Datasheet

PIC18F2221/2321/4221/4321 FAMILY
DS39689F-page 30 © 2009 Microchip Technology Inc.
TABLE 3-2: CAPACITOR SELECTION FOR
QUARTZ CRYSTALS
An external clock source may also be connected to the
OSC1 pin in the HS mode, as shown in Figure 3-2.
When operated in this mode, parameters D033 and
D043 apply.
FIGURE 3-2: EXTERNAL CLOCK INPUT
OPERATION (HS OSC
CONFIGURATION)
3.3 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 3-3 shows the pin connections for the EC
Oscillator mode.
FIGURE 3-3: 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 3-4 shows the pin connections
for the ECIO Oscillator mode. When operated in this
mode, parameters D033A and D043A apply.
FIGURE 3-4: EXTERNAL CLOCK
INPUT OPERATION
(ECIO CONFIGURATION)
Osc Type
Crystal
Freq
Typical Capacitor Values
Tested:
C1 C2
LP 32 kHz 22 pF 22 pF
XT 1 MHz
4 MHz
22 pF
22 pF
22 pF
22 pF
HS 4 MHz
10 MHz
20 MHz
25 MHz
22 pF
22 pF
22 pF
22 pF
22 pF
22 pF
22 pF
22 pF
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. Refer
to the following application notes for oscillator specific
information:
• AN588, “PIC
®
Microcontroller Oscillator Design
Guide”
• AN826, “Crystal Oscillator Basics and Crystal
Selection for rfPIC
®
and PIC
®
Devices”
• AN849, “Basic PIC
®
Oscillator Design”
• AN943, “Practical PIC
®
Oscillator Analysis and
Design”
• AN949, “Making Your Oscillator Work”
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: 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 values of external
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.
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