CHAPTER 8 CLOCK GENERATOR (
µ
PD789861 SUBSERIES)
User’s Manual U14826EJ5V0UD
82
8.6 Changing Setting of CPU Clock
8.6.1 Time required for switching CPU clock
The CPU clock can be selected by using bit 1 (PCC0) 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 (see
Table 8-2
).
Table 8-2. Maximum Time Required for Switching CPU Clock
Set Value Before Switching
Set Value After Switching
PCC0 PCC0
PCC0
0 1
0
4
clocks
1 2
clocks
Remark
Two clocks are the minimum instruction execution time of the CPU clock before switching.
8.6.2 Switching CPU clock
The following figure illustrates how the CPU clock is switched.
Figure 8-5. Switching Between System Clock and CPU Clock
CPU Clock
RESET
V
DD
Slow
operation
Fast operation
Wait (128 s: @1.0 MHz operation)
Internal reset operation
µ
<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 oscillation
stabilization time (2
7
/f
CC
) is automatically secured.
After that, the CPU starts instruction execution at the slow speed of the system clock (8.0
µ
s: @1.0 MHz
operation).
<2> After the time required for the V
DD
voltage to rise to the level at which the CPU can operate at the high speed
has elapsed, the processor clock control register (PCC) is rewritten so that the high-speed operation can be
selected.