Datasheet

PIC18F1220/1320
DS39605F-page 12 © 2007 Microchip Technology Inc.
TABLE 2-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
An external clock source may also be connected to the
OSC1 pin in the HS mode, as shown in Figure 2-2.
FIGURE 2-2: EXTERNAL CLOCK INPUT
OPERATION (HS OSC
CONFIGURATION)
2.3 HSPLL
A Phase Locked Loop (PLL) circuit is provided as an
option for users who wish to use a lower frequency
crystal oscillator circuit, or to clock the device up to its
highest rated frequency from a crystal oscillator. This
may be useful for customers who are concerned with
EMI due to high-frequency crystals.
The HSPLL mode makes use of the HS mode oscillator
for frequencies up to 10 MHz. A PLL then multiplies the
oscillator output frequency by 4 to produce an internal
clock frequency up to 40 MHz.
The PLL is enabled only when the oscillator configura-
tion bits are programmed for HSPLL mode. If
programmed for any other mode, the PLL is not
enabled.
FIGURE 2-3: PLL BLOCK DIAGRAM
Osc Type
Crystal
Freq
Typical Capacitor Values
Tested:
C1 C2
LP 32 kHz 33 pF 33 pF
200 kHz 15 pF 15 pF
XT 1 MHz 33 pF 33 pF
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:
32 kHz 4 MHz
200 kHz 8 MHz
1 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: R
S 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.
OSC1
OSC2
Open
Clock from
Ext. System
PIC18FXXXX
(HS Mode)
MUX
VCO
Loop
Filter
Crystal
Osc
OSC2
OSC1
PLL Enable
F
IN
FOUT
SYSCLK
Phase
Comparator
HS Oscillator Enable
÷4
(from Configuration Register 1H)