Information

© 2008 Microchip Technology Inc. DS39646C-page 169
PIC18F8722 FAMILY
EXAMPLE 13-1: IMPLEMENTING A REAL-TIME CLOCK USING A TIMER1 INTERRUPT SERVICE
TABLE 13-2: REGISTERS ASSOCIATED WITH TIMER1 AS A TIMER/COUNTER
Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Reset
Values
on page
INTCON GIE/GIEH PEIE/GIEL TMR0IE INT0IE RBIE TMR0IF INT0IF RBIF 57
PIR1
PSPIF ADIF RC1IF TX1IF SSP1IF CCP1IF TMR2IF TMR1IF 60
PIE1
PSPIE ADIE RC1IE TX1IE SSP1IE CCP1IE TMR2IE TMR1IE 60
IPR1 PSPIP ADIP RC1IP TX1IP SSP1IP CCP1IP TMR2IP TMR1IP 60
TMR1L Timer1 Register Low Byte 58
TMR1H Timer1 Register High Byte 58
T1CON RD16 T1RUN T1CKPS1 T1CKPS0 T1OSCEN T1SYNC
TMR1CS TMR1ON 58
Legend: Shaded cells are not used by the Timer1 module.
RTCinit
MOVLW 80h ; Preload TMR1 register pair
MOVWF TMR1H ; for 1 second overflow
CLRF TMR1L
MOVLW b’00001111’ ; Configure for external clock,
MOVWF T1CON ; 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
CLRF hours ; Reset hours
RETURN ; Done