>
Marco,  It is the DS18S20, with 3 leads.

For some reason the examples in the book used 2 ports:  Porta 4 to pull down
the line, and Portb x to read the line.  I didn't know about A4 being
special open drain at the time, so I switched the code to B0 & B1, and it
still worked perfectly!  Which was why I asked if another pin could be
substituted for A4.
/>


I'd say it was a programming convenience. It saves a certain amount of
thinking and port manipulation to have one pin driving something and another
reading the same line. This can ALMOST always be achieved with a single pin
and more work. The occasions when it can't be done with one pin would
include occasions where you are pushing the edge of your specification for
timing (and even then cunning tricks may allow it). In the case of using RA4
and some other pin, setting RA4 output high effectively removes it from
consideration as the open drain arrangement has no high side driver and the
pin is thereby made high impedance. If another pin was used and you wished
to remove its influence you would need to set the driving pin to input. If
you wanted as much speed as possible then two pins with one being RA4 may
allow this. As it works for you using other pins they may not be using such
an "optimisation".



        Russell McMahon

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