PIC18F1220ESLASHSOSQTP datasheet(15/308 Pages) MICROCHIP

15 / 308 page

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 (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 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

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

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

CEXT

REXT

PIC18FXXXX

OSC1

Internal

Clock

VDD

VSS

Recommended values: 3 k

Ω ≤ REXT ≤ 100 kΩ

CEXT > 20 pF

I/O (OSC2)

RA6

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