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2010 Microchip Technology Inc.
DS39616D-page 31
PIC18F2331/2431/4331/4431
3.4
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 3-4 shows the pin connections for the EC
Oscillator mode.
FIGURE 3-4:
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 3-5 shows the pin connections
for the ECIO Oscillator mode.
FIGURE 3-5:
EXTERNAL CLOCK INPUT
OPERATION
(ECIO CONFIGURATION)
3.5
RC Oscillator
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
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:
Normal manufacturing variation
Difference in lead frame capacitance between
package types (especially for low CEXT values)
Variations within the tolerance of limits of REXT
and CEXT
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.
FIGURE 3-6:
RC OSCILLATOR MODE
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
OSC1/CLKI
OSC2/CLKO
FOSC/4
Clock from
Ext. System
PIC18FXXXX
OSC1/CLKI
I/O (OSC2)
RA6
Clock from
Ext. System
PIC18FXXXX
OSC2/CLKO
CEXT
REXT
PIC18FXXXX
OSC1
FOSC/4
Internal
Clock
VDD
VSS
Recommended values: 3 k
REXT 100 k
CEXT > 20 pF
CEXT
REXT
PIC18FXXXX
OSC1
Internal
Clock
VDD
VSS
Recommended values: 3 k
REXT 100 k
CEXT > 20 pF
I/O (OSC2)
RA6
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