In SX Microcontrollers, SX/B Compiler and SX-Key Tool, Johnson Rod wrote:

hail
the project:
in a continous rotation drive one encoder returns command (where do i want the drive to point), another returns feedback (where the drive is pointing). the controller compares the 2 encoders w/ a simple algorithm and turns the drive so that the feedback matches the control. 

the algorithm: 
if command = feedback then dont move
if command - feedback < feedback - command then turn clockwise
if command - feedback > feedback - command then turn counterclockwise

the trouble:
the encoders have 512 bits of resolution, to simplify the project i ignored the lsb and voila. the algorithm applies directly to the sx because the byte sized variable wraps around itself. that is 
1 - 255 = 1 
255 - 1 = 254
so go clockwise.

but i think i can do better! i can do this with 9 bits.... ok i cant because i'm stuck on this snippet
; btw testcw = cmd 
mov   w, feedback0			; testcw = cmd - fbk	
sub     testcw0, w
mov    w, feedback1
sc
movsz w, ++feedback1
sub     testcw1, w


the result is true 16bit and wraps around 65535, but i need it to wrap around 511. my previous sub wrapping script would dec the variable in a loop and detect for overflow. but thats way to slow.

does anyone have any experiance with this?
any help would be well received.

a attached 2 files, 
apec256 is the working 8b program and is well commented
apec512 is the partial program in question and is poorly commented

future thanks for your generous reply
nickB

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