NEC mPD789101, User Manual

Page: 97 / 265

CHAPTER 7 CLOCK GENERATOR (
µ
PD789124, 789134 SUBSERIES)
User’s Manual U13045EJ2V0UM00
97
7.4 System Clock Oscillator
7.4.1 System clock oscillator
The system clock oscillator is oscillated by the resistor (R) and capacitor (C) (4.0 MHz TYP.) connected across
the CL1 and CL2 pins.
An external clock can also be input to the system clock oscillator. In this case, input the clock signal to the CL1
pin, and leave the CL2 pin open.
Figure 7-3 shows the external circuit of the system clock oscillator.
Figure 7-3. External Circuit of System Clock Oscillator
(a) RC oscillation (b) External clock
Caution When using the system clock oscillator, wire as follows in the area enclosed by the broken lines
in the above figures to avoid an adverse effect from wiring capacitance.
• Keep the wiring length as short as possible.
• Do not cross the wiring with the other signal lines. Do not route the wiring near a signal
line through which a high fluctuating current flows.
• Always make the ground point of the oscillator capacitor the same potential as V
SS
. Do
not ground the capacitor to a ground pattern through which a high current flows.
• Do not fetch signals from the oscillator.
Figure 7-4 shows incorrect examples of resonator connection.
V
SS
CL1
R C
CL2
External
clock
Open
CL1
CL2