Datasheet
PIC18F6525/6621/8525/8621
DS39612C-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 OSCILLATOR
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)
values and the operating temperature. In addition to
this, the oscillator frequency will vary from unit to unit
due to normal process parameter variation. Further-
more, the difference in lead frame capacitance
between package 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 32.0 kHz 33 pF 33 pF
XT 200 kHz 47-68 pF 47-68 pF
1.0 MHz 15 pF 15 pF
4.0 MHz 15 pF 15 pF
HS 4.0 MHz 15 pF 15 pF
8.0 MHz 15-33 pF 15-33 pF
20.0 MHz 15-33 pF 15-33 pF
25.0 MHz 15-33 pF 15-33 pF
These values are for design guidance only.
See notes following this table.
Crystals Used
32 kHz 4 MHz
200 kHz 8 MHz
1 MHz 20 MHz
Note 1: Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
2: R
S (see Figure 2-1) may be required in
HS mode, as well as XT mode, to avoid
overdriving crystals with low drive level
specification.
3: Since each resonator/crystal has its own
characteristics, the user should consult the
resonator/crystal manufacturer for appro-
priate values of external components or
verify oscillator performance.
OSC1
OSC2
Open
Clock from
Ext. System
PIC18F6X2X/8X2X
OSC2/CLKO
CEXT
REXT
PIC18F6X2X/8X2X
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 k REXT 100 k
C
EXT > 20 pF