Datasheet
2003-2013 Microchip Technology Inc. DS39609C-page 21
PIC18F6520/8520/6620/8620/6720/8720
2.0 OSCILLATOR
CONFIGURATIONS
2.1 Oscillator Types
The PIC18FXX20 devices can be operated in eight
different oscillator modes. The user can program three
configuration bits (FOSC2, FOSC1 and FOSC0) to
select one of these eight modes:
1. LP Low-Power Crystal
2. XT Crystal/Resonator
3. HS High-Speed Crystal/Resonator
4. HS+PLL High-Speed Crystal/Resonator
with PLL enabled
5. RC External Resistor/Capacitor
6. RCIO External Resistor/Capacitor with
I/O pin enabled
7. EC External Clock
8. ECIO External Clock with I/O pin
enabled
2.2 Crystal Oscillator/Ceramic
Resonators
In XT, LP, HS or HS+PLL 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 PIC18FXX20 oscillator design requires the use of
a parallel cut crystal.
FIGURE 2-1: CRYSTAL/CERAMIC
RESONATOR OPERATION
(HS, XT OR LP
CONFIGURATION)
TABLE 2-1: CAPACITOR SELECTION FOR
CERAMIC RESONATORS
Note: Use of a series cut crystal may give a fre-
quency out of the crystal manufacturer’s
specifications.
Note 1: See Table 2-1 and Table 2-2 for recommended
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
PIC18FXX20
RS
(2)
Internal
Ranges Tested:
Mode Freq C1 C2
XT 455 kHz
2.0 MHz
4.0 MHz
68-100 pF
15-68 pF
15-68 pF
68-100 pF
15-68 pF
15-68 pF
HS 8.0 MHz
16.0 MHz
10-68 pF
10-22 pF
10-68 pF
10-22 pF
These values are for design guidance only.
See notes following this table.
Resonators Used:
2.0 MHz Murata Erie CSA2.00MG 0.5%
4.0 MHz Murata Erie CSA4.00MG 0.5%
8.0 MHz Murata Erie CSA8.00MT 0.5%
16.0 MHz Murata Erie CSA16.00MX 0.5%
All resonators used did not have built-in capacitors.
Note 1: Higher capacitance increases the stability
of the 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 high
gain HS mode, try a lower frequency
resonator, 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, or verify oscillator
performance.