PIC18F2420/2520/4420/4520
An external clock source may also be connected to the
OSC1 pin in the HS mode, as shown in Figure 2-2.
TABLE 2-2:
CAPACITOR SELECTION FOR
CRYSTAL OSCILLATOR
Typical Capacitor Values
FIGURE 2-2:
EXTERNAL CLOCK INPUT
OPERATION (HS OSC
CONFIGURATION)
Crystal
Freq
Tested:
Osc Type
C1
C2
LP
XT
32 kHz
30 pF
30 pF
1 MHz
4 MHz
15 pF
15 pF
15 pF
15 pF
OSC1
Clock from
Ext. System
PIC18FXXXX
(HS Mode)
HS
4 MHz
10 MHz
20 MHz
25 MHz
25 MHz
15 pF
15 pF
15 pF
0 pF
15 pF
15 pF
15 pF
5 pF
OSC2
Open
15 pF
15 pF
2.3
External Clock Input
Capacitor values are for design guidance only.
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.
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
VDD and temperature range for the application.
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.
See the notes following this table for additional
information.
FIGURE 2-3:
EXTERNAL CLOCK
INPUT OPERATION
(EC CONFIGURATION)
Crystals Used:
32 kHz
25 MHz
1 MHz
4 MHz
10 MHz
20 MHz
OSC1/CLKI
Clock from
Ext. System
PIC18FXXXX
OSC2/CLKO
FOSC/4
Note 1: Higher capacitance increases the stability
of the oscillator but also increases the
start-up time.
The ECIO Oscillator mode functions like the EC mode,
except that the OSC2 pin becomes an additional gen-
eral 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.
2: When operating below 3V VDD, 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
FIGURE 2-4:
EXTERNAL CLOCK
INPUT OPERATION
(ECIO CONFIGURATION)
appropriate
values
of
external
components.
4: Rs may be required to avoid overdriving
OSC1/CLKI
PIC18FXXXX
I/O (OSC2)
Clock from
Ext. System
crystals with low drive level specification.
5: Always verify oscillator performance over
the VDD and temperature range that is
expected for the application.
RA6
DS39631E-page 24
© 2008 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
