Several weeks ago I came across an interesting tip that allowed you to
get trinary output from a single PIC pin.  I couldn't think of a use
for this at the time, but filed the idea away...

I'm currently flushing out some ideas I have for a project that will
require several H-bridges (somewhere between 4 and 8) and the trinary
output idea popped back into my head -- but now I cannot locate the
details.

I believe the rough idea was that you switched the pin to output mode
which would allow you to source or sink current.  When the pin was
switched to input mode, current was neither sourced nor sinked.

This seems like it might be a good way to control an H-bridge.  Not
only would it allow forward/reverse/stop control from a single pin,
but it provides a physical fail-safe against shorting out the bridge.
(If two pins were used, a software error could enable both forward and
reverse thereby shorting out the power supply and releasing lots of
magic smoke!)

I was wondering if anyone has done this before, though even a simple
reference on trinary outputs would help.  I believe my particular case
may be a little more complicated, since I want to build my H-Bridges
from discrete MOS components (unless someone can recomend a less
expensive MOS bridge) and I'll believe I'll need a current-controlled
device (bipolar transistor) to drive the power MOSFETS.  If anyone
happens to have any elegant drive solutions here, I'd love to hear of
them!  

-p.
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