Datasheet

PIC16F818/819
DS39598F-page 62  2001-2013 Microchip Technology Inc.
EXAMPLE 7-3: IMPLEMENTING A REAL-TIME CLOCK USING A TIMER1 INTERRUPT SERVICE
TABLE 7-2: REGISTERS ASSOCIATED WITH TIMER1 AS A TIMER/COUNTER
RTCinit BANKSEL TMR1H
MOVLW 0x80 ; 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
BANKSEL PIE1
BSF PIE1, TMR1IE ; Enable Timer1 interrupt
RETURN
RTCisr BANKSEL TMR1H
BSF TMR1H, 7 ; Preload for 1 sec overflow
BCF PIR1, TMR1IF ; Clear interrupt flag
INCF secs, F ; Increment seconds
MOVF secs, w
SUBLW .60
BTFSS STATUS, Z ; 60 seconds elapsed?
RETURN ; No, done
CLRF seconds ; Clear seconds
INCF mins, f ; Increment minutes
MOVF mins, w
SUBLW .60
BTFSS STATUS, Z ; 60 seconds elapsed?
RETURN ; No, done
CLRF mins ; Clear minutes
INCF hours, f ; Increment hours
MOVF hours, w
SUBLW .24
BTFSS STATUS, Z ; 24 hours elapsed?
RETURN ; No, done
CLRF hours ; Clear hours
RETURN ; Done
Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0
Value on
POR, BOR
Value on
all other
Resets
0Bh,8Bh,
10Bh,18Bh
INTCON GIE PEIE
TMR0IE INTE RBIE TMR0IF INTF RBIF 0000 000x 0000 000u
0Ch PIR1
— ADIF — — SSPIF CCP1IF TMR2IF TMR1IF -0-- 0000 -0-- 0000
8Ch PIE1
— ADIE — — SSPIE CCP1IE TMR2IE TMR1IE -0-- 0000 -0-- 0000
0Eh TMR1L Holding Register for the Least Significant Byte of the 16-bit TMR1 Register xxxx xxxx uuuu uuuu
0Fh TMR1H Holding Register for the Most Significant Byte of the 16-bit TMR1 Register xxxx xxxx uuuu uuuu
10h T1CON
— — T1CKPS1 T1CKPS0 T1OSCEN T1SYNC TMR1CS TMR1ON --00 0000 --uu uuuu
Legend: x = unknown, u = unchanged, - = unimplemented, read as ‘0’. Shaded cells are not used by the Timer1 module.