Wow, this is very nicely done!  I assume you are the author?
Thanks much for the great tool!

Cheers,
-Neil.



-----Original Message-----
From: Nikolai Golovchenko [mailto:nikolai@synaptics.com]
Sent: Monday, May 20, 2002 12:30 PM
To: piclist@mitvma.mit.edu
Cc: picdude@AVN-TECH.COM
Subject: Re:[PIC]: Division by a constant


source=Hi Jinx,

You just have to use this program:

http://www.piclist.com/cgi-bin/constdivmul.exe

:)

Division by a constant is equivalent to a multiplication by the reciprocal
constant, so all you have to do is to specify the multiplication constant
and the error. For example,

http://www.piclist.com/cgi-bin/constdivmul.exe?Acc=ACC&Bits=16&endian=little
&Const=0.00549378103986287&ConstErr=0.05&Temp=TEMP&cpu=pic16

does it in 36 instructions (7.2 us at 20 MHz clock) with the error of 0.01%.
Well, the error means the constant approximation error. That is, 1/182.024 =
0.00549378103986287 ~= 1/256+1/1024+1/2048+1/8192 = 0.0054931640625. But
there is also the rounding error. For a 9 bit result, an off by one number
may mean an error anywhere from 0.2% to infinity :), depending on the value:

error = 1/v*100, v=0..511

I would expect the result occasionally be off by one in that case, which may
be just fine. For example, if the result should be something like 360.01,
but the code produces 359. If this is not fine, try turning on the rounding
option in the code generator.

The code follows. Unfortunately, it seems to have a bug - the 9th bit of
result is lost. To fix it add this at the end:

clrf ACC1
rlcf ACC1, f

Hope it helps,
 Nikolai

; ACC = ACC * 0.00549378
; Temp = TEMP
; ACC size = 16 bits
; Error = 1 %
; Bytes order = little endian
; Round = no

; ALGORITHM:
; Clear accumulator
; Add input / 256 to accumulator
; Add input / 1024 to accumulator
; Add input / 2048 to accumulator
; Add input / 8192 to accumulator
; Move accumulator to result
;
; Approximated constant: 0.00549316, Error: 0.0112305 %

;     Input: ACC0 .. ACC1, 16 bits
;    Output: ACC0 .. ACC1, 9 bits
; Code size: 36 instructions

        cblock
        ACC0
        ACC1
        TEMP0
        TEMP1
        endc

;copy accumulator to temporary
        movf    ACC1, w
        movwf   TEMP1
        movf    ACC0, w
        movwf   TEMP0


;shift accumulator right 2 times
        clrc
        rrf     ACC1, f
        rrf     ACC0, f
        clrc
        rrf     ACC1, f
        rrf     ACC0, f

;add temporary to accumulator
        addwf   ACC0, f
        movf    TEMP1, w
        skpnc
        incfsz  TEMP1, w
        addwf   ACC1, f

;shift accumulator right 1 times
        rrf     ACC1, f
        rrf     ACC0, f

;add temporary to accumulator
        movf    TEMP0, w
        addwf   ACC0, f
        movf    TEMP1, w
        skpnc
        incfsz  TEMP1, w
        addwf   ACC1, f

;shift accumulator right 2 times
        rrf     ACC1, f
        rrf     ACC0, f
        clrc
        rrf     ACC1, f
        rrf     ACC0, f

;add temporary to accumulator
        movf    TEMP0, w
        addwf   ACC0, f
        movf    TEMP1, w
        skpnc
        incfsz  TEMP1, w
        addwf   ACC1, f

;shift accumulator right 8 times
        movf    ACC1, w
        movwf   ACC0

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body