Datasheet

PIC18F6520/8520/6620/8620/6720/8720
DS39609C-page 22  2003-2013 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, XT and LP modes, as shown in
Figure 2-2.
FIGURE 2-2: EXTERNAL CLOCK INPUT
OPERATION (HS, XT OR
LP OSC CONFIGURATION)
2.3 RC Oscillator
For timing insensitive applications, the “RC” and
“RCIO” device options offer additional cost savings.
The RC oscillator frequency is a function of the supply
voltage, the resistor (R
EXT) and capacitor (CEXT) val-
ues and the operating temperature. In addition to this,
the oscillator frequency will vary from unit to unit, due
to normal process parameter variation. Furthermore,
the difference in lead frame capacitance between pack-
age types will also affect the oscillation frequency,
especially for low C
EXT values. The user also needs to
take into account variation due to tolerance of external
R and C components used. Figure 2-3 shows how the
R/C combination is connected.
In the RC Oscillator mode, the oscillator frequency
divided by 4 is available on the OSC2 pin. This signal
may be used for test purposes or to synchronize other
logic.
FIGURE 2-3: RC OSCILLATOR MODE
The RCIO Oscillator mode functions like the RC mode,
except that the OSC2 pin becomes an additional
general purpose I/O pin. The I/O pin becomes bit 6 of
PORTA (RA6).
Ranges Tested:
Mode Freq C1 C2
LP 32 kHz
15-22 pF 15-22 pF
200 kHz
XT 1 MHz
15-22 pF 15-22 pF
4 MHz
HS 4 MHz
15-22 pF 15-22 pF8 MHz
20 MHz
Capacitor values are for design guidance only.
These capacitors were tested with the above crystal
frequencies 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.
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 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 manufac-
turer for appropriate values of external
components, or verify oscillator
performance.
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
PIC18FXX20
OSC2/CLKO
CEXT
REXT
PIC18FXX20
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 k  REXT  100 k
C
EXT > 20 pF