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5.6 Changing Setting of CPU Clock
5.6.1 Time required for switching CPU clock
The CPU clock can be selected by using bit 1 (PCC1) of the processor clock control register (PCC).
Actually, the specified clock is not selected immediately after the setting of PCC has been changed; the old clock
is used for the duration of several instructions after that (see Table 5-2).
Table 5-2. Maximum Time Required for Switching CPU Clock
Remark Before switching, the minimum instruction execution time of the CPU clock is two clocks.
5.6.2 Switching CPU clock
The following figure illustrates how the CPU clock switches.
RESET
CPU clock
<1>
The CPU is reset when the RESET pin is made low on power application. The effect of resetting is released
when the RESET pin is later made high, and the system clock starts oscillating. At this time, the time during
which oscillation stabilization (2
After that, the CPU starts instruction execution at the slow speed of the system clock (1.33 µ s: at 6.0 MHz
operation).
<2>
After the time required for the V
speed has elapsed, the processor clock control register (PCC) is rewritten so that the highest speed
operation can be selected.
78
CHAPTER 5
Set Value Before Switching
PCC1
0
1
Figure 5-5. Switching of CPU Clock
V
DD
Slow
operation
Wait (5.46 ms: at 6.0 MHz operation)
Internal reset operation
15
/f
) is automatically secured.
X
voltage to rise to the level at which the CPU can operate at the highest
DD
User's Manual U12978EJ3V0UD
CLOCK GENERATOR
Set Value After Switching
PCC1
0
2 clocks
Fastest operation
PCC1
1
4 clocks
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