There are many reasons to terminate all un-used inputs - some already
stated.

Relate it to a un-plugged air hose on an automobile.  It may have little or
no effect, but it may also cause serious problems or complete failure!  Best
to plug the hose.  In the case of a uP, it is perfectly acceptable to
configure un-used inputs as outputs or enable internal pull-ups (if
available) ASAP in the code.  This can save many parts in some chips.

In my experience, most inputs behave differently in the 'linear' range than
the digital range, even if driven to avoid oscillation (the exception is the
un-buffered inverter, which is usually a single transistor, and I have seen
it used as a linear gain stage).  If power consumption is an issue, this can
add up quickly.  Inputs circuits (including schmidt trigger) draw
considerably more power from the supply when the voltage strays from VDD or
GND.  Using  schmidt inverters with an RC to make an oscillator draws a
surprising amount of current, because the input connected to a capacitor is
always in the linear range.

I once inherited a design where current consumption was intended to be
extremely low (~=60uA), and draw varied by the amount of ambient light
falling of the PCB!  Ends up, an LED was tied to an open-drain output.  When
the LED was off (normal), light level hitting the LED would change the
voltage across it to the OD output (which was also an input, not unusual in
a uP), and take the pin through different areas of the linear region.  The
solution was to pull-up the pin, so it was at VDD when the LED was off (the
LED didn't care).

-----Original Message-----
From: Nicholas Cobb [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, March 28, 2001 6:15 AM
To: [EMAIL PROTECTED]
Subject: [PEDA] Reference


In a post about a month ago Mr. Lomax mentioned that leaving a CMOS input
open could cause problems on a board.  Are there any references that will
help me learn the details that might cause problems like this in commercial
devices?  Up to this point I have been making circuit boards that will be
used only by me.  I am just starting to work on some that will be mass
produced.  Any sort of help would be appreciated.
Thank you,
Nick Cobb

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