UP_BIT:
   DBIT  1                     ; bit for UP marker
DN_BIT:
   DBIT  1                     ; bit for DN marker

   ...
   ...

   MOV   A, ENC_PREV           ; look for bit 5 0->1
   CPL   A
   ANL   A, ENC_CURR
   JNB   ACC.5, FILT_DONE      ; no P3.5 edge
   MOV   A, ENC_CURR           ; test current bit 6
   JB    ACC.6, FILT_UP
   SETB  DN_BIT                ; show down to main code
   JMP   FILT_DONE
;
FILT_UP:
   SETB  UP_BIT                ; show up to main code
;  
FILT_DONE:
   ...
   ...