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CHAPTER 3 CPU
3.7.6
Oscillation Stabilization Wait Time
At power on or return from the stop mode when the oscillation clock is stopped, a time
until the oscillation clock stabilizes (oscillation stabilization wait time) is required after
starting an oscillation.The oscillation stabilization wait time is also required for
switching the clock mode from main clock mode to PLL clock or subclock mode and
from subclock mode to main clock or PLL clock mode.
I Operation During Oscillation Stabilization Wait Time
Ceramic and crystal oscillators require some tens of milliseconds to reach a stable oscillation frequency
after starting oscillation.Therefore when, immediately after an oscillation starts, once the CPU operation is
disabled and then an oscillation stabilizes after the elapse of oscillation stabilization wait time, the machine
clock is supplied to the CPU.
The oscillation stabilization wait time varies with the type of oscillator (ceramic, crystal, etc.).It is
necessary to select a oscillation stabilization wait time appropriate to an oscillator to be used.The
oscillation stabilization wait time can be selected using the clock select register (CKSCR).
When the clock mode changes from main clock mode to PLL clock or subclock mode or from subclock
mode to main clock or PLL clock mode, the CPU runs to the previous clock during the oscillation
stabilization wait time. When the oscillation stabilization wait time has elapsed, the CPU changes to the
specified clock mode.
Figure 3.7-7 shows the oscillating operation immediately after it starts.
Figure 3.7-7 Operation after Oscillation Stabilization Wait Time
X1
Oscillating start
Note:
There is no sub-clock in MB90F897S.
122
The oscillation time
for the oscillator
stabilization time
Oscillating stabilization
Start the normal operation or
Oscillation
change to PLL clock/subclock
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