PIC16F87 datasheet(75/230 Pages) MICROCHIP

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DS30487D-page 75

PIC16F87/88

7.6

Timer1 Oscillator

A crystal oscillator circuit is built between pins T1OSI

(input) and T1OSO (amplifier output). It is enabled by

setting control bit T1OSCEN (T1CON<3>). The oscilla-

tor is a low-power oscillator, rated up to 32.768 kHz. It

will continue to run during all power-managed modes.

It is primarily intended for a 32 kHz crystal. The circuit

for a typical LP oscillator is shown in Figure 7-3.

Table 7-1 shows the capacitor selection for the Timer1

oscillator.

The user must provide a software time delay to ensure

proper oscillator start-up.

FIGURE 7-3:

EXTERNAL

COMPONENTS FOR THE

TIMER1 LP OSCILLATOR

TABLE 7-1:

CAPACITOR SELECTION FOR

THE TIMER1 OSCILLATOR

7.7

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 7-3, 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 oscilla-

tor, a grounded guard ring around the oscillator circuit,

as shown in Figure 7-4, may be helpful when used on

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

FIGURE 7-4:

OSCILLATOR CIRCUIT

WITH GROUNDED

GUARD RING

Note:

The Timer1 oscillator shares the T1OSI

and T1OSO pins with the PGD and PGC

pins

used

for

programming

and

debugging.

When using the Timer1 oscillator, In-Circuit

Serial Programming™ (ICSP™) may not

function correctly (high voltage or low

voltage), or the In-Circuit Debugger (ICD)

may not communicate with the controller.

As a result of using either ICSP or ICD, the

Timer1 crystal may be damaged.

If ICSP or ICD operations are required, the

crystal should be disconnected from the

circuit (disconnect either lead) or installed

after programming. The oscillator loading

capacitors may remain in-circuit during

ICSP or ICD operation.

PIC16F87/88

T1OSI

T1OSO

33 pF

XTAL

32.768 kHz

Note:

See the Notes with Table 7-1 for additional

information about capacitor selection.

Osc Type

Freq

32 kHz

33 pF

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

external

components.

4: Capacitor values are for design guidance

only.

OSC1

VSS

OSC2

RB7

RB6

RB5

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