Functions Of Clock Generator - NEC 78K0 Series User Manual

6.1 Functions of Clock Generator

The clock generator generates the clock to be supplied to the CPU and peripheral hardware.
The following three kinds of system clocks and clock oscillators are selectable.
(1) Main system clock
<1> X1 oscillator
This circuit oscillates a clock of f
Oscillation can be stopped by executing the STOP instruction or using the main OSC control register
(MOC).
<2> Internal high-speed oscillator
This circuit oscillates a clock of f
operating with this internal high-speed oscillation clock. Oscillation can be stopped by executing the
STOP instruction or using the internal oscillation mode register (RCM).
An external main system clock (f
external main system clock input can be disabled by executing the STOP instruction or using RCM.
As the main system clock, a high-speed system clock (X1 clock or external main system clock) or internal high-
speed oscillation clock can be selected by using the main clock mode register (MCM).
(2) Subsystem clock
• Subsystem clock oscillator
This circuit oscillates at a frequency of f
and XT2. Oscillation can be stopped by using the processor clock control register (PCC) and clock
operation mode select register (OSCCTL).
An external subsystem clock (f
external subsystem clock input can be disabled by setting PCC and OSCCTL.
Remarks 1. f : X1 clock oscillation frequency
X
2. f : Internal high-speed oscillation clock frequency
RH
3. f : External main system clock frequency
EXCLK
4. f : XT1 clock oscillation frequency
XT
5. f : External subsystem clock frequency
EXCLKS
134
CHAPTER 6 CLOCK GENERATOR
= 1 to 20 MHz by connecting a resonator to X1 and X2.
X
= 8 MHz (TYP.). After a reset release, the CPU always starts
RH
= 1 to 20 MHz) can also be supplied from the EXCLK/X2/P122 pin. An
EXCLK
= 32.768 kHz by connecting a 32.768 kHz resonator across XT1
XT
= 32.768 kHz) can also be supplied from the EXCLKS/XT2/P124 pin. An
EXCLKS
Preliminary User's Manual U17260EJ3V1UD