NEC mPD780065 Series Preliminary User's Manual page 223

(3) MSB/LSB switching as the start bit
The 3-wire serial I/O mode has a function that enables it to select the transfer to start from either MSB or LSB.
Figure 14-6 shows the configuration of the serial I/O shift register 1 (SIO1) and 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 with bit 6 (DIR10) of the serial operating mode register
1 (CSIM1).
Figure 14-6. Switching Circuit in Transfer Bit Order
7
6
Internal Bus
1
0
LSB-first
MSB-first
SI1
SO1
SCK1
Start bit switching is realized by switching the bit order for data write to SIO1. The SIO1 shift order remains
unchanged.
Thus, MSB/LSB start bit must be switched before writing data to the shift register.
(4) Transfer start
Serial transfer is started by setting transfer data to the serial I/O shift register 1 (SIO1) when the following two
conditions are satisfied.
• Serial interface (SIO1) operation control bit (bit 7 (CSIE10) of the serial operating mode register 1 (CSIM1))
= 1
• Internal serial clock is stopped or SCK1 is at high level after 8-bit serial transfer.
Caution If CSIE10 is set to 1 after data write to SIO1, transfer does not start.
Upon termination of 8-bit transfer, serial transfer automatically stops and the interrupt request flag (CSIIF1)
is set.
CHAPTER 14 SERIAL INTERFACE (SIO1)
Read/write gate
Shift register 1 (SIO1)
Preliminary User's Manual U13420EJ2V0UM00
Read/write gate
SO1 Latch
D Q
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