PIC16F87XA
DS39582B-page 146
2003 Microchip Technology Inc.
TABLE 14-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
14.2.3
RC OSCILLATOR
For timing insensitive applications, the “RC” device
option offers additional cost savings. The RC oscillator
frequency is a function of the supply voltage, the
resistor (R
EXT
) and capacitor (C
EXT
) values and the
operating temperature. In addition to this, the oscillator
frequency will vary from unit to unit due to normal pro-
cess parameter variation. Furthermore, the difference
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 14-3 shows how the R/C
combination is connected to the PIC16F87XA.
FIGURE 14-3:
RC OSCILLATOR MODE
Osc Type
Crystal
Freq.
Cap. Range
C1
Cap. Range
C2
LP
32 kHz
33 pF
33 pF
200 kHz
15 pF
15 pF
XT
200 kHz
47-68 pF
47-68 pF
1 MHz
15 pF
15 pF
4 MHz
15 pF
15 pF
HS
4 MHz
15 pF
15 pF
8 MHz
15-33 pF
15-33 pF
20 MHz
15-33 pF
15-33 pF
These values are for design guidance only.
See notes following this table.
Crystals Used
32 kHz
Epson C-001R32.768K-A
± 20 PPM
200 kHz
STD XTL 200.000KHz
± 20 PPM
1 MHz
ECS ECS-10-13-1
± 50 PPM
4 MHz
ECS ECS-40-20-1
± 50 PPM
8 MHz
EPSON CA-301 8.000M-C
± 30 PPM
20 MHz
EPSON CA-301 20.000M-C
± 30 PPM
Note 1: Higher capacitance increases the stability
of oscillator but also increases the start-up
time.
2: Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate values of external
components.
3: R
s
may be required in HS mode, as well
as XT mode, to avoid overdriving crystals
with low drive level specification.
4: When migrating from other PICmicro
®
devices, oscillator performance should be
verified.
OSC2/CLKO
C
EXT
R
EXT
PIC16F87XA
OSC1
F
OSC
/4
Internal
Clock
V
DD
V
SS
Recommended values:
3 k
Ω
≤
R
EXT
≤
100 k
Ω
C
EXT
> 20 pF
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