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Ok David

 You can connect the +5V instead of GND, so sure that have 0V while the
button don«t press.
 I don«t know of military applications, but i think that you must be
utilized a debounce routine for prevent missing signal.

Francisco

David Olson wrote:
> 
> Francisco, thanks for the reply,
> 
> OK. Does your schematic show R1 in addition to the resistor network? Are you
> saying that I should limit the current running up to (and through) the
> switch as well as a resistor in series with the pin? If I interpret your
> ASCII schematic (which I'm terrible at) this is pulling the pin low when the
> switch is closed. I want to push the pin high when the switch is closed.
> Because my switches are located at the end of a cable where there can be
> chafing and other wire nastiness, I need the pin to be low until told
> otherwise.
> 
> Here's the situation: Let's say my PIC controls the firing of missiles and
> the button to do so is located remotely. If I get a short to ground, I don't
> want a missile to fire. I've got some safeguards in my code but, I'm
> thinking that I'll keep the low until forced high is the right approach. The
> most common short condition that I think I'll have is a short to ground and
> not two switch wires crossed. This also means that I've got 1 5v wire and 3
> wires to the pins instead of 3 5v wires to the pins and a ground.
> 
> Then again, I could be wrong. Good thing this isn't a missile firing
> application ;)
> 
> -DO
> 
> > -----Original Message-----
> > From: pic microcontroller discussion list
> > [mailto:PICLIST@mitvma.mit.edu]On Behalf Of Francisco Armenta
> > Sent: Tuesday, January 25, 2000 11:52 AM
> > To: PICLIST@mitvma.mit.edu
> > Subject: Re: [OT] Pulldown size
> >
> >
> > David the pin can sink 25 ma, but you must be limited the current to
> > less that 25 ma (whit a series resistor).
> >
> > example                    +5V
> >                             |
> >                             < RNWK
> >                      R1     >
> > pic pin ___--------/\/\/\---|---|
> >                                 |
> >                                   |_ SWITCH
> >                                   |
> >                                 |
> >                                 |
> >                                 _ (GND)
> >
> >  Francisco
> > David Olson wrote:
> > >
> > > I've got some switches externally located off my PIC board
> > (about 2 feet)
> > > and I'm purposely driving my port pins high when the switches are closed
> > > (due to the fact that I don't want a short to give me an
> > unwanted trigger)
> > > and I'm not sure how to size the pulldown resistors. I'm
> > thinking of using a
> > > resistor network since I need 8 resistors. Most of the networks
> > are or about
> > > 1/16 or less in wattage. If a pin can sink 25ma, is that what I need to
> > > consider in my power calculations? 5v X .025 is close to 1/8 watt.
> > >
> > > I'm locating the resistors on the PIC board and not with the switches.
> > >
> > > -DO
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