FUJITSU F2MCTM-16LX - Manual (Page 123)

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CHAPTER 5 CLOCKS
5.6 Oscillation Stabilization Wait Interval
When the power is turned on during the oscillation clock is stopped or when stop mode
is released, a time until the oscillation clock stabilizes (oscillation stabilization wait time
is required immediately after oscillation starts. Also, the oscillation stabilization wait
time is required when the clock mode is switched from main clock to PLL clock, main
clock to sub-clock, sub-clock to main clock, and sub-clock to PLL clock.
■ Oscillation Stabilization Wait Interval
Ceramic and crystal oscillators generally require several to dozens of ms 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 interval has elapsed, the machine clock is supplied to the CPU. Because the
oscillation stabilization wait time depends on the type of oscillator (crystal, ceramic, etc.), the proper
oscillation stabilization wait interval for the oscillator used must be selected. An oscillation stabilization
wait interval is selected by setting the clock selection register (CKSCR).
When clock mode is switched from main clock to PLL clock, main clock to subclock, subclock to main
clock, or subclock to PLL clock, the CPU runs in the clock mode set before switching for the oscillation
stabilization wait time. After the oscillation stabilization wait time has elapsed, the CPU changes to the
specified clock mode.
Figure 5.6-1 shows the operation immediately after oscillation starts.
Figure 5.6-1 Operation Immediately after Oscillation Stabilization Wait Time
X1
Oscillation time of
oscillator Oscillation stabilization
wait time
Oscillation start Oscillation stabilization
(Start normal operation or
switch to PLL clock/subclock