Datasheet

ManualsBrandsMICROCHIP TECHNOLOGY ManualsMicrocontrollers, Microprocessors & QuartzesEmbedded microcontroller SOIC 28 8-Bit 40 MHz I/O number 25

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PIC18F2220/2320/4220/4320

12.2 Timer1 Oscillator

A crystal oscillator circuit is built-in between pins,

T1OSI (input) and T1OSO (amplifier output). It is

enabled by setting control bit, T1OSCEN (T1CON<3>).

The oscillator is a low-power oscillator rated for 32 kHz

crystals. It will continue to run during all power-

managed modes. The circuit for a typical LP oscillator

is shown in Figure 12-3. Table 12-1 shows the

capacitor selection for the Timer1 oscillator.

The user must provide a software time delay to ensure

proper start-up of the Timer1 oscillator.

FIGURE 12-3: EXTERNAL COMPONENTS

FOR THE TIMER1 LP

OSCILLATOR

TABLE 12-1: CAPACITOR SELECTION FOR

THE TIMER OSCILLATOR

(2,3,4)

12.3 Timer1 Oscillator Layout

Considerations

The Timer1 oscillator circuit draws very little power

during operation. Due to the low-power nature of the

oscillator, it may also be sensitive to rapidly changing

signals in close proximity.

The oscillator circuit, shown in Figure 12-3, should be

located as close as possible to the microcontroller.

There should be no circuits passing within the oscillator

circuit boundaries other than V

SS or VDD.

If a high-speed circuit must be located near the oscilla-

tor (such as the CCP1 pin in output compare or PWM

mode, or the primary oscillator using the OSC2 pin), a

grounded guard ring around the oscillator circuit, as

shown in Figure 12-4, may be helpful when used on a

single-sided PCB or in addition to a ground plane.

FIGURE 12-4: OSCILLATOR CIRCUIT

WITH GROUNDED GUARD

RING

Osc Type Freq C1 C2

LP 32 kHz 22 pF

(1)

22 pF

(1)

Note 1: Microchip suggests this value as a starting

point in validating the oscillator circuit.

2: Higher capacitance increases the stability

of the oscillator but also increases the

start-up time.

3: Since each resonator/crystal has its own

characteristics, the user should consult

the resonator/crystal manufacturer for

appropriate values of external

components.

4: Capacitor values are for design guidance

only.

Note: See the Notes with Table 12-1 for additional

information about capacitor selection.

XTAL

PIC18FXXXX

T1OSI

T1OSO

32.768 kHz

22 pF

VDD

OSC1

OSC2

RC0

RC1

RC2

Note: Not drawn to scale.