
PIC16F872
DS30221A-page 98
Preliminary
1999 Microchip Technology Inc.
TABLE 11-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
11.2.3
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 (R
EXT
) and capacitor (C
EXT
) 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
C
EXT
values. The user also needs to take into account
variation due to tolerance of external R and C compo-
nents used. Figure 11-3 shows how the R/C combina-
tion is connected to the PIC16F872.
FIGURE 11-3: RC OSCILLATOR MODE
Osc
Type
Crystal
Freq
Cap. Range
C1
Cap.
Range
C2
LP
32 kHz
200 kHz
200 kHz
1 MHz
4 MHz
4 MHz
8 MHz
20 MHz
33 pF
15 pF
47-68 pF
15 pF
15 pF
15 pF
15-33 pF
15-33 pF
33 pF
15 pF
47-68 pF
15 pF
15 pF
15 pF
15-33 pF
15-33 pF
XT
HS
These values are for design guidance only.
See notes at bottom of page.
Crystals Used
Epson C-001R32.768K-A
200 kHz
STD XTL 200.000KHz
1 MHz
ECS ECS-10-13-1
4 MHz
ECS ECS-40-20-1
8 MHz
EPSON CA-301 8.000M-C
20 MHz
EPSON CA-301 20.000M-C
32 kHz
± 20 PPM
± 20 PPM
± 50 PPM
± 50 PPM
± 30 PPM
± 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 compo-
nents.
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
C
EXT
R
EXT
PIC16F872
OSC1
F
OSC
/4
Internal
Clock
V
DD
V
SS
Recommended values:
3 k
≤
R
EXT
≤
100 k
C
EXT
> 20pF