PIC16F870/871
11.2.3 RC OSCILLATOR
TABLE 11-2: CAPACITOR SELECTION FOR
CRYSTAL 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 11-3 shows how the R/C combina-
tion is connected to the PIC16F870/871.
Cap.
Range
C2
Crystal
Freq
Cap. Range
C1
Osc Type
LP
32 kHz
200 kHz
200 kHz
1 MHz
33 pF
15 pF
33 pF
15 pF
XT
47-68 pF
15 pF
47-68 pF
15 pF
4 MHz
15 pF
15 pF
HS
4 MHz
15 pF
15 pF
8 MHz
15-33 pF
15-33 pF
15-33 pF
15-33 pF
20 MHz
FIGURE 11-3: RC OSCILLATOR MODE
These values are for design guidance only.
See notes at bottom of page.
VDD
Crystals Used
32 kHz
Epson C-001R32.768K-A
± 20 PPM
Rext
Internal
OSC1
200 kHz STD XTL 200.000 kHz
± 20 PPM
± 50 PPM
± 50 PPM
± 30 PPM
Clock
1 MHz
4 MHz
8 MHz
20 MHz
ECS ECS-10-13-1
Cext
VSS
PIC16F870/871
ECS ECS-40-20-1
EPSON CA-301 8.000M-C
OSC2/CLKOUT
FOSC/4
Recommended values:
EPSON CA-301 20.000M-C ± 30 PPM
3 kΩ ≤ Rext ≤ 100 kΩ
Cext > 20pF
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.
DS30569A-page 92
Preliminary
1999 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
