473CHAPTER 19 SERIAL INTERFACE CHANNEL 2(3) MSB/LSB switching as the start bitThe 3-wire serial I/O mode enables to select transfer to start from MSB or LSB.Figure 19-13 shows the configuration of the transmit shift register (TXS/SIO2) and internal bus. As shownin the figure, MSB/LSB can be read/written in reverse form.MSB/LSB switching as the start bit can be specified with bit 2 (CSIM22) of the serial operating mode register2 (CSIM2).Figure 19-13. Circuit of Switching in Transfer Bit OrderStart bit switching is realized by switching the bit order for data write to TXS/SIO2. The TXS/SIO2 shift orderremains unchanged.Thus, switching between MSB-first and LSB-first must be performed before writing data to the TXS/SIO2.(4) Transfer startSerial transfer is started by setting transfer data to the transmission shift register (TXS/SIO2) when thefollowing two conditions are satisfied.• Serial interface channel 2 operation control bit (CSIE2) = 1• Internal serial clock is stopped or SCK2 is a high level after 8-bit serial transfer.Caution If CSIE2 is set to "1" after data write to TXS/SIO2, transfer does not start.Upon termination of 8-bit transfer, serial transfer automatically stops and the interrupt request flag (SRIF) isset.76Internal Bus10LSB-firstMSB-first Read/Write GateSI2 Transmit Shift Register (TXS/SIO2)Read/Write GateSO2SCK2D QSO2 Latch
