PIC18F6X2X Datasheet, PDF (142/386 Pages) Microchip Technology – 64/80-Pin High Performance, 64-Kbyte Enhanced FLASH Microcontrollers with A/D

PIC18F6X2X/8X2X
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
register is never preloaded or altered; doing so may
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
RTCisr
MOVLW
MOVWF
CLRF
MOVLW
MOVWF
CLRF
CLRF
MOVLW
MOVWF
BSF
RETURN
0x80
TMR1H
TMR1L
b’00001111’
T1OSC
secs
mins
.12
hours
PIE1, TMR1IE
; Preload TMR1 register pair
; for 1 second overflow
; Configure for external clock,
; Asynchronous operation, external oscillator
; Initialize timekeeping registers
;
; Enable Timer1 interrupt
BSF
BCF
INCF
MOVLW
CPFSGT
RETURN
CLRF
INCF
MOVLW
CPFSGT
RETURN
CLRF
INCF
MOVLW
CPFSGT
RETURN
MOVLW
MOVWF
RETURN
TMR1H,7
PIR1,TMR1IF
secs,F
.59
secs
secs
mins,F
.59
mins
mins
hours,F
.23
hours
.01
hours
; Preload for 1 sec overflow
; Clear interrupt flag
; Increment seconds
; 60 seconds elapsed?
; No, done
; Clear seconds
; Increment minutes
; 60 minutes elapsed?
; No, done
; clear minutes
; Increment hours
; 24 hours elapsed?
; No, done
; Reset hours to 1
; Done
DS39612A-page 140
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 2003 Microchip Technology Inc.