Datasheet

PIC16F7X
DS30325B-page 92  2002 Microchip Technology Inc.
TABLE 12-2: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
(FOR DESIGN GUIDANCE
ONLY)
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 (R
EXT) 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
C
EXT 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 PIC16F7X.
FIGURE 12-3: RC OSCILLATOR MODE
Osc Type
Crystal
Freq
Typical Capacitor Values
Tested:
C1 C2
LP 32 kHz 33 pF 33 pF
200 kHz 15 pF 15 pF
XT 200 kHz 56 pF 56 pF
1 MHz 15 pF 15 pF
4 MHz 15 pF 15 pF
HS 4 MHz 15 pF 15 pF
8 MHz 15 pF 15 pF
20 MHz 15 pF 15 pF
Capacitor values are for design guidance only.
These capacitors were tested with the crystals listed
below for basic start-up and operation. These values
were not optimized.
Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
V
DD and temperature range for the application.
See the notes following this table for additional
information.
Crystals Used:
32 kHz 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
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: Always verify oscillator performance over
the V
DD and temperature range that is
expected for the application.
OSC2/CLKOUT
CEXT
REXT
PIC16F7X
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20pF