PIC18F2420/2520/4420/4520
FIGURE 2-1:
CRYSTAL/CERAMIC
RESONATOR OPERATION
(XT, LP, HS OR HSPLL
CONFIGURATION)
2.0
2.1
OSCILLATOR
CONFIGURATIONS
Oscillator Types
(1)
C1
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:
OSC1
To
Internal
Logic
(3)
RF
XTAL
1. LP
2. XT
3. HS
Low-Power Crystal
Sleep
(2)
RS
Crystal/Resonator
(1)
PIC18FXXXX
C2
OSC2
High-Speed Crystal/Resonator
4. HSPLL High-Speed Crystal/Resonator
with PLL Enabled
Note 1: See Table 2-1 and Table 2-2 for initial values of
C1 and C2.
5. RC
External Resistor/Capacitor with
FOSC/4 Output on RA6
2: A series resistor (RS) may be required for AT
strip cut crystals.
6. RCIO
External Resistor/Capacitor with I/O
on RA6
3: RF varies with the oscillator mode chosen.
7. INTIO1 Internal Oscillator with FOSC/4 Output
on RA6 and I/O on RA7
TABLE 2-1:
CAPACITOR SELECTION FOR
CERAMIC RESONATORS
8. INTIO2 Internal Oscillator with I/O on RA6
and RA7
Typical Capacitor Values Used:
9. EC
External Clock with FOSC/4 Output
External Clock with I/O on RA6
10. ECIO
Mode
Freq
OSC1
OSC2
XT
3.58 MHz
4.19 MHz
4 MHz
15 pF
15 pF
30 pF
50 pF
15 pF
15 pF
30 pF
50 pF
2.2
Crystal Oscillator/Ceramic
Resonators
4 MHz
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.
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
VDD and temperature range for the application.
The oscillator design requires the use of a parallel cut
crystal.
See the notes following Table 2-2 for additional
information.
Note:
Use of a series cut crystal may give a fre-
quency out of the crystal manufacturer’s
specifications.
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 VDD 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 RS is 330Ω.
© 2008 Microchip Technology Inc.
DS39631E-page 23
MICROCHIP [ MICROCHIP ]
