PIC18F6X2X datasheet(142/386 Pages) MICROCHIP

142 / 386 page

PIC18F6X2X/8X2X

DS39612A-page 140

Advance Information

 2003 Microchip Technology Inc.

12.6

Using Timer1 as a Real-Time

Clock

Adding an external LP oscillator to Timer1 (such as the

one described in Section 12.2) gives users the option

to include RTC functionality to their applications. This is

accomplished with an inexpensive watch crystal to pro-

vide an accurate time base, and several lines of appli-

cation 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 incre-

ment a counter at one-second intervals using an Inter-

rupt Service Routine. Incrementing the TMR1 register

pair to overflow, triggers the interrupt and calls the rou-

tine 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 MSbit 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 rou-

tine, RTCinit. The Timer1 oscillator must also be

enabled and running at all times.

EXAMPLE 12-1:

IMPLEMENTING A REAL-TIME CLOCK USING A TIMER1 INTERRUPT SERVICE

RTCinit

MOVLW

0x80

; Preload TMR1 register pair

MOVWF

TMR1H

; for 1 second overflow

CLRF

TMR1L

MOVLW

b’00001111’

; Configure for external clock,

MOVWF

T1OSC

; Asynchronous operation, external oscillator

CLRF

secs

; Initialize timekeeping registers

CLRF

mins

;

MOVLW

.12

MOVWF

hours

BSF

PIE1, TMR1IE

; Enable Timer1 interrupt

RETURN

RTCisr

BSF

TMR1H,7

; Preload for 1 sec overflow

BCF

PIR1,TMR1IF

; Clear interrupt flag

INCF

secs,F

; Increment seconds

MOVLW

.59

; 60 seconds elapsed?

CPFSGT

secs

RETURN

; No, done

CLRF

secs

; Clear seconds

INCF

mins,F

; Increment minutes

MOVLW

.59

; 60 minutes elapsed?

CPFSGT

mins

RETURN

; No, done

CLRF

mins

; clear minutes

INCF

hours,F

; Increment hours

MOVLW

.23

; 24 hours elapsed?

CPFSGT

hours

RETURN

; No, done

MOVLW

.01

; Reset hours to 1

MOVWF

hours

RETURN

; Done

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