PIC18F2331/2431/4331/4431
An external clock source may also be connected to the
OSC1 pin in the HS mode, as shown in Figure 3-2.
TABLE 3-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
Typical Capacitor Values
FIGURE 3-2:
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
3.3
PLL Frequency Multiplier
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
oscillator circuit or to clock the device up to its highest
rated frequency from a crystal oscillator. This may be
useful for those concerned with EMI from high-
frequency crystals or users requiring higher clock
speeds from an internal oscillator.
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.
3.3.1
HSPLL OSCILLATOR MODE
See the notes following this table for additional
information.
The HSPLL mode uses the HS Oscillator mode for
frequencies up to 10 MHz. A PLL circuit then multiplies
the oscillator output frequency by four to produce an
internal clock frequency up to 40 MHz. The PLLEN bit
is not available in this oscillator mode.
Crystals Used:
32 kHz
200 kHz
1 MHz
4 MHz
8 MHz
20 MHz
The PLL is only available to the crystal oscillator when
the FOSC<3:0> Configuration bits are programmed for
HSPLL mode (‘0110’).
Note 1: Higher capacitance increases the
stability of oscillator, but also increases
the start-up time.
FIGURE 3-3:
PLL BLOCK DIAGRAM
HS Osc Enable
PLL Enable
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.
(from Configuration Register 1H)
OSC2
OSC1
Phase
Comparator
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
HS Mode
Crystal
Osc
FIN
FOUT
appropriate
components.
values
of
external
Loop
Filter
4: Rs may be required to avoid overdriving
crystals with low drive level specification.
4
VCO
5: Always verify oscillator performance over
the VDD and temperature range that is
expected for the application.
SYSCLK
DS39616D-page 30
2010 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
