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To all who provided advice on interfacing a rotary encoder:

Thanks very much, I got it working OK now!
The attached is the code fragment to detect movement and direction of an
encoder.
The key was the realisation that although these things bounce a lot, you can
start processing at the first change of state, and ignore the debounce as
long as you take care not to poll that encoder again within the next 5 mS or
so. After that time, the bouncing has stopped and it can be polled again.

Cheers, Jan Didden

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;--------System design notes-------------------------------------
;
; The encoders output a 2-bit value:
; the A-bit is used to detect rotation,
; the B-bit is used to determine direction of rotation.
; encoder movement is detected by polling PORTB.
;               =09
;               --------------------->cw
;       channel A     ___     ___     ___  [channel A leads channel B]
;                 ___|   |___|   |___|
;
;       channel B       ___     ___     ___
;                  |___|   |___|   |___|
;
;
;               CCW<------------------------------                              =09
;
;       channel A      ___     ___     ___ [channel A trails channel B]
;                  ___|   |___|   |___|
;=20
;       channel B    ___     ___     ___
;                 __|   |___|   |___|
;
;       A-bit           B-bit           Direction
;       0>1                     0                       CW
;                               1                       CCW
;       1>0                     0                       CCW
;                               1                       CW

;NewEncCmd holds the latewst polled PORTB inputs
;CurEncCmd holds the previously polled PORTB inputs
;There are 4 encoders, with the A-bits connected to PORTB and the A-bits =
to EncDirPort
;ProgA is one of the encoders we decode below

ProcEncInput                                            ;decode encoder

                movfw   NewEncCmd                       ;new setting
                xorwf   CurEncCmd,W                     ;previous setting, W now has changed bits set
                movwf   EncChanges
                btfsc   EncChanges, ProgA       ;prog enc change?
                goto    ProcProg                        ;yes, process it
                goto    ExEncInput

ProcProg

                btfss   NewEncCmd,ProgA         ;if 1, was 0>1
                goto    ProgFall

ProgRise
        =09
                btfss   EncDirPort,ProgB        ;if 0, was CW
                goto    ProgCW
                goto    ProgCCW

ProgFall
        =09
                btfss   EncDirPort,ProgB        ;if 0, was CCW
                goto    ProgCCW
                goto    ProgCW

ProgCCW
        =09
;do the processing work here
                goto    EncUpdate

ProgCW

;do the processing work here
        =09
EncUpdate
                movfw   NewEncCmd
                movwf   CurEncCmd                       ;update present enc positions

ExEncInput
                return
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