
CHAPTER 5 CLOCK GENERATOR
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User’s Manual U13952EJ3V1UD
5.6.2 Switching between system clock and CPU clock
The following figure illustrates how the CPU clock and system clock are switched.
Figure 5-8. Switching Between System Clock and CPU Clock
System clock
CPU clock
Interrupt request signal
RESET
VDD
fX
fXT
fX
Low-speed
operation
High-speed
operation
Subsystem clock
operation
High-speed
operation
Wait (6.55 ms: at 5.0 MHz operation)
Internal reset operation
<1> The CPU is reset when the RESET pin is made low on power application. Reset is released when the
RESET pin is later made high, and the main system clock starts oscillating. At this time, the oscillation
stabilization time (2
15/fX) is automatically secured.
After that, the CPU starts instruction execution at the low speed of the main system clock (1.6
s at
5.0 MHz operation).
<2> After the time required for the VDD voltage to rise to the level at which the CPU can operate at the high speed
has elapsed, bit 1 (PCC1) of the processor clock control register (PCC) and bit 4 (CSS0) of the subclock
control register (CSS) are rewritten so that the high-speed operation can be selected.
<3> A drop of the VDD voltage is detected by an interrupt request signal. The clock is switched to the subsystem
clock (at this moment, the subsystem clock must be in the stable oscillation status).
<4> Recovery of the VDD voltage is detected by an interrupt request signal. Bit 7 (MCC) of PCC is set to 0, and
the main system clock starts oscillating. After the time required for the oscillation to stabilize has elapsed,
PCC1 and CSS0 are rewritten so that high-speed operation can be selected again.
Caution
When the main system clock is stopped and the device is operating on the subsystem
clock, wait until the oscillation stabilization time has been secured by the program before
switching back to the main system clock.