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Sheet目录3855

13页 14页 15页 16页 [17页]18页 19页 20页 21页

PIC18F1230/1330

2009 Microchip Technology Inc.

DS39758D-page 113

13.2

Timer1 Oscillator

A crystal oscillator circuit is built-in between pins T1OSI

(input) and T1OSO/TICKI (amplifier output). The place-

ment of these pins depends on the value of Configuration

bit, T1OSCMX (see Section 20.1 “Configuration

Bits”

). 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 13-3. Table 13-1 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 13-3:

EXTERNAL

COMPONENTS FOR THE

TIMER1 LP OSCILLATOR

TABLE 13-1:

CAPACITOR SELECTION FOR

THE TIMER OSCILLATOR

13.2.1

USING TIMER1 AS A CLOCK

SOURCE

The Timer1 oscillator is also available as a clock source

in power-managed modes. By setting the System

Clock Select bits, SCS1:SCS0 (OSCCON<1:0>), to

‘01’, the device switches to SEC_RUN mode; both the

CPU and peripherals are clocked from the Timer1 oscil-

lator. If the IDLEN bit (OSCCON<7>) is cleared and a

SLEEP

instruction is executed, the device enters

SEC_IDLE mode. Additional details are available in

Section 4.0 “Power-Managed Modes”

Whenever the Timer1 oscillator is providing the clock

source, the Timer1 system clock status flag, T1RUN

(T1CON<6>), is set. This can be used to determine the

controller’s current clocking mode. It can also indicate

the clock source being currently used by the Fail-Safe

Clock Monitor. If the Clock Monitor is enabled and the

Timer1 oscillator fails while providing the clock, polling

the T1RUN bit will indicate whether the clock is being

provided by the Timer1 oscillator or another source.

13.3

Timer1 Oscillator Layout

Considerations

The oscillator circuit, shown in Figure 13-3, should be

located as close as possible to the microcontroller.

There should be no circuits passing within the oscillator

circuit boundaries other than VSS or VDD.

If a high-speed circuit must be located near the

oscillator (such as the PWM pin, or the primary

oscillator using the OSC2 pin), a grounded guard ring

around the oscillator circuit, as shown in Figure 13-4,

may be helpful when used on a single-sided PCB, or in

addition to a ground plane.

FIGURE 13-4:

OSCILLATOR CIRCUIT

WITH GROUNDED GUARD

RING

Osc Type

Freq

32 kHz

27 pF(1)

Note 1:

Microchip suggests this value as a starting

point in validating the oscillator circuit.

Higher capacitance increases the stability

of the oscillator, but also increases the

start-up time.

Since each resonator/crystal has its own

characteristics, the user should consult

the resonator/crystal manufacturer for

appropriate

values

external

components.

Capacitor values are for design guidance

only.

Note:

See the notes with Table 13-1 for addi-

tional information about capacitor selec-

XTAL

PIC18FXXXX

T1OSI

T1OSO/T1CKI

32.768 kHz

33 pF

RB3

RB2

OSC1

OSC2

VDD

Note:

Not drawn to scale.

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