NEC mPD789425, User Manual

Page: 109 / 321

CHAPTER 5 CLOCK GENERATOR
User’s Manual U15075EJ1V0UM00
109
5.4.2 Subsystem clock oscillator
The subsystem clock oscillator is oscillated by the crystal resonator (32.768 kHz TYP.) connected across the XT1
and XT2 pins.
An external clock can also be input to the circuit. In this case, input the clock signal to the XT1 pin, and input the
inverted signal to the XT2 pin.
Figure 5-6 shows the external circuit of the subsystem clock oscillator.
Figure 5-6. External Circuit of Subsystem Clock Oscillator
(a) Crystal oscillation (b) External clock
XT2
V
SS
XT1
32.768
kHz
Crystal resonator
External
clock
XT1
XT2
Caution When using the main system or subsystem clock oscillator, wire as follows in the area enclosed
by the broken lines in Figures 5-5 and 5-6 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.
When using the subsystem clock, particular care is required because the subsystem clock oscillator is designed
as a low-amplitude circuit for reducing current consumption.
Figure 5-7 shows examples of incorrect resonator connection.