NEC PD78214 - Manual (Page 321)

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PD78214 Sub-Series
10.7 NOTES
(1) Do not change CTXE from 0 to 1 and CRXE from 1 to 0, or vice versa, by means of a single instruction. If this
is attempted, the serial clock counter will malfunction and the first communication after the change will be
terminated before the eighth bit is sent. To change these statuses, use two instructions as shown below:
Example Changing CTXE from 1 to 0 and CRXE from 0 to 1
CLR1 CTXE
SET1 CRXE
(2) When switching the master and slave, the input and output of the serial clock line (SCK) are asynchronously
switched between the master and slave. The serial clock line (SCK) requires a pull-up resistor.
(3) Do not set ACKT before the transfer has been completed.