PIC18LF4520ESLASHSPQTP datasheet(27/390 Pages) MICROCHIP

27 / 390 page

 2004 Microchip Technology Inc.

Preliminary

DS39631A-page 25

PIC18F2420/2520/4420/4520

2.4

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, 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 2-5 shows how the R/C combination is

connected.

FIGURE 2-5:

RC OSCILLATOR MODE

The RCIO Oscillator mode (Figure 2-6) 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 2-6:

RCIO OSCILLATOR MODE

2.5

PLL Frequency Multiplier

A Phase Locked Loop (PLL) circuit is provided as an

option for users who wish to use a lower frequency

oscillator circuit or to clock the device up to its highest

rated frequency from a crystal oscillator. This may be

useful for customers who are concerned with EMI due

to high-frequency crystals or users who require higher

clock speeds from an internal oscillator.

2.5.1

HSPLL OSCILLATOR MODE

The HSPLL mode makes use of the HS mode oscillator

for frequencies up to 10 MHz. A PLL then multiplies the

oscillator output frequency by 4 to produce an internal

clock frequency up to 40 MHz. The PLLEN bit is not

available in this oscillator mode.

The PLL is only available to the crystal oscillator when

the FOSC3:FOSC0 configuration bits are programmed

for HSPLL mode (= 0110).

FIGURE 2-7:

PLL BLOCK DIAGRAM

(HS MODE)

2.5.2

PLL AND INTOSC

The PLL is also available to the internal oscillator block

in selected oscillator modes. In this configuration, the

PLL is enabled in software and generates a clock out-

put of up to 32 MHz. The operation of INTOSC with the

PLL is described in Section 2.6.4 “PLL in INTOSC

Modes”.

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

VCO

Loop

Filter

Crystal

Osc

OSC2

OSC1

PLL Enable

FIN

FOUT

SYSCLK

Phase

Comparator

HS Oscillator Enable

÷4

(from Configuration Register 1H)

HS Mode

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