I think the original thread was Degrees C to Degrees F.  I needed to
go the other way around, and I needed double precision numbers.  I'm
programming an oven controller that will run from 250F to 500 F
(120C to 260 C).  Someone else posted a nice, simple algorithm that
would only handle numbers up to 128 C - too small.

This algorithm checked OK with 32F, 212F, and 512 F with about 0.1%
or 1 digit  error at the upper end.  It will not work correctly below
32F or above 65,534F (pretty &*^^%&*^%  hot).

here goes:

I'm afraid my email package converts tabs to single spaces - you'll
need to clean this up a bit.


;*********************************************************************
**** ; ;                MACROS
;*********************************************************************
****


;
;******************************************************************* ;
Double Precision Addition ( ACCb + ACCa -> ACCb ) ;                     ; PRESER
VES ACCA
DOUBLEADD       MACRO   ACCALO, ACCAHI, ACCBLO, ACCBHI

     movf ACCaLO,W
    addwf ACCbLO, F ;add lsb
   btfsc STATUS,CARRY ;add in carry
   incf ACCbHI, F
   movf ACCaHI,W
   addwf ACCbHI, F ;add msb
   ENDM
;********************************************************


;******************************************************************* ;
Double Precision Subtraction ( ACCb - ACCa -> ACCb ) ;
; TRASHES
ACCA DOUBLESUB  MACRO   ACCALO, ACCAHI, ACCBLO, ACCBHI

 ; FIRST NEGATE ACCALO:
   comf ACCaLO, F ; negate ACCa ( -ACCa -> ACCa )
   incf ACCaLO, F
   btfsc STATUS,Z_bit
   decf ACCaHI, F
   comf ACCaHI, F
 ; THEN ADD:
 DOUBLEADD      ACCALO,ACCAHI, ACCBLO, ACCBHI
 ENDM

;

;
;*******************************************************************



;*********************************************************************
**** ; ;                SUBROUTINES
;*********************************************************************
****

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;
;               F_2_C
; DOUBLE BYTE DEGREES F TO DEGREES C CONVERTER
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;



F_2_C           ; SUBROUTINE
 ;DEG_F IS A NUMBER BETWEEN 32 AND 65,535 STORED IN
 ; DEG_F AND DEG_FHI
 ;DEG_C IS A NUMBER BETWEEN 0 AND 36,390 STORED IN
 ; DEG_C AND DEG_CHI
 ; WILL NOT WORK FOR DEG_F < 32 OR DEG_C < 0 !!

 ; ALGORITM:  DEG_C = DEG_F/2 + 1/16*DEG_F- 1/128*DEG_F+ 18
 ;                  = .55469F+19
 ;  ABOUT 0.1% ACCURATE OVER 0 TO 512 F RANGE
 ;  (DESIGNED TO BE GOOD FOR AN OVEN CONTROLLER)
 ;  212 F = 100 C   PERFECT
 ;  32 F = 0C       PERFECT
 ;  512 F = 266C    REALLY 266.7C 0.1% OFF


; REMEMBER TO DECLARE THESE VARIABLES:
 ;CBLOCK        <ADDRESS>
 ;      TEMP
 ;      TEMPHI
 ;      DEG_F
 ;      DEG_FHI
 ;      DEG_C
 ;      DEG_CHI
 ;      ENDC

 BCF STATUS, CARRY      ; CLEAR CARRY
 MOVF   DEG_F, W        ; RECALL DEG_F
 MOVWF  TEMP            ; REMEMBER DEG_F
 MOVF   DEG_FHI, W      ; RECALL DEG_FHI
 MOVWF  TEMPHI          ; REMEMBER IT
 BCF    STATUS, CARRY   ; CLEAR CARRY
 RRF    TEMPHI, W       ; TEMPHI/2 =>CARRY
 MOVWF  TEMPHI
 MOVWF  DEG_CHI
 RRF    TEMP, W         ; CARRY => TEMP/2
 MOVWF  TEMP
 MOVWF  DEG_C
 ;  DEG_C NOW = DEG_F/2

 BCF    STATUS, CARRY   ; CLEAR CARRY
 RRF    TEMPHI, F       ; TEMPHI/4 => CARRY
 RRF    TEMP, F         ; TEMP/4
 BCF    STATUS, CARRY   ; CLEAR CARRY
 RRF    TEMPHI, F       ; TEMPHI/8 => CARRY
 RRF    TEMP, F         ; TEMP/8
 BCF    STATUS, CARRY   ; CLEAR CARRY
 RRF    TEMPHI, F       ; TEMPHI/16 => CARRY
 RRF    TEMP, F         ; TEMP/16

 DOUBLEADD      TEMP, TEMPHI, DEG_C, DEG_CHI

 BCF    STATUS, CARRY   ; CLEAR CARRY
 RRF    TEMPHI, F       ; TEMPHI/32 => CARRY
 RRF    TEMP, F         ; TEMP/32
 BCF    STATUS, CARRY   ; CLEAR CARRY
 RRF    TEMPHI, F       ; TEMPHI/64 => CARRY
 RRF    TEMP, F         ; TEMP/64
 BCF    STATUS, CARRY   ; CLEAR CARRY
 RRF    TEMPHI, F       ; TEMPHI/128 => CARRY
 RRF    TEMP, F         ; TEMP/128

 DOUBLESUB      TEMP, TEMPHI, DEG_C, DEG_CHI
 ; REMEMBER DOUBLESUB TRASHES TEMP

 CLRF   TEMPHI
 MOVLW  .18
 MOVWF  TEMP

 DOUBLESUB      TEMP, TEMPHI, DEG_C, DEG_CHI

 ; DEG_C NOW CONTAINS CONVERTED NUMBER

  RETLW 0


;*********************************************************************
******** ;      MAIN ROUTINE ;
;*********************************************************************
********



MAIN
 MOVLW  .1
 MOVWF  DEG_F
 CLRF DEG_FHI


MAINLOOP

 CALL F_2_C


 NOP

 ; DOUBLE PRECISION INCREMENT DEG_F
 INCF   DEG_F, F
 BTFSC  STATUS, Z
 INCF   DEG_FHI, F


 GOTO MAINLOOP










 END
Best Regards,

Lawrence Lile