NEC mPD780851, Preliminary User'S Manual

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CHAPTER 5 CLOCK GENERATOR
Preliminary User’s Manual U14581EJ3V0UM00
5.4 System Clock Oscillator
5.4.1 Main system clock oscillator
The main system clock oscillator oscillates with a crystal resonator or a ceramic resonator (standard: 8.38 MHz)
connected to the X1 and X2 pins.
External clocks can be input to the main system clock oscillator. In this case, input a clock signal to the X1 pin
and an inverted clock signal to the X2 pin.
Figure 5-4 shows an external circuit of the main system clock oscillator.
Figure 5-4. External Circuit of Main System Clock Oscillator
(a) Crystal and ceramic oscillation (b) External clock
Crystal resonator or
ceramic resonator
X2
X1
PD74HCU04
External
clock
X2
X1
IC
µ
Cautions 1. Do not execute the STOP instruction while an external clock is input. This is because if the
STOP instruction is executed, the main system clock operation is stopped, and the X2 pin
is connected to V
DD
, via a pull-up resistor.
2. When using a main system clock oscillator, carry out wiring in the broken line area in Figure
5-4 as follows to avoid influence of wiring capacity.
• Keep the wiring length as short as possible.
• Do not cross the wiring with any other signal lines. Do not route the wiring in the vicinity
of a line through which a high alternating current flows.
• Always keep the ground of the capacitor of the oscillator at 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 5-5 shows examples of resonator having bad connection.