> If the four samples were
> A B
> 0  0 (oldest state)
> 0  1 (older state)
> 1  1 (last state)
> 0  0  (newest state)
> 
> Then this last state change from 1  1 to 0  0 could be 
> counted as 2, rather than thrown away as an illegal change.  
> This would take a 64 or
> 256 entry table, rather than 16 entries, but it would be just as fast.
>  Faster, even, since I wouldn't have to clear the top four 
> bits every time I got a new sample.

You could also record the direction as a simple 0 or one in the 5th bit
(over the top of the "older state" lsb) and then use a 32 entry table. Your
example above would become

 A B
 x  1 (oldest direction)
 x  1 (older direction)
 1  1 (last state)
 0  0  (newest state)

Or you could use a 16 entry table which returns an "invalid" value. When
"invalid" is returned, it causes the direction bit to be used to load a 2 or
-2 as appropriate. If you pick the invalid value to be 2 then a bit test and
negate are all that is required.

Actually... 

EncWait:
	mov w,Enc		;Port "Enc" 2 lsb's = encoder pins
	and w,#%00000011  ;get rid of all but 2 enc bits
	mov temp,w		;save the new reading
	xor w, old		;compare it to the old
	snz			;Any change?
	 jp EncWait  	; if not, go back and wait
	xor w,#%00000011
	snz
	 jp EncErr		;
;Table? TABLE!?!? We don't need no stinken table!
	clc			;get ready for right shift
	rl  old		;align bits
	xor old,temp	;check for direction and save new with old
EncCount
	jb  old.1,EncUp	;The second to lsb: 1=up direction, 0=down
EncDown:
	dec Pos		;we be going down: -1
	ret			;return so action is taken
EncUp:
	inc Pos		;we be mov'n on up: +1
      ret			;return so our caller can take action
Err:
	call EncCount	;just leave the direction as is and take action
	jp EncCount		;  ...twice

Sorry, parallax mnemonics. Based on code from Matthew Ballinger at
http://www.piclist.com/techref/microchip/qenc.htm

Old	Enc	old'
0000	0 0	0000
0000	0 1	0001
0001	1 1	0001	;note: old was rotated left before xor
0001	0 0	0001
0001	0 0   0000	;note: old.0 zeros out on first no change.
0001  1 0	0010	
0010  1 1	0111	


Old.1 (the lsb of Old) holds the direction at all times. The rotating old
left one bit and xoring with the new encoder reading is weird, but just
happens to work. (I'm pretty sure)

---
James.
 

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