PIC16F87X
DS30292C-page 122
2001 Microchip Technology Inc.
TABLE 12-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
12.2.3
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 resis-
tor (REXT) and capacitor (CEXT) values, and the operat-
ing 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
CEXT values. The user also needs to take into account
variation due to tolerance of external R and C compo-
nents used. Figure 12-3 shows how the R/C combina-
tion is connected to the PIC16F87X.
FIGURE 12-3:
RC OSCILLATOR MODE
Osc Type
Crystal
Freq.
Cap. Range
C1
Cap. Range
C2
LP
32 kHz
33 pF
200 kHz
15 pF
XT
200 kHz
47-68 pF
1 MHz
15 pF
4 MHz
15 pF
HS
4 MHz
15 pF
8 MHz
15-33 pF
20 MHz
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 appro-
priate values of external components.
3: Rs 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/CLKOUT
CEXT
REXT
PIC16F87X
OSC1
FOSC/4
Internal
Clock
VDD
VSS
Recommended values:
3 k
≤ REXT ≤ 100 k
CEXT > 20pF
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