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22.3.4
Operation in Synchronous Mode
In synchronous mode, data is transmitted or received in synchronization with clock pulses, making
it suitable for high-speed serial communication.
Inside the SCI, the transmitter and receiver are independent units, enabling full-duplex
communication by use of a common clock. Both the transmitter and the receiver also have a
double-buffered structure, so that data can be read or written during transmission or reception,
enabling continuous data transfer.
Figure 22.16 shows the general format for synchronous serial communication.
*
Synchronous
clock
Serial
data
Don't care
Note: * High except in continuous transfer
Figure 22.16 Data Format in Synchronous Communication
In synchronous serial communication, data on the transmission line is output from one falling edge
of the serial clock to the next. Data is guaranteed valid at the rising edge of the serial clock.
In synchronous serial communication, one character consists of data output starting with the LSB
and ending with the MSB. After the MSB is output, the transmission line holds the MSB state.
In synchronous mode, the SCI receives data in synchronization with the rising edge of the serial
clock.
• Data Transfer Format
A fixed 8-bit data format is used.
No parity or multiprocessor bits are added.
• Clock
Either an internal clock generated by the built-in baud rate generator or an external serial clock
input at the SCK pin can be selected, according to the setting of the C/$ bit in SMR1 and the
CKE1 and CKE0 bits in SCR1. For details on SCI clock source selection, see table 22.9.
When the SCI is operated on an internal clock, the serial clock is output from the SCK pin.
Eight serial clock pulses are output in the transfer of one character, and when no transfer is
performed the clock is fixed high. When only receive operations are performed, however, the
serial clock is output until an overrun error occurs or the RE bit is cleared to 0. To perform
receive operations in units of one character, select an external clock as the clock source.
Rev. 1.0, 02/00, page 442 of 1141
One unit of transfer data (character or frame)
LSB
Bit 0
Bit 1
Bit 2
Bit 3
Bit 4
Bit 5
*
MSB
Bit 6
Bit 7
Don't care
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