CHAPTER 5 CLOCK GENERATORS
User’s Manual U18172EJ2V0UD
70
5.4 System
Clock
Oscillators
The following three types of system clock oscillators are available.
•
High-speed internal oscillator:
Internally oscillates a clock of 8 MHz (TYP.).
•
Crystal/ceramic oscillator:
Oscillates a clock of 2 MHz to 10 MHz.
•
External clock input circuit:
Supplies a clock of 2 MHz to 10 MHz to the X1 pin.
5.4.1
High-speed internal oscillator
The 78K0S/KU1+ include a high-speed internal oscillator (8 MHz (TYP.)).
If the high-speed internal oscillation is selected by the option byte as the clock source, the X1 and X2 pins can be
used as I/O port pins.
For details of the option byte, refer to
CHAPTER 15 OPTION BYTE
. For details of I/O ports, refer to
CHAPTER 4
PORT FUNCTIONS
.
5.4.2 Crystal/ceramic oscillator
The crystal/ceramic oscillator oscillates using a crystal or ceramic resonator connected between the X1 and X2
pins.
If the crystal/ceramic oscillator is selected by the option byte as the system clock source, the X1 and X2 pins are
used as crystal or ceramic resonator connection pins.
For details of the option byte, refer to
CHAPTER 15 OPTION BYTE
. For details of I/O ports, refer to
CHAPTER 4
PORT FUNCTIONS
.
Figure 5-6 shows the external circuit of the crystal/ceramic oscillator.
Figure 5-6. External Circuit of Crystal/Ceramic Oscillator
V
SS
X1
X2
Crystal resonator
or ceramic resonator
Caution When using the crystal/ceramic oscillator, wire as follows in the area enclosed by the broken
lines in Figure 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.
