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ManualsBrandsMICROCHIP TECHNOLOGY ManualsMicrocontrollers, Microprocessors & QuartzesEmbedded microcontroller SPDIP 28 8-Bit 40 MHz I/O number 22
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PIC18CXX2
DS39026D-page 18 1999-2013 Microchip Technology Inc.
TABLE 2-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATORS
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)
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 package
types will also affect the oscillation frequency, espe-
cially 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
15-33 pF 15-33 pF
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 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.