PIC16F870/871
TABLE 11-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
Cap. Range
C1
33 pF
15 pF
47-68 pF
15 pF
15 pF
15 pF
15-33 pF
15-33 pF
Cap.
Range
C2
33 pF
15 pF
47-68 pF
15 pF
15 pF
15 pF
15-33 pF
15-33 pF
11.2.3
RC OSCILLATOR
For timing insensitive applications, the 鈥淩C鈥?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 PIC16F870/871.
Osc Type
LP
XT
Crystal
Freq
32 kHz
200 kHz
200 kHz
1 MHz
4 MHz
HS
4 MHz
8 MHz
20 MHz
These values are for design guidance only.
See notes at bottom of page.
FIGURE 11-3: RC OSCILLATOR MODE
V
DD
Crystals Used
32 kHz
200 kHz
1 MHz
4 MHz
8 MHz
20 MHz
Epson C-001R32.768K-A
STD XTL 200.000 kHz
ECS ECS-10-13-1
ECS ECS-40-20-1
EPSON CA-301 8.000M-C
EPSON CA-301 20.000M-C
卤 20 PPM
卤 20 PPM
卤 50 PPM
卤 50 PPM
卤 30 PPM
卤 30 PPM
Cext
V
SS
Rext
OSC1
Internal
Clock
PIC16F870/871
OSC2/CLKOUT
3 k鈩?/div>
鈮?/div>
Rext
鈮?/div>
100 k鈩?/div>
Cext > 20pF
F
OSC
/4
Recommended values:
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
漏
1999 Microchip Technology Inc.
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