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CHAPTER 13
SERIAL INTERFACE SIO0
User’s Manual U12790EJ2V0UD
(4) MSB/LSB switching as the start bit
In the 3-wire serial I/O mode, transfer can be selected to start from the MSB or LSB.
Figure 13-10 shows the configuration of serial I/O shift register 0 (SIO0) and the internal bus. As shown in
the figure, MSB/LSB can be read/written in reverse form.
MSB/LSB switching as the start bit can be specified by bit 2 (CSIM02) of serial operating mode register 0
(CSIM0).
Figure 13-10. Circuit for Switching Transfer Bit Order
Start bit switching is realized by switching the bit order for data written to SIO0. The SIO0 shift order remains
unchanged.
Thus, switching between MSB-first and LSB-first must be performed before writing data to the shift register.
(5) Transfer start
Serial transfer is started by setting transfer data to serial I/O shift register 0 (SIO0) when the following two
conditions are satisfied.
Serial interface SIO0 operation control bit (CSIE0) = 1.
Internal serial clock is stopped or SCK0 is high level after 8-bit serial transfer.
Caution
If CSIE0 is set to “1” after writing data to SIO0, transfer does not start.
Remark
CSIE0: Bit 7 of serial operating mode register 0 (CSIM0)
Upon termination of 8-bit transfer, serial transfer automatically stops and the interrupt request flag (CSIIF0)
is set.
7
6
Internal bus
1
0
LSB-first
MSB-first
Read/write gate
SI0
Shift register 0 (SIO0)
Read/write gate
SO0
SCK0
DQ
SO0 latch