FWIW,

I re-tried the 2-I/O pin and also 1-I/O pin a2d converter again and got
really good results. (The last time I tried a couple of years ago I
failed miserably.) The circuit is this:


                    +-----------> PIC (not used in 1-pin version)
                    |
Vin  o---/\/\/\---+-+-/\/\/\----< PIC
                   |
                  ===
                   |
                  ---
                  ///

The two resistors were 10k and the capacitor was a 0.56uF mylar film. I
didn't experiment much with these values, I basically took some stuff
out of one my junk piles.

I was able to fairly easily get 12-bits of dynamic range with the 2-I/O
pin version and about 8 with the 1-I/O pin version. I haven't attempted
to verify linearity, temperature dependence, long term stability, etc.
My conclusions on the dynamic range are subjectively based on how much
flickering I can observe in the least significant bit. YMMV

If you want to see the code go here:

http://www.interstice.com/~sdattalo/technical/software/pic/a2d.asm

but beware, the code looks almost as bad as my ratsnest prototype :).

The trick I employed this time is a pre-charge algorithm. In other
words, before beginning the actual a2d conversion I make sure that the
voltage on the capacitor is right at the switching threshold of the pic
I/O pin.

Oh, one more thing; I used only a 4Mhz crystal.

and one more thing: The 2 pin version could easily be scaled up for 4
analog inputs. The 1-pin version doesn't scale as nicely, but since it
only has 8-bits of dynamic range it is certainly a candidate for
vertical counters...



Scott


PS, the asin and arctan routines are now on my pic page:

http://www.interstice.com/~sdattalo/technical/software/software.html