PIC18F2331/2431/4331/4431
3.4
External Clock Input
3.5
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 actual
oscillator frequency is a function of several factors:
• Supply voltage
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 3-4 shows the pin connections for the EC
Oscillator mode.
• Values of the external resistor (REXT) and capacitor
(CEXT)
• Operating temperature
Given the same device, operating voltage and tempera-
ture, and component values, there will also be unit-to-unit
frequency variations. These are due to factors, such as:
FIGURE 3-4:
EXTERNAL CLOCK INPUT
OPERATION
• Normal manufacturing variation
• Difference in lead frame capacitance between
package types (especially for low CEXT values)
(EC CONFIGURATION)
• Variations within the tolerance of limits of REXT
and CEXT
OSC1/CLKI
Clock from
Ext. System
PIC18FXXXX
OSC2/CLKO
In the RC Oscillator mode (Figure 3-6), 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.
FOSC/4
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 3-5 shows the pin connections
for the ECIO Oscillator mode.
FIGURE 3-6:
RC OSCILLATOR MODE
VDD
REXT
Internal
OSC1
FIGURE 3-5:
EXTERNAL CLOCK INPUT
OPERATION
Clock
(ECIO CONFIGURATION)
CEXT
VSS
PIC18FXXXX
OSC2/CLKO
OSC1/CLKI
PIC18FXXXX
I/O (OSC2)
Clock from
Ext. System
FOSC/4
Recommended values: 3 k REXT 100 k
RA6
CEXT > 20 pF
The RCIO Oscillator mode (Figure 3-7) 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 3-7:
RCIO OSCILLATOR MODE
VDD
REXT
Internal
OSC1
Clock
CEXT
PIC18FXXXX
VSS
I/O (OSC2)
RA6
Recommended values: 3 k REXT 100 k
CEXT > 20 pF
2010 Microchip Technology Inc.
DS39616D-page 31
MICROCHIP [ MICROCHIP ]
