PIC18F2220/2320/4220/4320
2.4
External Clock Input
2.5
RC Oscillator
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.
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 (REXT) 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 CEXT 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 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)
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.
OSC1/CLKI
Clock from
Ext. System
PIC18FXXXX
OSC2/CLKO
FOSC/4
FIGURE 2-6:
RC OSCILLATOR MODE
VDD
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.
REXT
Internal
OSC1
Clock
CEXT
VSS
PIC18FXXXX
FIGURE 2-5:
EXTERNAL CLOCK INPUT
OPERATION
(ECIO CONFIGURATION)
OSC2/CLKO
FOSC/4
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
OSC1/CLKI
PIC18FXXXX
I/O (OSC2)
Clock from
Ext. System
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).
RA6
FIGURE 2-7:
RCIO OSCILLATOR MODE
VDD
REXT
Internal
OSC1
Clock
CEXT
PIC18FXXXX
VSS
I/O (OSC2)
RA6
Recommended values: 3 kΩ ≤ REXT ≤ 100 kΩ
CEXT > 20 pF
2003 Microchip Technology Inc.
DS39599C-page 21
MICROCHIP [ MICROCHIP ]
