PIC18F2420/2520/4420/4520
monitoring TMR1L within the interrupt routine until it
12.7 Considerations in Asynchronous
increments, and then updating the TMR1H:TMR1L
register pair while the clock is low, or one-half of the
period of the clock source. Assuming that Timer1 is
being used as a Real-Time Clock, the clock source is a
32.768 kHz crystal oscillator; in this case, one half
period of the clock is 15.25 μs.
Counter Mode
Following a Timer1 interrupt and an update to the
TMR1 registers, the Timer1 module uses a falling edge
on its clock source to trigger the next register update on
the rising edge. If the update is completed after the
clock input has fallen, the next rising edge will not be
counted.
The Real-Time Clock application code in Example 12-1
shows a typical ISR for Timer1, as well as the optional
code required if the update cannot be done reliably
within the required interval.
If the application can reliably update TMR1 before the
timer input goes low, no additional action is needed.
Otherwise, an adjusted update can be performed
following a later Timer1 increment. This can be done by
EXAMPLE 12-1:
IMPLEMENTING A REAL-TIME CLOCK USING A TIMER1 INTERRUPT SERVICE
RTCinit
MOVLW
MOVWF
CLRF
80h
TMR1H
TMR1L
; Preload TMR1 register pair
; for 1 second overflow
MOVLW
MOVWF
CLRF
b’00001111’
T1CON
secs
; Configure for external clock,
; Asynchronous operation, external oscillator
; Initialize timekeeping registers
;
CLRF
mins
MOVLW
MOVWF
BSF
.12
hours
PIE1, TMR1IE
; Enable Timer1 interrupt
RETURN
RTCisr
; Start ISR here
; Insert the next 4 lines of code when TMR1
; can not be reliably updated before clock pulse goes low
; wait for TMR1L<0> to become clear
; (may already be clear)
BTFSC
BRA
TMR1L,0
$-2
BTFSS
BRA
TMR1L,0
$-2
; wait for TMR1L<0> to become set
; TMR1 has just incremented
; If TMR1 update can be completed before clock pulse goes low
; Preload for 1 sec overflow
; Clear interrupt flag
; Increment seconds
; 60 seconds elapsed?
BSF
BCF
INCF
MOVLW
TMR1H, 7
PIR1, TMR1IF
secs, F
.59
CPFSGT secs
RETURN
; No, done
CLRF
INCF
MOVLW
secs
mins, F
.59
; Clear seconds
; Increment minutes
; 60 minutes elapsed?
CPFSGT mins
RETURN
; No, done
CLRF
INCF
MOVLW
mins
hours, F
.23
; clear minutes
; Increment hours
; 24 hours elapsed?
CPFSGT hours
RETURN
; No, done
; Reset hours
; Done
CLRF
hours
RETURN
© 2008 Microchip Technology Inc.
DS39631E-page 131
MICROCHIP [ MICROCHIP ]
