Datasheet

PIC18F6585/8585/6680/8680
DS30491D-page 24  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 gen-
eral 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
200 kHz 15 pF 15 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 TBD TBD
These values are for design guidance only.
See notes following this table.
Crystals Used
32.0 kHz Epson C-001R32.768K-A ± 20 PPM
200 kHz STD XTL 200.000KHz ± 20 PPM
1.0 MHz ECS ECS-10-13-1 ± 50 PPM
4.0 MHz ECS ECS-40-20-1 ± 50 PPM
8.0 MHz Epson CA-301 8.000M-C ± 30 PPM
20.0 MHz Epson CA-301 20.000M-C ± 30 PPM
Note 1: Higher capacitance increases the stability
of the oscillator, but also increases the
start-up time.
2: Rs (see Figure 2-1) may be required in
HS mode, as well as XT mode, to avoid
overdriving crystals with low drive level
specifications.
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
PIC18FXX80/XX85
OSC2/CLKO
CEXT
REXT
PIC18FXX80/XX85
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 k  REXT  100 k
C
EXT > 20pF
18F8680.book Page 24 Tuesday, January 29, 2013 1:32 PM