the SCIF, the transmitter and receiver are independent units, enabling full-duplex
communication. Both the transmitter and receiver have a 64-stage FIFO buffer structure, so that
data can be read or written during transmission or reception, enabling continuous data
transmission and reception.
Figure 21.7 shows the general format for asynchronous serial communication.
In asynchronous serial communication, the transmission line is usually held in the mark state (high
level). The SCIF monitors the transmission line, and when it goes to the space state (low level),
recognizes a start bit and starts serial communication.
One character in serial communication consists of a start bit (low level), followed by
transmit/receive data (LSB first; from the lowest bit), a parity bit (high or low level), and finally
stop bits (high level).
In reception in asynchronous mode, the SCIF synchronizes with the fall of the start bit. Receive
data can be latched at the middle of each bit because the SCIF samples data at the eighth clock
with frequency of 16 times the bit rate.
LSB
Start
bit
MSB
Idle state
(mark state)
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 21.7 Data Format in Asynchronous Communication
(Example with 8-Bit Data, Parity, and Two Stop Bits)