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12.3.4
Synchronous Operation
In synchronous mode, the SCI transmits and receives data in synchronization with clock pulses.
This mode is suitable for high-speed serial communication.
The SCI transmitter and receiver share the same clock but are otherwise independent, so full-
duplex communication is possible. The transmitter and the receiver are also double-buffered, so
continuous transmitting or receiving is possible by reading or writing data while transmitting or
receiving is in progress.
Figure 12.14 shows the general format in synchronous serial communication.
*
Serial clock
Serial data
Don't care
Note: * High except in continuous transmitting or receiving
Figure 12.14 Data Format in Synchronous Communication
In synchronous serial communication, each data bit is placed on the communication line from one
falling edge of the serial clock to the next. Data is guaranteed valid at the rise of the serial clock.
In each character, the serial data bits are transferred in order from LSB (first) to MSB (last). After
output of the MSB, the communication line remains in the state of the MSB. In synchronous
mode the SCI receives data by synchronizing with the rise of the serial clock.
Communication Format: The data length is fixed at 8 bits. No parity bit or multiprocessor bit
can be added.
Clock: An internal clock generated by the on-chip baud rate generator or an external clock input
from the SCK pin can be selected by means of the C/A bit in SMR and the CKE1 and CKE0 bits
in SCR. See table 12.6 for details of SCI clock source selection.
When the SCI operates on an internal clock, it outputs the clock source at the SCK pin. Eight
clock pulses are output per transmitted or received character. When the SCI is not transmitting or
receiving, the clock signal remains in the high state. If receiving in single-character units is
required, an external clock should be selected.
One unit (character or frame) of transfer data
L SB
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
*
MSB
Bit 6
Bit 7
Don't care
403
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