Tobie Horswill <PICLIST@MITVMA.MIT.EDU> wrote:

> A = current angle (compass heading)
> B = desired angle (GPS waypoint heading)
>
> if A < B then
>     if ABS(A-B) > 180 then TURN COUNTER CLOCK-WISE else TURN
>     CLOCK-WISE
> else
> if A > B then
>     if ABS(A-B) < 180 then TURN COUNTER CLOCK-WISE else TURN
>     CLOCK-WISE
> else
>     NO CORRECTION;
>
> This works fine in a little Windows App I made and I think I could
> port it to PIC assembly fairly easily or program it directly in C.
> However, I feel it will take a lot of program memory to directly
> translate every conditional and math operator (the values are 16
> bits wide since there are 360 degrees) to the PIC. Any suggestions
> on how to simplify this procedure in PIC assembly?

Tobie:

First, you can divide all your angular measurements by 2 to get the
numbers into the range [0-179]; that'll remove the need to do 16-bit
math without really affecting performance.

Next, you could rewrite it as:

    W = A-B

    if W = 0 then NO CORRECTION
    if W < 0 then W = W + 180
    if W < 90 then TURN CCW else TURN CW

In PIC code, this works out to something like:

        MOVF    B,W     ;W = A-B [A and B are in the range 0-179].
        SUBWF   A,W     ;

        BZ      DONE    ;If W = 0 then NO CORRECTION.

        SKPC            ;If W >= 0 then skip ahead.
        ADDLW   180     ;Otherwise, W = 180 + W.

        SUBLW   89      ;W = 89 - W.
        BC      TURNCCW ;If W >= 0 (in other words, if the old W was
                        ;less than 90), go TURN CCW.

    TURNCW:

        ....            ;Otherwise, TURN CW.

        GOTO    DONE

    TURNCCW:

        ....

    DONE:

-Andy


=== Andrew Warren --- aiw@cypress.com
=== Cypress Semiconductor Corporation
=== Interface Products Division, S.D.
===
=== The opinions expressed above do
=== not necessarily represent those of
=== Cypress Semiconductor Corporation.

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