Aaron Hickman <hickmaab@DUNX1.OCS.DREXEL.EDU> wrote:
>To all,
>
>        My previous message might have been unclear as stated.  What I am
>looking for is a method of converting a 16 bit ADC value into an
>appropriate voltage value (ie. 20 V * (ADC value/65535).  I feel certain
>that I need to use floating point methods, but have little experience in
>using them.  Can anyone help?


Aaron,

   First of all, the maximum input to your A/D is probably equal
   to the reference voltage, typically 2.5 or 5.  Second, your
   question implies a range of 20 volts.

   You will need a voltge divider with a divisor of F.  For example,
   if you used 10K series and 75K shunt, F = (10+47)/10 = 8.5

   ;The input in millivolts to the a/d for a 20V input is:
   ; 10 * 20000 / 85.  Note X10 in numerator and denominator
   ;Change the divisor here if the resistor voltage divider changes.

max_a2d_input    equ 10 * 20000 / 85   ; in mv

   ;Assuming a count of 65535 (16 bit converter) for 2500 mv (Vref) in:

count_for_20     equ 2500 * 65535 / max_a2d_input

counts_per_volt  equ       count_for_20 / 20

Now, given an A/D count, the voltage is   count/counts_per_volt

Using the above numbers:

mas_a2d_input =  2.35 mv
count_for_20 = 27887
counts_per_volt = 1394

Most of these calculations are done at compile time.  To convert an
A/D count to volts you only need one division which does not have to
be floating point.  The following routine will do the trick:

;*******************************************************************
;  Division:  Divide 16 bits by 16 bits,
;             16 bit quotient and remainder.
;  Standard shift and subtract algorithm.
;
;  q_2:q_1 / n_2:n_1 -> q_2:q_1   Remainder in t_2:t_1
;
;  Uses count
;
;  Execution time: 271 to 361 clock cycles
;  Program memory: 27
;
;  (This is a cleaned up version of the routine in AN526)
;
;*******************************************************************

div16b16: macro
   local  loop, check_sign, check_count
   movlw   16
   movwf   count
   clrf    t_2
   clrf    t_1
div16_loop:
   rlf     q_1, f
   rlf     q_2, f
   rlf     t_1, f
   rlf     t_2, f
   movf    n_2, w
   subwf   t_2, w
   skpz
   goto    check_sign
   movf    n_1,w
   subwf   t_1,w
check_sign:
   skpc
   goto    check_count
   movf    n_1,w
   subwf   t_1, f
   skpc
   decf    t_2, f
   movf    n_2,w
   subwf   t_2, f
   setc
check_count:
   decfsz  count, f
   goto    loop
   rlf     q_1, f
   rlf     q_2, f

   endm




--
Bob Fehrenbach    Wauwatosa, WI     bfehrenb@execpc.com