----------
> From: Norm Cramer <cramer@DSEG.TI.COM>
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Best way to read a POT without the A/D?
> Date: Wednesday, January 07, 1998 11:42 AM
>
> What is the best way to read a POT without using an A/D converter?  I
need
> to generate values between 0 and 128 from the reading.  It also needs to
be
> repeatable i.e. when the POT is not moved, the same number is generated.
> The time limit for reading is not too critical, maybe 250 msec or less.
> Idealy it should not need "calibration" so that multiple units would
behave
> the same.
>
> What I am currently doing is the method shown at:
> http://www.geocities.com/SiliconValley/2499/answers.html#PIC00065
> and measuring the charge time.  The times seem to vary quite a bit when
the
> pot is in the same position.
>
> Is there a better way to get more stable readings?


           --------------|   Output Bit
          |                 |
         /  Rh            |
         \                  |
          |--------------|   Vin Sense bit
         /         |        |
         \  Rl === C   |
          |        |        |
           -------
              |
            GND


    Vin the sense bit is at the threshold of the gate going into the
    micro controller. Rh and Rl are the two resistors parts of the POT
    The output bit is PWM by software to act like a variable voltage
    for the resistor chain.
                 Rl                  Th
   Vin = -------------- *  ------------ * K
             Rl  + Rh         Th  + Tl

   "Th"  is the high time of the output bit and "Tl" is the low time of the

   output bit. The software to measure the pot position toggles the
   output bit to keep the Vin at the gate threshold. Solving the above
   equation will show that there is a direct relationship between the
   software PWM times and the pot position.

   Practical implementations will have two fixed value resistors on
   each end of the POT so the POT will operate over its entire range.
   The C value is non critical. It is only used for filtering.


Walter Bankls