At 05:19 PM 8/5/02 -0400, Michael A. Powers wrote:

>  bsf PORTB, 4
>  bsf PORTB, 1  ;---------------------
>  ; delay for at least 2ms
>  movlw 0x95
>  movwf Dlay
>  movlw 0x02
>  movwf Dlay + 1
>  decfsz Dlay
>   goto $ - 1
>  decfsz Dlay + 1
>   goto $ - 3
>
>  bcf PORTB, 4 ; --------------------
>
>When I measured this on an oscilloscope, the actual delay came to be 1.2 ms.
>If anything, it should be greater than 2 ms because I did not take the extra
>cycles that occur each borrow.  Does each subtraction that does not borrow
>take 3us to complete? (1us for decfsz, 2us for goto)

The problem is your last 'goto' - you jump back to 'decfsz Dlay', you
should be jumping back to 'movlw 0x02'.

But - you should be using labels in your code!  This business of 'goto $-3'
is BAD PROGRAMMING!  What happens when you add 1 extra line of code?

I cheat too - I will sometimes use 'goto $-1'.  But NO more than 1!

Here is how I think the code should be written:

    movlw     0x95
    movwf     Dlay
Delay_loop
    movlw     0x02
    movwf     Dlay + 1
    decfsz    Dlay
      goto    $ - 1
    decfsz    Dlay + 1
      goto    Delay_loop

dwayne

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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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