Datasheet

PIC18FXX8
DS41159E-page 18 © 2006 Microchip Technology Inc.
TABLE 2-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
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. Furthermore, the differ-
ence 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 2-2 shows how the RC
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 2-2: 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).
Osc Type
Crystal
Freq
Cap. Range
C1
Cap. Range
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 15-33 pF 15-33 pF
These values are for design guidance only.
See notes on this page.
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: Recommended values of C1 and C2 are
identical to the ranges tested (Table 2-1).
2: Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
3: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate values of external
components.
4: Rs may be required in HS mode, as well
as XT mode, to avoid overdriving crystals
with low drive level specification.
Note: If the oscillator frequency divided by 4
signal is not required in the application, it
is recommended to use RCIO mode to
save current.
OSC2/CLKO
CEXT
REXT
PIC18FXX8
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
C
EXT > 20 pF