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CHAPTER 5 CLOCK GENERATOR
User’s Manual U19780EJ2V0UD
147
Figure 5-14. Clock Generator Operation When Power Supply Voltage Is Turned On
(When 2.7 V/1.59 V POC Mode Is Set (Option Byte: POCMODE = 1))
Internal high-speed
oscillation clock (f
RH
)
CPU clock
High-speed
system clock (f
XH
)
(when X1 oscillation
selected)
Internal high-speed
oscillation clock
High-speed system clock
Switched by
software
Subsystem clock (f
SUB
)
(when XT1 oscillation
selected)
Note 2
Subsystem clock
X1 clock
oscillation stabilization time:
2
11
/f
X
to 2
16
/f
X
Note 1
Starting X1 oscillation
is set by software.
Starting XT1 oscillation
is set by software.
Waiting for oscillation accuracy
stabilization (86 to 361 s)
Internal reset signal
0 V
2.7 V (TYP.)
Power supply
voltage (V
DD
)
<1>
<3>
<2>
<4>
<5>
Reset processing
(11 to 45 s)
<4>
<5>
μ
μ
<1> When the power is turned on, an internal reset signal is generated by the power-on-clear (POC) circuit.
<2> When the power supply voltage exceeds 2.7 V (TYP.), the reset is released and the internal high-speed
oscillator automatically starts oscillation.
<3> After the reset is released and reset processing is performed, the CPU starts operation on the internal high-
speed oscillation clock.
<4> Set the start of oscillation of the X1 or XT1 clock via software (see
(1)
in
5.6.1 Example of controlling high-
speed system clock
and
(1)
in
5.6.3 Example of controlling subsystem clock
).
<5> When switching the CPU clock to the X1 or XT1 clock, wait for the clock oscillation to stabilize, and then set
switching via software (see
(3)
in
5.6.1 Example of controlling high-speed system clock
and
(2)
in
5.6.3
Example of controlling subsystem clock
).
Notes 1.
When releasing a reset (above figure) or releasing STOP mode while the CPU is operating on the
internal high-speed oscillation clock, confirm the oscillation stabilization time for the X1 clock using the
oscillation stabilization time counter status register (OSTC). If the CPU operates on the high-speed
system clock (X1 oscillation), set the oscillation stabilization time when releasing STOP mode using the
oscillation stabilization time select register (OSTS).
2.
The 78K0/KB2-A is not provided with a subsystem clock.
Cautions 1. A voltage oscillation stabilization time of 1.93 to 5.39 ms is required after the supply voltage
reaches 1.59 V (TYP.). If the supply voltage rises from 1.59 V (TYP.) to 2.7 V (TYP.) within 1.93
ms, the power supply oscillation stabilization time of 0 to 5.39 ms is automatically generated
before reset processing.
2. It is not necessary to wait for the oscillation stabilization time when an external clock input
from the EXCLK pin is used.