Datasheet

 2009-2011 Microchip Technology Inc. DS39957D-page 47
PIC18F87K90 FAMILY
TABLE 3-2: CAPACITOR SELECTION FOR
CERAMIC RESONATORS
TABLE 3-3: CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
FIGURE 3-2: CRYSTAL/CERAMIC
RESONATOR OPERATION
(HS OR HSPLL
CONFIGURATION)
3.5 RC Oscillator
For timing-insensitive applications, the RC and RCIO
Oscillator modes offer additional cost savings. The
actual oscillator frequency is a function of several
factors:
• Supply voltage
• Values of the external resistor (R
EXT) and
capacitor (C
EXT)
• Operating temperature – Given the same device,
operating voltage and temperature and
component values, there will also be unit-to-unit
frequency variations. These are due to factors,
such as:
- Normal manufacturing variation
- Difference in lead frame capacitance
between package types (especially for low
C
EXT values)
- Variations within the tolerance of limits of
R
EXT and CEXT
Typical Capacitor Values Used:
Mode Freq. OSC1 OSC2
HS 8.0 MHz
16.0 MHz
27 pF
22 pF
27 pF
22 pF
Capacitor values are for design guidance only.
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. Refer
to the following application notes for oscillator-specific
information:
• AN588, “PIC
®
Microcontroller Oscillator Design
Guide”
• AN826, “Crystal Oscillator Basics and Crystal
Selection for rfPIC
®
and PIC
®
Devices”
• AN849, “Basic PIC
®
Oscillator Design”
• AN943, “Practical PIC
®
Oscillator Analysis and
Design”
• AN949, “Making Your Oscillator Work”
See the notes following Table 3-3 for additional
information.
Osc Type
Crystal
Freq.
Typical Capacitor Values
Tested:
C1 C2
HS 4 MHz 27 pF 27 pF
8 MHz 22 pF 22 pF
20 MHz 15 pF 15 pF
Capacitor values are for design guidance only.
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.
Refer to the Microchip application notes cited in
Table 3-2 for oscillator-specific information. Also see
the notes following this table for additional
information.
Note 1: Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
2: Since each resonator/crystal has its
own characteristics, the user should
consult the resonator/crystal manufac-
turer for appropriate values of external
components.
3: Rs may be required 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.
Note 1: See Table 3-2 and Table 3-3 for initial values of
C1 and C2.
2: A series resistor (R
S) may be required for AT
strip cut crystals.
3: R
F varies with the oscillator mode chosen.
C1
(1)
C2
(1)
XTAL
OSC2
OSC1
RF
(3)
Sleep
To
Logic
RS
(2)
Internal
PIC18F87K90