Hi,

   recently someone asked if it was possible to use the fact that PIC pins
can be tri-stated to produce a different kind of DAC. (Using ideal
components of coures.)  The answer is that it requires an inverting op-amp
with the +ve input at Vsupply/2.

If you then use a R-3R-9R-27R network to sink and source current it is
possible to get a reasonably linear response. (depending on how little
current the tri-state input sinks) Each output can occupy three states
Hi,Lo, or Tri. Here are the first few codes. Generating the remainder is
left as an exercise for the three-fingered student from Sirius.

        MSB     2      3        LSB
        (R)     (3R)   (9R)     (27R)

0       Hi      Hi      Hi      Hi
1       Hi      Hi      Hi      Tri
2       Hi      Hi      Hi      Lo
3       Hi      Hi      Tri     Hi
4       Hi      Hi      Tri     Tri
5       Hi      Hi      Tri     Lo
6       Hi      Hi      Lo      Hi
7       Hi      Hi      Lo      Tri
8       Hi      Hi      Lo      Lo
9       Hi      Tri     Hi      Hi

It's probably not worth generating these codes from scratch, just use an 81
byte lookup table with one nibble containing the Port bits and the other the
Tris bits.

Have fun,

Keith.

P.S. The three fingered alien came from Sirius because everyone knows the
little green men come from Alpha Centauri ;-)

==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: kdowsett@rpms.ac.uk
   WWW: http://kd.rpms.ac.uk/index.html