PIC18F44J10-ISLASHML datasheet(124/368 Pages) MICROCHIP

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PIC18F45J10 FAMILY

DS39682E-page 122

TABLE 12-1:

CAPACITOR SELECTION FOR

THETIMEROSCILLATOR(2,3,4)

12.3.1

USING TIMER1 AS A

CLOCK SOURCE

The Timer1 oscillator is also available as a clock source

in power-managed modes. By setting the clock select

bits, SCS<1:0> (OSCCON<1:0>), to ‘01’, the device

switches to SEC_RUN mode; both the CPU and

peripherals are clocked from the Timer1 oscillator. 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.

12.3.2

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 VSS 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

12.4

Timer1 Interrupt

The TMR1 register pair (TMR1H:TMR1L) increments

from 0000h to FFFFh and rolls over to 0000h. The

Timer1 interrupt, if enabled, is generated on overflow

which is latched in interrupt flag bit, TMR1IF

(PIR1<0>). This interrupt can be enabled or disabled

by setting or clearing the Timer1 Interrupt Enable bit,

TMR1IE (PIE1<0>).

Oscillator

Type

Freq.

32 kHz

27 pF(1)

Note 1:

Microchip suggests these values 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.

VDD

OSC1

VSS

OSC2

RC0

RC1

RC2

Note:

Not drawn to scale.

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