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'What to do with PINs not used'
1999\09\22@084141 by David Williams

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What should I do with the pins on the PIC that I'm not using?  Should
I leave them open ro tie them to ground?

Dave

1999\09\22@092756 by Marcelo Yamamoto

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Dave wrote


>What should I do with the pins on the PIC that I'm not using?  Should
>I leave them open ro tie them to ground?
>
>Dave
>

You should tie them to VDD or VSS. Pay attention to set them properly as
inputs.
Marcelo

1999\09\22@094024 by Harrison Cooper

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Why does this come up every 6 months or so???

NO!!!  Don't tie them to anything!  Define them as OUTPUTS, and set them to
default as low.

Why?  Because, these are I/O pins.  Unless its a dedicated input pin, don't
tie it because at somepoint two things might happen.  First, an unprogrammed
chip might be put into the circuit, and I don't recall if by default the
pins are input or outputs.  If tied either way, they could might drive Vcc
to Vss, or the other way around.

Second, if you need to use that pin for something else down the road, you
have to then isolate it first and wire it up.

Most programmable logic devices define unused pins as no connects, as they
tristate or isolate the pins when they burn them.

Just my .02 worth.  Everyone has their own ideas on what to do....

1999\09\22@095904 by David Covick

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Harrison,

What do I do with the statement in the Microchip data sheet that says "with
all I/O pins in hi-impedance state and tied to Vdd or Vss"??

David



> Why does this come up every 6 months or so???
>
> NO!!!  Don't tie them to anything!  Define them as OUTPUTS, and set them
to
> default as low.
>
> Why?  Because, these are I/O pins.  Unless its a dedicated input pin,
don't
> tie it because at somepoint two things might happen.  First, an
unprogrammed
{Quote hidden}

1999\09\22@100528 by Harrison Cooper

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I think its taken out of context....when they say that, its for testing
conditions I believe.  For worst case power consumption's and such, or to
measure prop delays, they must state those conditions such that a designer
could duplicate it.

What page of the data sheet did you read that on?

{Original Message removed}

1999\09\22@100738 by John Hansen

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<x-flowed>At 06:55 AM 9/22/99 -0700, you wrote:
>Harrison,
>
>What do I do with the statement in the Microchip data sheet that says "with
>all I/O pins in hi-impedance state and tied to Vdd or Vss"??
>
>David
>
>
>
> > Why does this come up every 6 months or so???
> >

This is why this comes up every six months or so.

John

</x-flowed>

1999\09\22@101806 by David Covick

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PIC12C5XX, page 80, note 5.

I am working on a low power battery project, and it so happens this topic
has peaked my interest.....so maybe, the "application" is what dictates how
the I/O and pins are configured.  Could this be the confusion?




> I think its taken out of context....when they say that, its for testing
> conditions I believe.  For worst case power consumption's and such, or to
> measure prop delays, they must state those conditions such that a designer
> could duplicate it.
>
> What page of the data sheet did you read that on?
>
> {Original Message removed}

1999\09\22@103501 by bill

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The safest thing to do is to tie all unused pins to either ground or the positiv
e supply voltage using resistors, and leave them tri-stated.  Anything
from about 1K to about 10K will work fine.  This way, if the pins get set to an
unintended state (due to software bugs or electrical noise or
whatever), the chip will not be damaged because the current will be limited by t
he resistor.  You could get rid of the resistors by just leaving the
pins unconnected and set as outputs, but this leaves the possibility of the pins
floating while the chip is initializing, or in a fault situation in which
the pins are tristated unintentionally.

The important thing is to avoid having a tri-stated (input) pin floating, and to
avoid having an output pin driven against an external connection so
that it exceeds its rated source/sink capability (or the combined source/sink ca
pability of the chip).  Pull up or pull down resistors eliminate both
possibilities in all conditions.  The worst than can happen is accidentally driv
ing an output against a pull up/down and thus wasting power.

> What should I do with the pins on the PIC that I'm not using?  Should
> I leave them open ro tie them to ground?
>
> Dave
>


---
                                       Peace,
                                       William Kitchen
                                       spam_OUTbillTakeThisOuTspamiglobal.net

The future is ours to create.

1999\09\22@132702 by Byron A Jeff

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>
> What should I do with the pins on the PIC that I'm not using?  Should
> I leave them open ro tie them to ground?

Leave them open and program them as outputs.

BAJ

1999\09\22@151413 by Barry King

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>From:                  "William J. Kitchen" <.....billKILLspamspam@spam@iglobal.net>
> The safest thing to do is to tie all unused pins to either ground or the posit
ive supply voltage using resistors

William is right.  That's the safest thing to do.

In addition to the safety against misprogrammed TRIS bits, there are
are two other advantages to setting them as inputs and tieing them
low.

First, at reset, the pin WILL be an input until you program it
otherwise.  I know, setting up TRIS is the first thing you do in your
program, but how long is the chip in reset from Vdd on until the
program runs?  Power up timer, etc, adds up.  During that time, the
pin condition is undefined.  If something (stray capacitance, dirty
PCB, "cosmic rays", etc...) tends to pull the pin into the invalid
voltage range, the input buffer will draw excessive current for some
time.  If you reset the chip a lot in your application, and or its a
battery powered system, this could add up to shorter battery life.
Plus, its icky :).

Second, the current consumption for an enabled output driver even
with no load, may be slightly higher than an input tied low.  (Due to
leakage in the large driver FETs, I think.)  Emphasis on *slightly*.
I'm not sure the difference is significant at room temperature, but
might be more so across the temperature range.  The Microchip
specifications for shutdown power consumption are for the case
where all I/Os are set as inputs and tied to Vdd or Vss.  This gives
the lowest possible quiescent current.

For experimental boards, I leave unused pins unconnected and
program then as outputs, set low, so its easy to prototype new hair-
brained circuits.

But in production boards, I set them as inputs and tie them low (via
resistor packs) if possible.  If board space or cost are so critical
you can't afford the resistors, then leave them as outputs set low,
subject to the (probably slight) disadvantages.

------------
Barry King, KA1NLH
Engineering Manager
NRG Systems "Measuring the Wind's Energy"
Hinesburg, Vermont, USA
http://www.nrgsystems.com

------- End of forwarded message -------

1999\09\23@160727 by paulb

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William J. Kitchen wrote:

> You could get rid of the resistors by just leaving the pins
> unconnected and set as outputs, but this leaves the possibility of the
> pins floating while the chip is initializing,

 This is the prototypical example of muddled thinking.  So the pins
float for a millisecond during initialization.  So what?  We are talking
about currents of perhaps hundreds of microamps (per input) *at most*
here, if it really mattered more, then *all transitions* would be
prohibited!  That is absurd.

 The situation is: An input in the "switching" voltage range *whether
"floating" or biassed deliberately by the application circuit* causes
some conduction of both input FETs, resulting in a current from rail to
rail (temporary class A operation - class A amplification always has a
quiescent current).  The chip is designed to tolerate this current, but
it represents wasted current in battery-powered applications.

 There has been a suggestion that inputs "floating" in this range may
cause faulty operation.  Think carefully!  If this were so, then *none*
of those nifty time-constant circuits to measure analog voltages or
capacitance would be possible.  There is *one* caveat to this - it *may*
cause problems if you are using the ADC.  That's all.

> or in a fault situation in which the pins are tristated
> unintentionally.

 Oh dear, what a nasty possibility!  *However*, nowhere near as nasty
as the equally probable exact opposite - a pin accidentally turned into
an output whilst tied directly to rail.  Even a 10 kilohm resistor will
I suspect, draw more "leakage" current than an input held at ¸Vcc.

 In summary, leave them open, initialize them as outputs, use shadow
registers, rest easy.  If the TRIS registers subsequently get scrambled
despite all this, well, tough luck, just be glad they weren't tied to a
rail.

 Here endeth the FAQ.
--
 Cheers,
       Paul B.

1999\09\23@175452 by paulb

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part 0 44 bytes content-type:text/plain; charset="us-ascii" (decoded 7bit)

REMOVE


1999\09\23@182442 by Sean H. Breheny

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hahaha. How about

EXPUNGE PUKELIST

or better yet

EXPECTORATE PUKELIST

:-)


Sean

At 07:52 AM 9/24/99 +1000, you wrote:
>Now *that's* an option I hadn't considered!
>--
>  Cheers,
>        Paul B.X-POP3-Rcpt: paulb@ecpport2
[SNIP]
>
>REMOVE
>
>
|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
Save lives, please look at http://www.all.org
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