PIC18LF252-ISP Datasheet, PDF (20/332 Pages) Microchip Technology – High-Performance, Enhanced Flash Microcontrollers with 10-Bit A/D

PIC18FXX2
TABLE 2-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
Ranges Tested:
Mode
Freq
C1
C2
LP
32.0 kHz
33 pF
200 kHz
15 pF
33 pF
15 pF
XT
200 kHz 22-68 pF 22-68 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
32.0 kHz
200 kHz
1.0 MHz
4.0 MHz
Epson C-001R32.768K-A
STD XTL 200.000KHz
ECS ECS-10-13-1
ECS ECS-40-20-1
± 20 PPM
± 20 PPM
± 50 PPM
± 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 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.
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)
Clock from
Ext. System
Open
OSC1
PIC18FXXX
OSC2
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 package
types will also affect the oscillation frequency, espe-
cially 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.
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.
Note:
If the oscillator frequency divided by 4 sig-
nal is not required in the application, it is
recommended to use RCIO mode to save
current.
FIGURE 2-3:
RC OSCILLATOR MODE
VDD
REXT
CEXT
VSS
OSC1
Internal
Clock
PIC18FXXX
OSC2/CLKO
FOSC/4
Recommended values:3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20pF
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).
DS39564C-page 18
© 2006 Microchip Technology Inc.