NEC PD78058FY(A) - Manual (Page 431)

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CHAPTER 18 SERIAL INTERFACE CHANNEL 1
(b) When the automatic transmit/receive function is used by the external clock
If bit 1 (CSIM11) of serial operation mode register 1 (CSIM1) is cleared to 0, external clock operation is
set.
When the automatic transmit/receive function is used by the external clock, it must be selected so that
the interval may be longer than the values shown as follows.
Table 18-3. Interval Timing Through CPU Processing (When the External Clock Is Operating)
CPU Processing Interval Time
When using multiplication instruction 13T CPU
When using division instruction 20T CPU
External access 1 wait mode 9T CPU
Other than above 7T CPU
T CPU : 1/f CPU
f CPU : CPU clock (set by the bits 0 to 2 (PCC0 to PCC2) of the processor clock control
register (PCC) and bit 0 (MCS) of the oscillation mode selection register (OSMS))