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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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