PIC18F2220/2320/4220/4320
An external clock source may also be connected to the
OSC1 pin in the HS mode, as shown in Figure 2-2.
TABLE 2-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
Typical Capacitor Values
FIGURE 2-2:
EXTERNAL CLOCK INPUT
OPERATION (HS OSC
CONFIGURATION)
Crystal
Freq
Tested:
Osc Type
C1
C2
LP
XT
HS
32 kHz
200 kHz
1 MHz
4 MHz
4 MHz
8 MHz
20 MHz
33 pF
15 pF
33 pF
27 pF
27 pF
22 pF
15 pF
33 pF
15 pF
33 pF
27 pF
27 pF
22 pF
15 pF
OSC1
Clock from
Ext. System
PIC18FXXXX
(HS Mode)
OSC2
Open
2.3
HSPLL
Capacitor values are for design guidance only.
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.
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
VDD and temperature range for the application.
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.
See the notes following this table for additional
information.
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.
Crystals Used:
32 kHz
200 kHz
1 MHz
4 MHz
8 MHz
20 MHz
FIGURE 2-3:
PLL BLOCK DIAGRAM
HS Osc Enable
PLL Enable
(from Configuration Register 1H)
Note 1: Higher capacitance increases the stability
of the 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.
OSC2
OSC1
Phase
Comparator
HS Mode
Crystal
Osc
FIN
FOUT
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
Loop
Filter
appropriate
values
of
external
components.
÷4
VCO
4: RS may be required to avoid overdriving
SYSCLK
crystals with low drive level specification.
5: Always verify oscillator performance over
the VDD and temperature range that is
expected for the application.
DS39599C-page 20
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
