PIC16F818/819
4.0
4.1
OSCILLATOR
CONFIGURATIONS
Oscillator Types
TABLE 4-1:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR (FOR
DESIGN GUIDANCE ONLY)
Crystal
Freq
32 kHz
200 kHz
200 kHz
1 MHz
4 MHz
Typical Capacitor Values
Tested:
C1
C2
33 pF
15 pF
56 pF
15 pF
15 pF
15 pF
15 pF
15 pF
The PIC16F818/819 can be operated in eight different
oscillator modes. The user can program three configu-
ration bits (FOSC2:FOSC0) to select one of these eight
modes (modes 5-8 are new PIC16 oscillator
configurations):
1.
2.
3.
4.
5.
6.
7.
8.
LP
XT
HS
RC
RCIO
INTIO1
INTIO2
ECIO
Low-Power Crystal
Crystal/Resonator
High-Speed Crystal/Resonator
External Resistor/Capacitor with
F
OSC
/4 output on RA6
External Resistor/Capacitor with
I/O on RA6
Internal Oscillator with F
OSC
/4
output on RA6 and I/O on RA7
Internal Oscillator with I/O on RA6
and RA7
External Clock with I/O on RA6
Osc Type
LP
XT
33 pF
15 pF
56 pF
15 pF
15 pF
15 pF
15 pF
15 pF
HS
4 MHz
8 MHz
20 MHz
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.
Note 1:
Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
2:
Since each crystal has its own character-
istics, the user should consult the crystal
manufacturer for appropriate values of
external components.
3:
R
S
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.
4.2
Crystal Oscillator/Ceramic
Resonators
In XT, LP or HS modes, a crystal or ceramic resonator
is connected to the OSC1/CLKI and OSC2/CLKO pins
to establish oscillation (see Figure 4-1 and Figure 4-2).
The PIC16F818/819 oscillator design requires the use
of a parallel cut crystal. Use of a series cut crystal may
give a frequency out of the crystal manufacturer’s
specifications.
FIGURE 4-1:
CRYSTAL OPERATION
(HS, XT OR LP OSC
CONFIGURATION)
OSC1
PIC16F818/819
C1
(1)
XTAL
OSC2
R
S
(2)
C2
(1)
R
F
(3)
Sleep
To Internal
Logic
Note 1:
See Table 4-1 for typical 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 crystal chosen (typically
between 2 MΩ to 10 MΩ).
2004 Microchip Technology Inc.
DS39598E-page 33
MICROCHIP [ MICROCHIP TECHNOLOGY ]
