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CHAPTER 18 SERIAL INTERFACE CHANNEL 1
(3) MSB/LSB switching as the start bit
The 3-wire serial I/O mode enables to select transfer to start from MSB or LSB.
Figure 18-7 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 (DIR) of the serial operating mode register 1
(CSIM1).
Figure 18-7. Circuit of Switching in Transfer Bit Order
Start bit switching is realized by switching the bit order for data write to SIO1. The SIO1 shift order remains
unchanged.
Thus, switching between MSB-first and LSB-first must be performed before writing data to the SIO1.
(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 channel 1 operation control bit (CSIE1) = 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 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.
7
6
Internal Bus
1
0
LSB-first
MSB-first Read/Write Gate
SI1 Serial IO Shift Register 1 (SIO1)
Read/Write Gate
SO1
SCK1
DQ
SO1 Latch