Gennady Palitsky <PICLIST@MITVMA.MIT.EDU> wrote:

> I believe this should be simple, but don't have an idea as of right
> now. To generate delay I am using standard procedure:
>
> LOOP
>     decfsz    COUNTER,1
>     goto        LOOP
> ------------------
> As far as I understand, both DECFSZ and GOTO are 2 cycle
> instructions, so 1 point change in COUNTER value gives me 4 cycles
> change in delay time (am I wrong?).

    Gennady:

    Yes, you're wrong... The DECFSZ only takes two cycles on the
    FINAL iteration of the loop (when the GOTO is skipped); every
    other time through the loop, it takes just one cycle.

    The GOTO is a two-cycle instruction, so the total delay is X*3-1,
    where X is the value loaded into the counter before the loop.

> Is there any way to generate delay which will give me 1:1 ratio (1
> point change in COUNTER value : 1 cycle change in delay time). Or
> do I want too much?

    Try this; it generates delays with a resolution of one cycle,
    although it's limited to the range [20-271].

        MOVLW   X
        CALL    DELAY

        ....

    ; This routine delays X cycles.  Enter with X (in the range
    ; [20-271]) in the W register.
    ;
    ; Note that the delay is inclusive of the "MOVLW X", "CALL
    ; DELAY", and "RETURN" overhead, so a sequence like:
    ;
    ;     MOVLW   100
    ;     CALL    DELAY
    ;     MOVLW   200
    ;     CALL    DELAY
    ;
    ; will delay EXACTLY 300 cycles.

    DELAY:

        MOVWF   COUNTER

        BTFSC   COUNTER, 0
        GOTO    $+1

        BTFSS   COUNTER, 1
        GOTO    SKIP
        NOP
        GOTO    $+1

    SKIP:

        RRF     COUNTER
        RRF     COUNTER

        MOVLW   4
        SUBWF   COUNTER

        BCF     COUNTER,6
        BCF     COUNTER,7

    LOOP:

        NOP
        DECFSZ  COUNTER
        GOTO    LOOP

        RETURN

    As is always the case with code that I write online, this code
    has neither been tested nor even assembled.

    -Andy

=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
=== http://www.geocities.com/SiliconValley/2499