
PIC18F2480/2580/4480/4580
DS39637A-page 24
Preliminary
2004 Microchip Technology Inc.
TABLE 2-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
An external clock source may also be connected to the
OSC1 pin in the HS mode, as shown in Figure 2-2.
FIGURE 2-2:
EXTERNAL CLOCK
INPUT OPERATION
(HS OSCILLATOR
CONFIGURATION)
2.3
External Clock Input
The EC and ECIO Oscillator modes require an external
clock source to be connected to the OSC1 pin. There is
no oscillator start-up time required after a Power-on
Reset or after an exit from Sleep mode.
In the EC Oscillator mode, the oscillator frequency
divided by 4 is available on the OSC2 pin. This signal
may be used for test purposes or to synchronize other
logic. Figure 2-3 shows the pin connections for the EC
Oscillator mode.
FIGURE 2-3:
EXTERNAL CLOCK
INPUT OPERATION
(EC CONFIGURATION)
The ECIO Oscillator mode functions like the EC mode,
except that the OSC2 pin becomes an additional
general purpose I/O pin. The I/O pin becomes bit 6 of
PORTA (RA6). Figure 2-4 shows the pin connections
for the ECIO Oscillator mode.
FIGURE 2-4:
EXTERNAL CLOCK
INPUT OPERATION
(ECIO CONFIGURATION)
Osc Type
Crystal
Freq
Typical Capacitor Values
Tested:
C1
C2
LP
32 kHz
200 kHz
1 MHz
4 MHz
4 MHz
8 MHz
20 MHz
33 pF
15 pF
33 pF
27 pF
27 pF
22 pF
15 pF
33 pF
15 pF
33 pF
27 pF
27 pF
22 pF
15 pF
XT
HS
Capacitor values are for design guidance only.
These capacitors were tested with the crystals listed
below for basic start-up and operation.
These values
are 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.
Crystals Used:
32 kHz
200 kHz
1 MHz
4 MHz
8 MHz
20 MHz
Note 1:
Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
2:
When operating below 3V V
DD
, or when
using certain ceramic resonators at any
voltage, it may be necessary to use the
HS mode or switch to a crystal oscillator.
3:
Since each resonator/crystal has its own
characteristics, the user should consult
the resonator/crystal manufacturer for
appropriate
components.
4:
Rs may be required to avoid overdriving
crystals with low drive level specification.
5:
Always verify oscillator performance over
the V
DD
and temperature range that is
expected for the application.
values
of
external
OSC1
OSC2
Open
Clock from
Ext. System
PIC18FXXXX
(HS Mode)
OSC1/CLKI
OSC2/CLKO
F
OSC
/4
Clock from
Ext. System
PIC18FXXXX
OSC1/CLKI
I/O (OSC2)
RA6
Clock from
Ext. System
PIC18FXXXX