CHAPTER 5 CLOCK GENERATOR
User’s Manual U16899EJ2V0UD
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5.5 Clock Generator Operation
The clock generator generates the following clocks and controls the operation modes of the CPU, such as standby
mode.
•
High-speed system clock f
XP
•
Ring-OSC clock f
R
•
Subsystem clock f
XT
•
CPU clock f
CPU
•
Clock to peripheral hardware
The CPU starts operation when the on-chip Ring-OSC oscillator starts outputting after reset release in the
78K0/KE1+, thus enabling the following.
(1) Enhancement of security function
When the high-speed system clock is set as the CPU clock by the default setting, the device cannot operate if the
high-speed system clock is damaged or badly connected and therefore does not operate after reset is released.
However, the start clock of the CPU is the on-chip Ring-OSC clock, so the device can be started by the Ring-
OSC clock after reset release by the clock monitor (detection of high-speed system clock stop). Consequently,
the system can be safely shut down by performing a minimum operation, such as acknowledging a reset source
by software or performing safety processing when there is a malfunction.
(2) Improvement of performance
Because the CPU can be started without waiting for the high-speed system clock oscillation stabilization time, the
total performance can be improved.
A timing diagram of the CPU default start using Ring-OSC is shown in Figure 5-12.
