Clock Generator Operation - NEC mPD78F0730 Preliminary User's Manual

5.5 Clock Generator Operation

The clock generator generates the following clocks and controls the operation modes of the CPU, such as standby
mode (see Figure 5-1).
• Main system clock f
• High-speed system clock f
• Internal high-speed oscillation clock f
• Internal low-speed oscillation clock f
• CPU clock f
• USB clock f
• Peripheral hardware clock f
The CPU starts operation when the internal high-speed oscillator starts outputting after a reset release in the
µ
PD78F0730, thus enabling the following.
(1) Enhancement of security function
When the X1 clock is set as the CPU clock by the default setting, the device cannot operate if the X1 clock is
damaged or badly connected and therefore does not operate after reset is released. However, the start clock of
the CPU is the internal high-speed oscillation clock, so the device can be started by the internal high-speed
oscillation clock after a reset release. 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 X1 clock oscillation stabilization time, the total
performance can be improved.
When the power supply voltage is turned on, the clock generator operation is shown in Figure 5-13.
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XP
XH
X1 clock f
X
External main system clock f
CPU
USB
PRS
CHAPTER 5 CLOCK GENERATOR
EXCLK
RH
RL
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