PIC18FXX20
TABLE 2-2:
CAPACITOR SELECTION FOR
FIGURE 2-2:
EXTERNAL CLOCK INPUT
OPERATION (HS, XT OR
LP OSC
CRYSTAL OSCILLATOR
Ranges Tested:
CONFIGURATION)
Mode
Freq
C1
C2
LP
32.0 kHz
200 kHz
200 kHz
1.0 MHz
4.0 MHz
4.0 MHz
8.0 MHz
20.0
MHz
25.0
MHz
33 pF
15 pF
47-68 pF
15 pF
15 pF
15 pF
33 pF
15 pF
47-68 pF
15 pF
15 pF
15 pF
OSC1
PIC18FXX20
OSC2
Clock from
Ext. System
Open
XT
HS
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 (REXT) 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 CEXT 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.
15-33 pF
15-33 pF
15-33 pF
15-33 pF
TBD
TBD
These values are for design guidance only.
See notes following this table.
Crystals Used
32.0 kHz
200 kHz
1.0 MHz
4.0 MHz
8.0 MHz
Epson C-001R32.768K-A ± 20 PPM
STD XTL 200.000KHz
ECS ECS-10-13-1
ECS ECS-40-20-1
± 20 PPM
± 50 PPM
± 50 PPM
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.
Epson CA-301 8.000M-C ± 30 PPM
20.0 MHz Epson CA-301 20.000M-C ± 30 PPM
FIGURE 2-3:
RC OSCILLATOR MODE
Note 1: Higher capacitance increases the stability
of the oscillator, but also increases the
start-up time.
VDD
REXT
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
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.
Internal
OSC1
Clock
CEXT
VSS
PIC18FXX20
OSC2/CLKO
FOSC/4
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
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.
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).
DS39609A-page 22
Advance Information
2003 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
