PIC18F23K20_10 Datasheet, PDF (164/456 Pages) Microchip Technology – 28/40/44-Pin Flash Microcontrollers with nanoWatt XLP Technology

PIC18F2XK20/4XK20
13.9 Using Timer1 as a Real-Time Clock
Adding an external LP oscillator to Timer1 (such as the
one described in Section 13.6 “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 13-1, demonstrates a simple method to
increment a counter at one-second intervals using an
Interrupt Service Routine. Incrementing the TMR1
register pair to overflow triggers the interrupt and calls
the routine, which increments the seconds counter by
one; additional counters for minutes and hours are
incremented on overflows of the less significant
counters.
Since the register pair is 16 bits wide, a 32.768 kHz
clock source will take 2 seconds to count up to over-
flow. To force the overflow at the required one-second
intervals, it is necessary to preload it; the simplest
method is to set the MSb of TMR1H with a BSF instruc-
tion. 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
routine, RTCinit. The Timer1 oscillator must also be
enabled and running at all times.
EXAMPLE 13-1: IMPLEMENTING A REAL-TIME CLOCK USING A TIMER1 INTERRUPT SERVICE
RTCinit
RTCisr
MOVLW
MOVWF
CLRF
MOVLW
MOVWF
CLRF
CLRF
MOVLW
MOVWF
BSF
RETURN
80h
TMR1H
TMR1L
b’00001111’
T1CON
secs
mins
.12
hours
PIE1, TMR1IE
BSF
BCF
INCF
MOVLW
CPFSGT
RETURN
CLRF
INCF
MOVLW
CPFSGT
RETURN
CLRF
INCF
MOVLW
CPFSGT
RETURN
CLRF
RETURN
TMR1H, 7
PIR1, TMR1IF
secs, F
.59
secs
secs
mins, F
.59
mins
mins
hours, F
.23
hours
hours
; Preload TMR1 register pair
; for 1 second overflow
; Configure for external clock,
; Asynchronous operation, external oscillator
; Initialize timekeeping registers
;
; Enable Timer1 interrupt
; 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
; Done
DS41303G-page 164
 2010 Microchip Technology Inc.