Datasheet

© 2007 Microchip Technology Inc. DS39605F-page 13
PIC18F1220/1320
2.4 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 2-4 shows the pin connections for the EC
Oscillator mode.
FIGURE 2-4: EXTERNAL CLOCK INPUT
OPERATION
(EC CONFIGURATION)
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 2-5 shows the pin connections
for the ECIO Oscillator mode.
FIGURE 2-5: EXTERNAL CLOCK INPUT
OPERATION
(ECIO CONFIGURATION)
2.5 RC Oscillator
For timing insensitive applications, the “RC” and
“RCIO” device options offer additional cost savings.
The RC oscillator frequency is a function of the supply
voltage, the resistor (R
EXT) and capacitor (CEXT) val-
ues and the operating temperature. In addition to this,
the oscillator frequency will vary from unit to unit due to
normal manufacturing variation. Furthermore, the dif-
ference in lead frame capacitance between package
types will also affect the oscillation frequency, espe-
cially for low C
EXT values. The user also needs to take
into account variation, due to tolerance of external
R and C components used. Figure 2-6 shows how the
R/C combination is connected.
In the RC 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 2-6: RC OSCILLATOR MODE
The RCIO Oscillator mode (Figure 2-7) functions like
the RC 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 2-7: RCIO OSCILLATOR MODE
OSC1/CLKI
OSC2/CLKO
F
OSC/4
Clock from
Ext. System
PIC18FXXXX
OSC1/CLKI
I/O (OSC2)
RA6
Clock from
Ext. System
PIC18FXXXX
OSC2/CLKO
CEXT
REXT
PIC18FXXXX
OSC1
F
OSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
C
EXT > 20 pF
CEXT
REXT
PIC18FXXXX
OSC1
Internal
Clock
VDD
VSS
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
C
EXT > 20 pF
I/O (OSC2)
RA6