PIC18F2680ISPSQTP datasheet(156/482 Pages) MICROCHIP

156 / 482 page

PIC18F2585/2680/4585/4680

DS39625C-page 154

Preliminary

12.3.3

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 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>).

12.5

Resetting Timer1 Using the CCP1

Special Event Trigger

If either of the CCP1 modules is configured in Compare

mode

generate

special

event

trigger

(CCP1M3:CCP1M0

CCP2M3:CCP2M0 = 1011),

this signal will reset Timer1. The trigger from ECCP1

will also start an A/D conversion if the A/D module is

enabled (see Section 15.3.4 “Special Event Trigger”

for more information.).

The module must be configured as either a timer or a

synchronous counter to take advantage of this feature.

When used this way, the CCPR1H:CCPR1L register

pair effectively becomes a period register for Timer1.

If Timer1 is running in Asynchronous Counter mode,

this Reset operation may not work.

In the event that a write to Timer1 coincides with a

special event trigger, the write operation will take

precedence.

12.6

Using Timer1 as a Real-Time

Clock

Adding an external LP oscillator to Timer1 (such as the

one described in Section 12.3 “Timer1 Oscillator”

above) gives users the option to include RTC function-

ality to their applications. This is accomplished with an

inexpensive watch crystal to provide an accurate time

base and several lines of application code to calculate

the time. When operating in Sleep mode and using a

battery or supercapacitor as a power source, it can

completely eliminate the need for a separate RTC

device and battery backup.

The application code routine, RTCisr, shown in

Example 12-1, demonstrates a simple method to

increment a counter at one-second intervals using an

Interrupt Service Routine. Incrementing the TMR1

the routine, which increments the seconds counter by

one; additional counters for minutes and hours are

incremented as the previous counter overflow.

Since the register pair is 16 bits wide, counting up to

overflow the register directly from a 32.768 kHz clock

would take 2 seconds. To force the overflow at the

required one-second intervals, it is necessary to pre-

load it. The simplest method is to set the MSb of

TMR1H with a BSF instruction. Note that the TMR1L

introduce cumulative error over many cycles.

For this method to be accurate, Timer1 must operate in

Asynchronous mode and the Timer1 overflow interrupt

must be enabled (PIE1<0> = 1) as shown in the routine,

RTCinit

. The Timer1 oscillator must also be enabled

and running at all times.

VDD

OSC1

VSS

OSC2

RC0

RC1

RC2

Note: Not drawn to scale.

Note:

The special event triggers from the

ECCP1 module will not set the TMR1IF

interrupt flag bit (PIR1<0>).

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