Elementary Offset problem. Please help!

[PotatoPeeler]

Hello I am a student that are a complete novice.

I have written a code in Assembler that works, except for one thing.

The offset value that I have entered for TMR0 never appears in TMR0 if I simulate! So I am doing it wrong.

I am using P16F84A (Use MPLAB and PIC Simulator to write code.).
I am using TMR0 is timer and not the WDT.

If have never noticed before that my offset values are not entered in TMR0; because I have only had to work with very small offset values, so I didn't notice errors. I can not find anything on the net to explain how to enter offset correctly.

Here is part of my code with an example of how I entered all my offset values:

START BSF STATUS,RP0
CLRF TRISA ;Make all outputs
CLRF TRISB ;Redandant part.

MOVLW B'00010110' ;INPUTS RB1, RB2 AND RB4!
MOVWF TRISB

MOVLW B'10000001'
MOVWF OPTION_REG ; Redundant part

MOVLW B'10101000'
MOVWF INTCON ;Interupt Setup

BCF STATUS,RP0
CLRF PORTA
CLRF PORTB ;PORT A & B start at 0

BSF STATUS,RP0
MOVLW B'10000001'
MOVWF OPTION_REG ;PRESCALER=4
BCF STATUS,RP0
MOVLW B'00001000' ;******This part doesnt work!
MOVWF TMR0 ;******OFFSET=8

MAIN GOTO MAIN


END

 

[btbass]

I have simulated the code and it works just fine?????

MOVLW B'00001000' ;******This part does work!
MOVWF TMR0 ;******OFFSET=8

 

[PotatoPeeler]

I have simulated the code and it works just fine?????

MOVLW B'00001000' ;******This part does work!
MOVWF TMR0 ;******OFFSET=8

Thanks.

Got the problem. It was in when I entered the offset is the interrupt service routine.

I still do not understand how offset works though.

If I enter offset just before I return from interrupt it works just fine, If I enter it early in my ISR it does not8O.

 

[btbass]

The timer does not stop incrementing while you are in the interrupt.
If you pre-load the timer at the beginning of the interrupt, depending on the pre-scale value, it may have incremented by the end of it.