Fujitsu MB90390 Series, Hardware Manual

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CHAPTER 5 CLOCKS
5.5 Oscillation Stabilization Wait Time
When the power is turned on or when stop mode is released an oscillation stabilization
wait time is required after oscillation begins because there is no oscillation. When
switching from the main clock to a PLL clock occurs, an oscillation stabilization wait
time is also required after PLL oscillation starts.
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Oscillation Stabilization Wait Time
Ceramic and crystal oscillators generally require several milliseconds to stabilize at their natural frequency
(oscillation frequency) when oscillation starts. For this reason, CPU operation is not allowed immediately
after oscillation starts but is allowed only after full oscillation stabilization. After the oscillation
stabilization wait time has elapsed, the clock is supplied to the CPU. Because the oscillation stabilization
time depends on the type of oscillator (crystal, ceramic, etc.), the proper oscillation stabilization wait time
for the oscillator used must be selected. An oscillation stabilization wait time is selected by setting the
clock selection register (CKSCR).
When switching from the main clock to a PLL clock occurs, the CPU continues to operate on the main
clock during the PLL oscillation stabilization wait time. After this interval, the operating clock switches to
the PLL clock.
Figure 5.5-1 shows the operation immediately after oscillation starts.
Figure 5.5-1 Operation Immediately after Oscillation Starts
X1
Start of oscillation Sta b le oscillation
Oscillator-acti v ated
oscillation time
Oscillation sta b iliza-
tion wait inter v al
N ormal operation
start or switching to
PLL clock