Transmission starts when the master, the transmitting side, writes transmit data to SOTB1.
Here, data transfer starts regardless of the state of the receiving side.
On the other hand, if transfer start has not been set on the receiving side (in the case of transmit/receive mode, dummy data has not been written to SOTB), the receiving side ignores the serial clocks, however many are sent.
That is, the master must start the transfer after transfer start has been set on the slave side.
If this order is not observed, normal transfer cannot be executed.