> Van: Dave & Ann Scott <andscott@LOGANTELE.COM>
> Aan: PICLIST@MITVMA.MIT.EDU
> Onderwerp: False HI on Pin, Trying to use pin for both input & output
> Datum: vrijdag 13 augustus 1999 19:37
>
> I'm trying to use one I/O for both input and output (RA1 on a 16C84).

<Snip>

> Hardware - The pin has a 470 ohm resistor and (Smurf-) blue LED to
ground.
> Also a pushbutton switch and 10k resistor to V+.
>
> What should happen:
> If pushbutton not pressed - LED should just blink every 8 sec.
> If pushbutton is pressed - start other routine.
>
> What does happen:  Other routine starts by itself.  Even if 10k resistor
is
> pulled out of the circuit and circuit left open (so pushbutton is not
> malfunctioning).

Hello Dave,

  You are testing the the I/O while the LED is Off.  This means that the
I/O line is floating (or rather High, due to the Chip-design).  Reason ?
Think of a LED as a Zener-diode.  If it's ON, current flows.  If it's OFF
allmost _NO_ current flows (leakage only !), so it can't be used as a
pull-down.   Solution :  Put a resistor parallel to the LED.

How come I know this ?  Simple:  I stepped in the same pit-fall you did :-)

Greetz,
  Rudy Wieser