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CHAPTER 14
SERIAL INTERFACE SIO1
User’s Manual U12790EJ2V0UD
(3) 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 14-7 shows the configuration of serial I/O shift register 1 (SIO1) 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 6 (DIR1) of serial operating mode register 1 (CSIM1).
Figure 14-7. Circuit for Switching Transfer Bit Order
Start bit switching is realized by switching the bit order for data written to SIO1. The SIO1 shift order remains
unchanged.
Thus, switching between MSB-first and LSB-first must be performed before writing data to the shift register.
(4) Transfer start
Serial transfer is started by setting transfer data to serial I/O shift register 1 (SIO1) when the following two
conditions are satisfied.
Serial interface SIO1 operation control bit (bit 7 (CSIE1) of serial operating mode register 1 (CSIM1)) =
1
Internal serial clock is stopped or SCK1 is a high level after 8-bit serial transfer.
Caution
If CSIE1 is set to 1 after data is written to SIO1, transfer does not start.
Upon termination of 8-bit transfer, serial transfer automatically stops and the interrupt request flag (CSIIF1)
is set.
7
6
Internal bus
1
0
LSB-first
MSB-first
Read/write gate
SI1
Shift register 1 (SIO1)
Read/write gate
SO1
SCK1
DQ
SO1 latch