NEC uPD78P078 - Manual (Page 456)

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CHAPTER 19 SERIAL INTERFACE CHANNEL 1
(b) When the automatic transmit/receive function is used by the external clock
If bit 1 (CSIM11) of serial operating 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 19-3. Interval Timing through CPU Processing (when the External Clock is Operating)
CPU Processing Interval Time
When using multiplication instruction 13T CPU or longer
When using division instruction 20T CPU or longer
External access 1 wait mode 9T CPU or longer
Other than above 7T CPU or longer
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))