Hardware manual

Rev. 3.0, 09/98, page 218 of 361
10.3.3 Synchronous Mode
(1) Overview: In clocked 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 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 10.12 shows the general format in clocked synchronous serial communication.
Serial clock
Serial data Bit 0 Bit 1 Bit 2 Bit 3 Bit 4 Bit 5 Bit 6 Bit 7
LSB MSB
Don't care Don't care
One unit (character or frame) of serial data
* *
Note: * High except in continuous transmitting or receiving
Figure 10.12 Data Format in Clocked Synchronous Communication
In clocked synchronous serial communication, each data bit is sent on the communication line
from one falling edge of the serial clock to the next. Data are received in synchronization with the
rising edge of the serial clock.
In each character, the serial data bits are transmitted in order from LSB (first) to MSB (last). After
output of the MSB, the communication line remains in the state of the MSB.
• Communication Format: The data length is fixed at eight 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 clearing or setting the CKE1 bit in the serial control
register (SCR). See table 10.6.
When the SCI operates on an internal clock, it outputs the clock signal 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 at the high level.