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102
CHAPTER 5 CLOCK GENERATOR
Preliminary User’s Manual U13420EJ2V0UM00
5.6.2 System clock and CPU clock switching procedure
This section describes switching procedure between the system clock and CPU clock.
Figure 5-8. System Clock and CPU Clock Switching
<1>
The CPU is reset by setting the RESET signal to low level after power-on. After that, when reset is released
by setting the RESET signal to high level, main system clock starts oscillation. At this time, oscillation
stabilization time (2
17
/f
X
) is secured automatically.
After that, the CPU starts executing the instruction at the minimum speed of the main system clock (3.81
µ
s
@ 8.38-MHz operation).
<2>
After the lapse of a sufficient time for the V
DD
voltage to increase to enable operation at maximum speeds, the
PCC is rewritten and maximum-speed operation is carried out.
<3>
Upon detection of a decrease of the V
DD
voltage due to an interrupt request signal, the main system clock is
switched to the subsystem clock (which must be in an oscillation stable state).
<4>
Upon detection of V
DD
voltage reset due to an interrupt, 0 is set to the MCC and oscillation of the main system
clock is started. After the lapse of time required for stabilization of oscillation, the PCC is rewritten and the
maximum-speed operation is resumed.
Caution
When subsystem clock is being operated while the main system clock is stopped, if switching
to the main system clock is done again, be sure to switch after securing oscillation stabilization
time by software.
System clock
CPU clock
Interrupt request signal
RESET
V
DD
f
X
f
X
f
XT
f
X
Lowest-
speed
operation
Highest-
speed
operation
Subsystem
clock
operation
High-speed
operation
Wait (15.6 ms: @8.38-MHz operation)
Internal reset operation
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