PIC18F2420/2520/4420/4520
2.0
2.1
OSCILLATOR
CONFIGURATIONS
Oscillator Types
FIGURE 2-1:
CRYSTAL/CERAMIC
RESONATOR OPERATION
(XT, LP, HS OR HSPLL
CONFIGURATION)
OSC1
To
Internal
Logic
Sleep
PIC18F2420/2520/4420/4520 devices can be operated
in ten different oscillator modes. The user can program
the Configuration bits, FOSC<3:0>, in Configuration
Register 1H to select one of these ten modes:
Low-Power Crystal
Crystal/Resonator
High-Speed Crystal/Resonator
High-Speed Crystal/Resonator
with PLL Enabled
5. RC
External Resistor/Capacitor with
F
OSC
/4 Output on RA6
6. RCIO
External Resistor/Capacitor with I/O
on RA6
7. INTIO1 Internal Oscillator with F
OSC
/4 Output
on RA6 and I/O on RA7
8. INTIO2 Internal Oscillator with I/O on RA6
and RA7
9. EC
External Clock with F
OSC
/4 Output
10. ECIO
External Clock with I/O on RA6
1.
2.
3.
4.
LP
XT
HS
HSPLL
C1
(1)
XTAL
R
S
(2)
C2
(1)
Note 1:
2:
3:
OSC2
R
F
(3)
PIC18FXXXX
See Table 2-1 and Table 2-2 for initial values of
C1 and C2.
A series resistor (R
S
) may be required for AT
strip cut crystals.
R
F
varies with the oscillator mode chosen.
TABLE 2-1:
CAPACITOR SELECTION FOR
CERAMIC RESONATORS
Typical Capacitor Values Used:
Mode
XT
Freq
3.58 MHz
4.19 MHz
4 MHz
4 MHz
OSC1
15 pF
15 pF
30 pF
50 pF
OSC2
15 pF
15 pF
30 pF
50 pF
2.2
Crystal Oscillator/Ceramic
Resonators
In XT, LP, HS or HSPLL Oscillator modes, a crystal or
ceramic resonator is connected to the OSC1 and
OSC2 pins to establish oscillation. Figure 2-1 shows
the pin connections.
The oscillator design requires the use of a parallel cut
crystal.
Note:
Use of a series cut crystal may give a fre-
quency out of the crystal manufacturer’s
specifications.
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.
See the notes following Table 2-2 for additional
information.
Note:
When using resonators with frequencies
above 3.5 MHz, the use of HS mode,
rather than XT mode, is recommended.
HS mode may be used at any V
DD
for
which the controller is rated. If HS is
selected, it is possible that the gain of the
oscillator will overdrive the resonator.
Therefore, a series resistor should be
placed between the OSC2 pin and the
resonator. As a good starting point, the
recommended value of R
S
is 330Ω.
©
2008 Microchip Technology Inc.
DS39631E-page 23
MICROCHIP [ MICROCHIP TECHNOLOGY ]
