PIC18CXX2_13 Datasheet, PDF (18/304 Pages) Microchip Technology – High Performance Microcontrollers with 10-bit A/D

PIC18CXX2
TABLE 2-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATORS
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
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
8.0 MHz
20.0 MHz
Epson C-001R32.768K-A ± 20 PPM
STD XTL 200.000kHz ± 20 PPM
ECS ECS-10-13-1
± 50 PPM
ECS ECS-40-20-1
± 50 PPM
Epson CA-301 8.000M-C ± 30 PPM
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
appropriate values of external compo-
nents or verify oscillator performance.
An external clock source may also be connected to the
OSC1 pin in these modes, as shown in Figure 2-2.
FIGURE 2-2:
EXTERNAL CLOCK INPUT
OPERATION (HS, XT OR
LP CONFIGURATION)
Clock from
Ext. System
Open
OSC1
PIC18CXXX
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.
FIGURE 2-3:
RC OSCILLATOR MODE
VDD
REXT
OSC1
Internal
Clock
CEXT
PIC18CXXX
VSS
OSC2/CLKO
FOSC/4
Recommended values:3 k  REXT  100 k
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).
DS39026D-page 18
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