Information

© 2008 Microchip Technology Inc. DS39646C-page 31
PIC18F8722 FAMILY
2.0 OSCILLATOR
CONFIGURATIONS
2.1 Oscillator Types
The PIC18F8722 family of devices can be operated in
ten different oscillator modes. The user can program the
Configuration bits, FOSC<3:0>, in Configuration
Register 1H to select one of these ten modes:
1. LP Low-Power Crystal
2. XT Crystal/Resonator
3. HS High-Speed Crystal/Resonator
4. HSPLL High-Speed Crystal/Resonator
with PLL enabled
5. RC External Resistor/Capacitor with
F
OSC/4 output on RA6
6. RCIO External Resistor/Capacitor with I/O
on RA6
7. INTIO1 Internal Oscillator with F
OSC/4 output
on RA6 and I/O on RA7
8. INTIO2 Internal Oscillator with I/O on RA6
and RA7
9. EC External Clock with F
OSC/4 output
10. ECIO External Clock with I/O on RA6
2.2 Crystal Oscillator/Ceramic
Resonators
In XT, LP, HS or HSPLL Oscillator modes, a crystal or
ceramic resonator is connected to the OSC1 and
OSC2 pins to establish oscillation. Figure 2-1 shows
the pin connections.
The oscillator design requires the use of a parallel cut
crystal.
FIGURE 2-1: CRYSTAL/CERAMIC
RESONATOR OPERATION
(XT, LP, HS OR HSPLL
CONFIGURATION)
TABLE 2-1: CAPACITOR SELECTION FOR
CERAMIC RESONATORS
Note: Use of a series cut crystal may give a
frequency out of the crystal manufacturer’s
specifications.
Typical Capacitor Values Used:
Mode Freq OSC1 OSC2
XT 3.58 MHz 22 pF 22 pF
Capacitor values are for design guidance only.
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. Refer
to the following application notes for oscillator specific
information:
• AN588 – PIC
®
Microcontroller Oscillator Design
Guide
• AN826 – Crystal Oscillator Basics and Crystal
Selection for rfPIC
®
and PIC
®
Devices
• AN849 – Basic PIC
®
Oscillator Design
• AN943 – Practical PIC
®
Oscillator Analysis and
Design
• AN949 – Making Your Oscillator Work
See the notes following Table 2-2 for additional
information.
Note: When using resonators with frequencies
above 3.5 MHz, the use of HS mode,
rather than XT mode, is recommended.
HS mode may be used at any V
DD for
which the controller is rated. If HS is
selected, it is possible that the gain of the
oscillator will overdrive the resonator.
Therefore, a series resistor may be placed
between the OSC2 pin and the resonator.
As a good starting point, the
recommended value of R
S is 330Ω.
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