Just go through the table backwards.  Ie Step 4, Step 3, Step 2, Step 1,
Step 4....

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Harry H. Arends
Sent: Saturday, September 12, 2009 4:04 AM
To: 'Microcontroller discussion list - Public.'
Subject: [PIC] Steppermotor using lookuptable

LS,

I am using this contruction to control a unipolair steppermotor.
The connections of the motor are as follows:
A - RA3
B - RA2
A'- RA1
B'- RA0

The lookup table is here:
[code]
Full_Step:                  ; Used with RS 191-8299
    movf    STEP, w
    andlw   0x03
    addwf   PCL, f
    retlw   b'00001001'     ; Step-1
    retlw   b'00001100'     ; Step-2
    retlw   b'00000110'     ; Step-3
    retlw   b'00000011'     ; Step-4
[/code]
This is the stepping routine:
[code]
    movlw   .50             ; Loop 50 Time For 1.8 Degree/Step 
                            ; 4x50x1.8 = 360
    movwf   COUNT  
Go_Reverse_Full:
    movlw   .4
    movwf   STEP
GoReverseFull_Loop:
    call    Full_Step       ; Get the value from a table
    movwf   PORTA
    call    Delay           ; do some delay between steps
    decfsz  STEP, f         ; count steps
    goto    GoReverseFull_Loop
    decfsz  COUNT, f        ; Do one revolution
    goto    Go_Reverse_Full
[/code]

If i wanted to go forward i could use a second lookup table but can it
be done with one table and if so how?

Regards

Harry

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