[SOLVED] Unexplained action of TMR0 in PIC

;MAIN PROGRAM ************************
start
	bcf		GPIO,0		;ensure port is off
		;wait for switch on
	btfsc 	GPIO,1             
	goto	start		;bit is set - input is high - switch is off
		;switch is on  -  start with reference number = 0
	clrf 		ref		;start ramp at zero
	
		;initial 2sec delay
	bcf		flags,running		;allow normal timer int	
	call 		timer2sec
	bsf 		GPIO,2		;##### test LED ##########
twosecdelay	
	btfss 	flags,tim
	goto 	twosecdelay		;loop until end of delay
		;end of 2sec delay
	bcf		flags,newstart		
  	bsf		flags,running	;show interrupt that prog is in normal running mode
    	       ;set clock counter to 13
  	movlw	.13
    	movwf	count     
    	
    	  ;-----------program loops here between interrupts--------
mainloop    	 ;wait for interrupt which will switch on port
	btfss	flags,newstart	      ;flag to send prog back to give 2sec time delay again
	goto	 mainloop			;normal measure in interrupt routine
	goto 	prestart    		;meter has gone to zero - new start required	
	
	  
measure	;'''''''''call to here from interrupt program''''''''''''''
			;pulse width ramps up form zero to a ratio of 231 in 256, in approx 6secs
			;ref = turn-off time.  ramps up to maxref 
					
	bcf		GPIO,4		;####### test LED ###########	
	bcf		GPIO,5		;####### test LED ###########	
	bsf 		GPIO,0		;turn on port	
	  ;measure 'on' period  (port is now on)
		;look for  tmr0 = ref to turn port off
int2		
	movwf	ref		;get ref number
	subwf	TMR0,0
	btfss	STATUS,Z
	goto	int2
	bcf		GPIO,0		;TMR0 = ref.  Turn off port			
			;port-on period ended		
	  
	decfsz	count,1		;1 in 13 ramp rate
	return		;loop until time to step up/down ref register (13 x 2ms)	
		;reload counter after 13 x 2.048ms = 26.62ms
		;(25.62ms x 231 = 6.15sec)
	movlw	.13
	movwf	count
	
		    ;test switch input			    
	btfsc 	GPIO,1             
	goto	s1		;bit is set - input is high - switch is off
	goto	s2		;input is low - switch is on

s1
	bsf		GPIO,4		;####### test LED #########
	movwf	ref		;switch is off - test file for 0
	btfss	STATUS,Z
	goto	s1a		;not zero
	bsf		flags,newstart		;gauge has run back to zero. loop back to include 2sec delay
	return	
s1a
	decf 	ref,1		;not at 0, so decrement	
	return	

s2
	bsf		GPIO,5		;####### test LED ##########		
	movwf	ref		;test if ref = maximum allowed
	subwf	maxref,0
	btfsc	STATUS,C	
	return		;ref = >maxref
	incf		ref,1		;ref was <maxref  
	return
		;end of 'measure' routine from timer interrupt

;end of main prog ********************
	

;SUBROUTINES ***************************

;Timer  (prescaler = div 8.  TMR0 clocks tim1 every 2.048ms. 
				;tim1 clocks tim2 every 524ms)				
				;set timer for 2sec count of   t1 =209;  t2 = 3
				;(209 x 2.048ms + 3 x 524ms = 2secs)
timer2sec				
	movlw	.209		;count of 209 x tim1
	movwf 	tim1  		;store
	movlw	3    		;count of 3 x tim2
	movwf	tim2
	bcf		flags,tim	;clear flag
	clrf		TMR0     	;reset timer counter
	bcf       	INTCON,2      ;T01F - Clear TMR0 flag bit to allow interrupt
	return
	
	;********end of timer subs**********

	ORG     0x004             ; interrupt vector location
	movwf   w_temp            ; save off current W register contents
	movf	STATUS,w          ; move status register into W register
	movwf	status_temp       ; save off contents of STATUS register
	

	bcf       	INTCON,2      ;T01F - Clear TMR0 flag bit to allow interrupt and keep timer running	
	btfss     	flags,running   ;jump if not in delay mode
	goto	normal_int		;in 2sec delay  mode
	call		measure		;sub to prog to measure port turn-on time
		;go to interrupt routine to measure TMR0
	goto	endtimint		;exit	
	
normal_int
  	  		 ;2sec timer interrupt routine *******************			
	decfsz 	tim1,1		;decrement reg
	goto	endtimint
	decfsz	tim2,1		;decrement reg
	goto	endtimint
	bsf		flags,tim   	;set flag to show end of 2secs		
	
endtimint
	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                    ; return from interrupt