Datasheet

PIC18F2450/4450
© 2008 Microchip Technology Inc. DS39760D-page 25
2.2.2 CRYSTAL OSCILLATOR/CERAMIC
RESONATORS
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.
The oscillator design requires the use of a parallel cut
crystal.
FIGURE 2-2: CRYSTAL/CERAMIC
RESONATOR OPERATION
(XT, HS OR HSPLL
CONFIGURATION)
TABLE 2-1: CAPACITOR SELECTION FOR
CERAMIC RESONATORS
TABLE 2-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
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
Configuration bits. Users may select a clock frequency
of the oscillator frequency, or 1/2, 1/3 or 1/4 of the
frequency.
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).
Note: Use of a series cut crystal may give a fre-
quency out of the crystal manufacturer’s
specifications.
Typical Capacitor Values Used:
Mode Freq OSC1 OSC2
XT 4.0 MHz 33 pF 33 pF
HS 8.0 MHz
16.0 MHz
27 pF
22 pF
27 pF
22 pF
Capacitor values are for design guidance only.
These capacitors were tested with the resonators
listed below for basic start-up and operation. These
values are not optimized.
Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
V
DD and temperature range for the application.
See the notes following Table 2-2 for additional
information.
Resonators Used:
4.0 MHz
8.0 MHz
16.0 MHz
Note 1: See Table 2-1 and Table 2-2 for initial values of
C1 and C2.
2: A series resistor (R
S) may be required for AT
strip cut crystals.
3: R
F varies with the oscillator mode chosen.
C1
(1)
C2
(1)
XTAL
OSC2
OSC1
RF
(3)
Sleep
To
Logic
PIC18FXXXX
RS
(2)
Internal
Osc Type
Crystal
Freq
Typical Capacitor Values
Tested:
C1 C2
XT 4 MHz 27 pF 27 pF
HS 4 MHz 27 pF 27 pF
8 MHz 22 pF 22 pF
20 MHz 15 pF 15 pF
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.
Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
V
DD and temperature range for the application.
See the notes following this table for additional
information.
Crystals Used:
4 MHz
8 MHz
20 MHz
Note 1: Higher capacitance increases the stability
of oscillator but also increases the start-up
time.
2: When operating below 3V V
DD, or when
using certain ceramic resonators at any
voltage, it may be necessary to use the
HS mode or switch to a crystal oscillator.
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate values of external
components.
4: Rs may be required to avoid overdriving
crystals with low drive level specification.
5: Always verify oscillator performance over
the V
DD and temperature range that is
expected for the application.