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CHAPTER 4 CLOCKS
MB90620 series
4.5
Oscillation Stabilization Wait Interval
When the system changes operation from a state in which the main clock is stopped
(such as at power-on, in stop mode and at watch-dog reset), a delay (an oscillation
stabilization wait interval) is required to stabilize the clock oscillation before operation
starts. When the switch from the main clock to a PLL clock occurs, an oscillation
stabilization wait interval is also required when PLL oscillation starts.
I
Oscillation stabilization wait interval
Ceramic and crystal oscillators generally take several milliseconds to 20 milliseconds to stabilize
at their natural frequency when oscillation starts.
For this reason, CPU operation is not allowed as soon as oscillation starts and is allowed only
after full stabilization of oscillation. After the oscillation stabilization wait interval has elapsed, the
clock is supplied to the CPU.
Because the oscillation stabilization time depends on the type of the 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).
In a switch from the main clock to a PLL clock, the CPU continues to operate on the main clock
during the oscillation stabilization wait interval for PLL. After this interval, the operating clock
switches to the PLL clock.
Figure 4.5-1 shows the operation after oscillation starts.
Figure 4.5-1 Operation when oscillation starts
Oscillator-activated Oscillation stabilization Normal operation start
oscillation time
wait interval
Start of oscillation
Stable oscillation
or change to PLL clock