Information

TIMER1 MODULE

EXAMPLE 1: PIC12/PIC14/PIC16 INTERRUPT SERVICE ROUTINE

EXAMPLE 2: PIC18 HIGH PRIORITY INTERRUPT SERVICE ROUTINE

EXAMPLE 3: PIC18 LOW PRIORITY INTERRUPT SERVICE ROUTINE

Note: These instructions are required based on the function of the ISR. If the only code in the ISR is to reload

Timer1, then they are not required, but may be required if additional code is added.

ISR @ 0x004 ; (3-4Tcy), fixed interrupt latency

MOVWF w_temp ; (1Tcy), save off current W register contents

MOVF STATUS,w ; (1Tcy), move status register into W register

MOVWF status_temp ; (1Tcy), save off contents of STATUS register

BANKSEL PIR ; (2Tcy), choose correct SFR bank

BTFSS PIR,TMRIF ; (2Tcy), did a Timer1 overflow occur?

GOTO EXIT ; No

RELOAD

BSF TMR1H,7 ; (1Tcy) Yes, reload for a 1 second overflow

EXIT

MOVF status_temp,w ; retrieve copy of STATUS register

MOVWF STATUS ; restore pre-isr STATUS register contents

SWAPF w_temp,f

SWAPF w_temp,w ; restore pre-isr W register contents

RETFIE

Total = 11-12 TCY (if Timer1 overflow occurred)

ISR @ 0x0008 ; (3-4Tcy), fixed interrupt latency

BRA HIGHINT ; (2Tcy), go to high priority interrupt routine

HIGHINT

BTFSC PIR,TMRIF ; (1Tcy), did a Timer1 overflow occur?

BSF TMR1H,7 ; (1Tcy) Yes, reload for a 1 second overflow

RETFIE FAST

Total = 7-8 TCY (if Timer1 overflow occurred)

ISR @ 0x0018 ; (3-4Tcy), fixed interrupt latency

MOVFF STATUS,STATUS_TEMP ; (2Tcy), save STATUS register

MOVFF WREG,WREG_TEMP ; (2Tcy), save working register, refer to note 1

MOVFF BSR,BSR_TEMP ; (2Tcy), save BSR register, refer to note 1

BTFSS PIR,TMRIF ; (2Tcy), did a Timer1 overflow occur?

BRA EXIT ; No

RELOAD

BSF TMR1H,7 ; (1Tcy) Yes, reload for a 1 second overflow

EXIT

MOVFF BSR_TEMP,BSR ;restore BSR register, refer to note 1

MOVFF WREG_TEMP,WREG ;restore working register, refer to note 1

MOVFF STATUS_TEMP,STATUS ;restore STATUS register

RETFIE

Total = 12-13 TCY (if Timer1 overflow occurred)