PIC18F2450/4450
2.2.2
CRYSTAL OSCILLATOR/CERAMIC
RESONATORS
TABLE 2-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
In HS, HSPLL, XT and XTPLL Oscillator modes, a
crystal or ceramic resonator is connected to the OSC1
and OSC2 pins to establish oscillation. Figure 2-2
shows the pin connections.
Typical Capacitor Values
Crystal
Freq
Tested:
Osc Type
C1
C2
XT
HS
4 MHz
4 MHz
8 MHz
20 MHz
27 pF
27 pF
22 pF
15 pF
27 pF
27 pF
22 pF
15 pF
The oscillator design requires the use of a parallel cut
crystal.
Note:
Use of a series cut crystal may give a fre-
quency out of the crystal manufacturer’s
specifications.
Capacitor values are for design guidance only.
These capacitors were tested with the crystals listed
below for basic start-up and operation. These values
are not optimized.
FIGURE 2-2:
CRYSTAL/CERAMIC
RESONATOROPERATION
(XT, HS OR HSPLL
Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
VDD and temperature range for the application.
CONFIGURATION)
(1)
C1
OSC1
To
Internal
Logic
See the notes following this table for additional
information.
(3)
RF
XTAL
Crystals Used:
Sleep
(2)
RS
(1)
4 MHz
8 MHz
20 MHz
PIC18FXXXX
C2
OSC2
Note 1: See Table 2-1 and Table 2-2 for initial values of
C1 and C2.
2: A series resistor (RS) may be required for AT
strip cut crystals.
3: RF varies with the oscillator mode chosen.
Note 1: Higher capacitance increases the stability
of oscillator but also increases the start-
up time.
2: When operating below 3V VDD, or when
using certain ceramic resonators at any
voltage, it may be necessary to use the
HS mode or switch to a crystal oscillator.
TABLE 2-1:
CAPACITOR SELECTION FOR
CERAMIC RESONATORS
Typical Capacitor Values Used:
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
Mode
Freq
OSC1
OSC2
XT
HS
4.0 MHz
33 pF
33 pF
8.0 MHz
16.0 MHz
27 pF
22 pF
27 pF
22 pF
appropriate
values
of
external
components.
Capacitor values are for design guidance only.
4: Rs may be required to avoid overdriving
crystals with low drive level specification.
These capacitors were tested with the resonators
listed below for basic start-up and operation. These
values are not optimized.
5: Always verify oscillator performance over
the VDD and temperature range that is
expected for the application.
Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
VDD and temperature range for the application.
An internal postscaler allows users to select a clock
frequency other than that of the crystal or resonator.
Frequency division is determined by the CPUDIV Con-
figuration bits. Users may select a clock frequency of
the oscillator frequency, or 1/2, 1/3 or 1/4 of the
frequency.
See the notes following Table 2-2 for additional
information.
Resonators Used:
An external clock may also be used when the micro-
controller is in HS Oscillator mode. In this case, the
OSC2/CLKO pin is left open (Figure 2-3).
4.0 MHz
8.0 MHz
16.0 MHz
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 25
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