Changing Setting Of Cpu Clock; Time Required For Switching Cpu Clock; Switching Cpu Clock - NEC mPD789101 User Manual

CHAPTER 6 CLOCK GENERATOR ( µ PD789104, 789114 SUBSERIES)

6.6 Changing Setting of CPU Clock

6.6.1 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, and the old
clock is used for the duration of several instructions after that (refer to Table 6-2).
Table 6-2. Maximum Time Required for Switching CPU Clock
Remark Two clocks are the minimum instruction execution

6.6.2 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 stabilizes (2
After that, the CPU starts instruction execution at the low speed of the system clock (1.6 µ s: at 5.0-MHz
operation).
<2> After the time during which the V
speed has elapsed, the processor clock control register (PCC) is rewritten so that the highest speed can
be selected.
Set Value before Switching
PCC1
0
1
time of the CPU clock before switching.
Figure 6-5. Switching CPU Clock
V
DD
Slow
operation
Wait (6.55 ms: at 5.0-MHz operation)
Internal reset operation
15 /f ) is automatically secured.
X
voltage rises to the level at which the CPU can operate at the highest
DD
User's Manual U13045EJ2V0UM00
Set Value after Switching
PCC1 PCC1
0 1
4 clocks
2 clocks
Fastest
operation
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