PIC16F7X
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 (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 PIC16F7X.
Typical Capacitor Values
Crystal
Freq
Tested:
Osc Type
C1
C2
LP
XT
32 kHz
200 kHz
200 kHz
1 MHz
33 pF
15 pF
56 pF
15 pF
15 pF
15 pF
15 pF
15 pF
33 pF
15 pF
56 pF
15 pF
15 pF
15 pF
15 pF
15 pF
4 MHz
HS
4 MHz
FIGURE 12-3:
RC OSCILLATOR MODE
8 MHz
VDD
20 MHz
Capacitor values are for design guidance only.
REXT
Internal
OSC1
These capacitors were tested with the crystals listed
below for basic start-up and operation. These values
were not optimized.
Clock
CEXT
VSS
PIC16F7X
Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
VDD and temperature range for the application.
OSC2/CLKOUT
FOSC/4
Recommended values:
3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20pF
See the notes following this table for additional
information.
Crystals Used:
32 kHz
200 kHz
1 MHz
Epson C-001R32.768K-A
STD XTL 200.000KHz
ECS ECS-10-13-1
4 MHz
ECS ECS-40-20-1
8 MHz
EPSON CA-301 8.000M-C
EPSON CA-301 20.000M-C
20 MHz
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 VDD and temperature range that is
expected for the application.
DS30325B-page 92
2002 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
