Datasheet

78K0/Kx2-L CHAPTER 5 CLOCK GENERATOR
R01UH0028EJ0400 Rev.4.00 217
Sep 27, 2010
5.5 Clock Generator Operation
The clock generator generates the following clocks and controls the operation modes of the CPU, such as standby
mode (refer to Figures 5-1 and 5-2).
• Main system clock f
XP
• High-speed system clock f
XH
X1 clock fX
External main system clock f
EXCLK
• Internal high-speed oscillation clock f
IH
• Subsystem clock fSUB
Note
• XT1 clock f
XT
• External subsystem clock f
EXCLKS
• Internal low-speed oscillation clock fIL
• CPU clock f
CPU
• Peripheral hardware clock f
PRS
Note 78K0/KC2-L only
The CPU starts operation when the internal high-speed oscillator starts outputting after a reset release in the
78K0/Kx2-L microcontrollers, 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 Figures 5-16 and 5-17.