UPD784938 Datasheet, PDF (114/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 4 CLOCK GENERATOR
(2) When the device is powered on, and when restoring from the STOP mode, sufficient time must be allowed for the oscillation
to stabilize. Generally speaking, the time required for oscillation stabilization is several milliseconds when a crystal
resonator is used, and several hundred microseconds when a ceramic resonator is used.
An adequate oscillation stabilization period should be secured by the following means:
<1> When powered-on: RESET input (reset period)
<2> When returning from STOP mode:
(i) RESET input (reset period)
(ii) Time of the oscillation stabilization timer that automatically starts at the valid edge of NMI, INTP4, or INTP5
signal^Note (set by the oscillation stabilization time specification register (OSTS))
Note For INTP4 and INTP5, when masking is released and macro service is disabled.
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Preliminary User’s Manual U13987EJ1V0UM00