Searching \ for '[PIC]: Unused pins' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/microchip/devices.htm?key=pic
Search entire site for: 'Unused pins'.

Exact match. Not showing close matches.
PICList Thread
'[PIC]: Unused pins'
2002\08\28@192537 by Brendan Moran

flavicon
face
Just wondering what you folks consider standard procedure for
handling unused PIC pins.  I can see two decent options, myself.  1st
is to leave them as inputs, and ground them.  2nd is to set them to
outputs, and leave them disconnected.  Both should use minimal power, but I
don't know the other pros and cons related to the option.

--Brendan

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email spam_OUTlistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@193440 by Byron A Jeff

face picon face
On Wed, Aug 28, 2002 at 04:23:43PM -0700, Brendan Moran wrote:
> Just wondering what you folks consider standard procedure for
> handling unused PIC pins.  I can see two decent options, myself.  1st
> is to leave them as inputs, and ground them.  2nd is to set them to
> outputs, and leave them disconnected.  Both should use minimal power, but I
> don't know the other pros and cons related to the option.

You've pretty much described it. Setting to outputs works just as well and
requires less hardware. Also it has less of chance of screwing up if its
TRIS bit flips. Consider if you have a grounded input and it flips to a high
output because of a glitch. You now have one burned output port. However if
a disconnected output flips to an input, You'll probably be OK.

Just some thoughts.

BAJ

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@193656 by Sid Weaver

picon face
In a message dated 08/28/2002 19:26:44 Eastern Daylight Time,
bmoranspamKILLspamMILLENNIUM.CA writes:


> Just wondering what you folks consider standard procedure for
> handling unused PIC pins.  I can see two decent options, myself.  1st
> is to leave them as inputs, and ground them.  2nd is to set them to
> outputs, and leave them disconnected.  Both should use minimal power, but I
> don't know the other pros and cons related to the option.
>
> --Brendan
>

Brendan, I simply ignore unused pins.
Sid Weaver - W4EKQ

< < < <
Remember - Fumpus tegit .

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email .....listservKILLspamspam.....mitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@194537 by Brendan Moran

flavicon
face
> Brendan, I simply ignore unused pins.
> Sid Weaver - W4EKQ

That's what I *have* been doing, but since I'm committing this project to a
PCB, I want it to be done right.  And I know that if the pins are ignored,
then there could be some issues with power consumption. CMOS is worse than
TTL for this.

--Brendan

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email EraseMElistservspam_OUTspamTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@194833 by Tony Nixon

flavicon
picon face
The problem with grounding directly, is if something goes wrong and
changes them to outputs at the oppostie polarity.

The cheapest is to set them as outputs, but again, if something goes
wrong and sets them as inputs, then they float and "may" cause problems.

The best (??) is to set them as low outputs with pull down resistors, or
high with pullups.  If something goes wrong, only a slight increase in
power consumption occurs.



Brendan Moran wrote:
{Quote hidden}

--
Best regards

Tony

mICros
http://www.bubblesoftonline.com
@spam@salesKILLspamspambubblesoftonline.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email KILLspamlistservKILLspamspammitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@195234 by David Duffy

flavicon
face
At 04:44 PM 28/08/2002 -0700, you wrote:
> > Brendan, I simply ignore unused pins.
> > Sid Weaver - W4EKQ

Not a good idea! Floating inputs will cause havoc given half a chance.
You can do without the increased input current and glitches. :-)

>That's what I *have* been doing, but since I'm committing this project to a
>PCB, I want it to be done right.  And I know that if the pins are ignored,
>then there could be some issues with power consumption. CMOS is worse than
>TTL for this.

As another poster said, set them as output and leave the not connected.
If you really want, you can leave the mas input and use pull-up/down resistors
but that's really a waste of space & money most of the time.
Regards...
___________________________________________
David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: http://www.audiovisualdevices.com.au
___________________________________________

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email RemoveMElistservTakeThisOuTspammitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@195441 by Tony Nixon

flavicon
picon face
Sid Weaver wrote:

> Brendan, I simply ignore unused pins.

Problems caused by ignoring unused pins will not go away. They will
stick around and haunt you. Might be fine one day, next day ........

Hard to diagnose the problems too if you are unaware of a floating pin.

--
Best regards

Tony

mICros
http://www.bubblesoftonline.com
spamBeGonesalesspamBeGonespambubblesoftonline.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email TakeThisOuTlistservEraseMEspamspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@200144 by Sid Weaver
picon face
In a message dated 08/28/2002 19:46:38 Eastern Daylight Time,
RemoveMEbmoranspamTakeThisOuTMILLENNIUM.CA writes:


> That's what I *have* been doing, but since I'm committing this project to a
> PCB, I want it to be done right.  And I know that if the pins are ignored,
> then there could be some issues with power consumption. CMOS is worse than
> TTL for this.
>

Brendan, I've made a lot of PC boards where I ignored the unused pins.  Power
consumption was really never an issue because I was always running of a wall
wart.  Even so, I can't imagine that the power consumed by an unused pin
would be significant.  And I have followed the same procedure with the
"other" micro-processor.

Sid

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listservEraseMEspam.....mitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@202427 by David Duffy

flavicon
face
At 08:00 PM 28/08/2002 -0400, you wrote:
>In a message dated 08/28/2002 19:46:38 Eastern Daylight Time,
>EraseMEbmoranspamMILLENNIUM.CA writes:
>
> > That's what I *have* been doing, but since I'm committing this project to a
> > PCB, I want it to be done right.  And I know that if the pins are ignored,
> > then there could be some issues with power consumption. CMOS is worse than
> > TTL for this.
> >
>
>Brendan, I've made a lot of PC boards where I ignored the unused pins.  Power
>consumption was really never an issue because I was always running of a wall
>wart.  Even so, I can't imagine that the power consumed by an unused pin
>would be significant.  And I have followed the same procedure with the
>"other" micro-processor.

Hi Sid,
It's not the power consumption as such, but the whole chip can oscillate and
behave badly when you let inputs float. It's just like leaving unused inputs on
CMOS gates floating - just asking for trouble. They (CMOS) have a habit of
latching up, heating up and dying. The increased power consumption is just a
symptom of the problem and should not be ignored.
Regards...
___________________________________________
David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: http://www.audiovisualdevices.com.au
___________________________________________

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email RemoveMElistservEraseMEspamEraseMEmitvma.mit.edu with SET PICList DIGEST in the body


2002\08\28@210747 by Jim

flavicon
face
I would at least attach the pin to a
pad - where a resistor or a jumper
could be placed optionally at build
(board-stuff) time ...

RF Jim

{Original Message removed}

2002\08\28@210908 by Leonard Gabrielson

picon face
Hi,

Oscillation is not a problem if you simply make them outputs and connect
them to nothing.  I've been doing it for years.

Len
{Original Message removed}

2002\08\28@235846 by David Duffy

flavicon
face
Len wrote:
>Oscillation is not a problem if you simply make them outputs and connect
>them to nothing.  I've been doing it for years.

That's right. The oscillation or such occurs when the pin floats (when input).
It can be biased into its linear region, which makes it draw excessive current
and generally becoming unhappy. (This is what I said the first time isn't it?)
OK, I think we have this clear for the OP now! Time to get back to work. :-)

<snip>
> > Hi Sid,
> > It's not the power consumption as such, but the whole chip can oscillate
> > and behave badly when you let inputs float. It's just like leaving unused
> > inputs on
> > CMOS gates floating - just asking for trouble. They (CMOS) have a habit of
> > latching up, heating up and dying. The increased power consumption is just
> >a symptom of the problem and should not be ignored.

___________________________________________
David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: http://www.audiovisualdevices.com.au
___________________________________________

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email RemoveMElistservspam_OUTspamKILLspammitvma.mit.edu with SET PICList DIGEST in the body


2002\08\29@074933 by Olin Lathrop

face picon face
> Just wondering what you folks consider standard procedure for
> handling unused PIC pins.

I set them as outputs driving low and leave them disconnected because:

1  -  It is easier to make a manual change to use a pin if it is not
connected to anything.

2  -  A firmware bug won't accidentally cause a lot of current draw thru the
pin.

3  -  Driving low by default is better than driving high because this works
for open drain outputs too.

My PIC assembler preprocessor and the PORT module automatically set all
unused pins as outputs driving low.  See the /INBIT and /OUTBIT directives
in PREPIC and the PORT module at http://www.embedinc.com/pic.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
RemoveMEpiclist-unsubscribe-requestTakeThisOuTspamspammitvma.mit.edu


2002\08\29@075347 by Olin Lathrop

face picon face
> Brendan, I simply ignore unused pins.

Bad idea.

CMOS inputs are very high impedence and can pick up noise if not driven.
Noise on an input will increase overall current draw from the extra clock
edges, and can be worse if the input is held in the mid region.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspamspamspamBeGonemitvma.mit.edu


2002\08\29@080010 by Olin Lathrop

face picon face
From: "Sid Weaver" <RemoveMENEWZEDKILLspamspamAOL.COM>
> Brendan, I've made a lot of PC boards where I ignored the unused pins.
Power
> consumption was really never an issue because I was always running of a
wall
> wart.  Even so, I can't imagine that the power consumed by an unused pin
> would be significant.  And I have followed the same procedure with the
> "other" micro-processor.

Sooner or later you'll get bitten.  Yes, you can get away with floating
inputs most of the time.  If you don't want to learn from everyone here that
you shouldn't leave inputs floating, that's your business, but please stop
advising others to do so.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
piclist-unsubscribe-requestSTOPspamspamspam_OUTmitvma.mit.edu


2002\08\29@082927 by Sid Weaver

picon face
In a message dated 08/28/2002 20:25:56 Eastern Daylight Time,
spamBeGonedavidSTOPspamspamEraseMEAUDIOVISUALDEVICES.COM.AU writes:


> Hi Sid,
> It's not the power consumption as such, but the whole chip can oscillate and
> behave badly when you let inputs float. It's just like leaving unused
> inputs on
> CMOS gates floating - just asking for trouble. They (CMOS) have a habit of
> latching up, heating up and dying. The increased power consumption is just a
> symptom of the problem and should not be ignored.
> Regards...
>

For David

When I said I ignore them, what I really meant was to set them as outputs
then forget about them.  In a finished PC you you will probably never need
them, but it is nice to have an uncluttered pin available "just in case".
There have been times in the past when a wire jumper and a small program
change saved a complete circuit.

Ever been to Cook?

Cheers

Sid

--
http://www.piclist.com hint: To leave the PICList
KILLspampiclist-unsubscribe-requestspamBeGonespammitvma.mit.edu


2002\08\29@104324 by Ken Crismon

flavicon
face
David,

A newbie addition to this question.  What should the unused pins be "tied" to.  Ground???

Thanks,

Ken Crismon

{Original Message removed}

2002\08\29@104600 by 4HAZ

flavicon
face
My $0.02
I am working with a single pcb layout at present that covers a line of
products, minor differences in components and microcontroller code and what
is externally connected and viola a new and different product.
I personally leave the unused pins programmed as inputs / outputs depending
on their normal usage, the unused inputs are either tied high or low via a
10k or so resistor.
For example one of the inputs normally comes from another device whos output
is an npn open collector configuration, the pull-up resistor on my pcb will
prevent oscilation even if that function is not being used.
The important thing to remember here is not to directly ground any "unused"
pin.

Lonnie - KF4HAZ -

--
http://www.piclist.com hint: To leave the PICList
EraseMEpiclist-unsubscribe-requestspamEraseMEmitvma.mit.edu


2002\08\29@162224 by Russell McMahon

face
flavicon
face
Write this in your "Things must know about microprocessors" book.

       **** Ignoring unused pins is bad engineering. ****

It's asking for trouble.
Sooner or later you will be sorry.
You may already be sorry and wonder why :-)
This applies to almost every microprocessor - it's NOT a PIC specific
problem.

The only situation where it wouldn't matter (much) would be if the default
port setting was inputs with pullups enabled.

If an input floats it will likely spend a significant part of its time near
the middle of the voltage range. Near is a relative term depending somewhat
on processor. Unless you input is Schmitt triggered, having it float mid
supply will do BAD things. A Schmitt will draw less extra current (but still
more than normal). Regardless, it will not only draw higher current but MAY
noise up A2Ds, make the processor act funny like, create EMI and more. Often
it mightn't. Sometimes it will. Don't do it.

Choices in approximate order of decreasing goodness (IMHO) BUT this will
vary with your level of discipline and documentation.

1 Input with internal pullup.
2 Output set high or low.
3 Input with external pull up/down
4 Input tied to ground.supply

1 is zero cost but can be destroyed by software change.

2 is zero cost but has a pin doing something - not usually a problem unless
someone equates unused with don care.

3 Costs the pull up/down but is well defined.
This is the safest option but annoying.

4 Is free BUT can cause bad problems if software is changed.

       Russell McMahon

--
http://www.piclist.com hint: To leave the PICList
@spam@piclist-unsubscribe-request@spam@spamspam_OUTmitvma.mit.edu


2002\08\29@163652 by lexandre_Guimar=E3es?=

flavicon
face
Hi,

> 4 Input tied to ground.supply

> 4 Is free BUT can cause bad problems if software is changed.

   I would NEVER do that ! I have seen a product ( Thanks God it was not
mine ! ) from a client that just blew 200 boards in a cold month because of
that !!!! It was poorly designed in every way I can think of and was very
sensitive to EMI. The ports changed state from input to output and the chips
just blew apart. I agree that if the other aspects of the design were better
thought that should not happen often but I can't see any advantages of tying
the pin to ground !!

   Pull-ups or down are the best if you can afford it, if not just leave
the ports as outputs. There is nothing to be gained by relying on the
internal pullups or tying the pin to ground or VCC.

Best regards,
Alexandre Guimaraes

--
http://www.piclist.com hint: To leave the PICList
spamBeGonepiclist-unsubscribe-requestspamKILLspammitvma.mit.edu


2002\08\29@164054 by mike

flavicon
face
On Thu, 29 Aug 2002 17:37:21 -0300, you wrote:

>Hi,
>
>> 4 Input tied to ground.supply
>
>> 4 Is free BUT can cause bad problems if software is changed.
>
>    I would NEVER do that ! I have seen a product ( Thanks God it was not
>mine ! ) from a client that just blew 200 boards in a cold month because of
>that !!!! It was poorly designed in every way I can think of and was very
>sensitive to EMI. The ports changed state from input to output and the chips
>just blew apart. I agree that if the other aspects of the design were better
>thought that should not happen often but I can't see any advantages of tying
>the pin to ground !!
>
>    Pull-ups or down are the best if you can afford it, if not just leave
>the ports as outputs. There is nothing to be gained by relying on the
>internal pullups or tying the pin to ground or VCC.

Have you ever designed complex single-layer PCBs ?
--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestspam_OUTspammitvma.mit.edu


2002\08\29@165353 by hard Prosser

flavicon
face
I just looked at my last 3 main projects - Not an unused pin on any of
them!
But if there were, they'd be outputs. - if there's space it can be useful
to bring them out to some sort of pad & they can be real helpful when
debugging.

Richard P




Write this in your "Things must know about microprocessors" book.

       **** Ignoring unused pins is bad engineering. ****

It's asking for trouble.
Sooner or later you will be sorry.
You may already be sorry and wonder why :-)
This applies to almost every microprocessor - it's NOT a PIC specific
problem.

The only situation where it wouldn't matter (much) would be if the default
port setting was inputs with pullups enabled.

If an input floats it will likely spend a significant part of its time near
the middle of the voltage range. Near is a relative term depending somewhat
on processor. Unless you input is Schmitt triggered, having it float mid
supply will do BAD things. A Schmitt will draw less extra current (but
still
more than normal). Regardless, it will not only draw higher current but MAY
noise up A2Ds, make the processor act funny like, create EMI and more.
Often
it mightn't. Sometimes it will. Don't do it.

Choices in approximate order of decreasing goodness (IMHO) BUT this will
vary with your level of discipline and documentation.

1 Input with internal pullup.
2 Output set high or low.
3 Input with external pull up/down
4 Input tied to ground.supply

1 is zero cost but can be destroyed by software change.

2 is zero cost but has a pin doing something - not usually a problem unless
someone equates unused with don care.

3 Costs the pull up/down but is well defined.
This is the safest option but annoying.

4 Is free BUT can cause bad problems if software is changed.

       Russell McMahon

--
http://www.piclist.com hint: To leave the PICList
TakeThisOuTpiclist-unsubscribe-request.....spamTakeThisOuTmitvma.mit.edu

--
http://www.piclist.com hint: To leave the PICList
TakeThisOuTpiclist-unsubscribe-requestKILLspamspamspammitvma.mit.edu


2002\08\29@200709 by David Duffy

flavicon
face
At 09:33 AM 29/08/02 -0500, you wrote:
>David,
>
>A newbie addition to this question.  What should the unused pins be "tied"
>to.  Ground???

Ken,
As previously advised, don't tie them to anything. Leave them not connected
& set as output.
This is not a "floating" pin as the PIC is driving the output to a known
state. The problem is
when the pin is set to *input* and left not connected. Then, external
forces (the way you hold
your tongue, phase of the moon, etc!) will let the pin wander into the
undefined region (neither
high nor low) and the PIC pin input circuitry will draw more current and
possible behave in a
less than predictable manner. As others have pointed out, if you have spare
port pins, you
can always add pull up/down resistors to them for future expansion. I hate
having PIC pins on
a board that are unused! I usually think of extra features to use them at
minimal cost or bring
them out to a connector (or pads) and chuck a series resistor in there (for
driving a LED) or
a pull-up resistor for adding a switch later on. A lot of the boards I
design are multi-purpose.
Regards...

--
http://www.piclist.com hint: To leave the PICList
.....piclist-unsubscribe-requestspamRemoveMEmitvma.mit.edu


2002\08\29@230505 by Ken Crismon

flavicon
face
Thanks for the info David.  After reading further it became clear that the "set to output" was the most common response.

I am a newbie and having fun learning a lot about the PIC and the 8051 at the same time.  Building two proto-types one for each controller and the engineering team will decide later which is better for our application.  My take is it will come down to cost and maintainability.

Regardless...  Sure am having a lot of fun.  My past life I was a Z-80 dude but things have certainly changed!!!

Thanks again for the answer and I'll have lots more stupid newbie questions but fortunately this list seems to deal with them very nicely!!!

Cheers,

Ken

{Original Message removed}


'[PIC]: Unused pins'
2002\09\01@110329 by Dan Marchesani
picon face
Hi Brendan,

That is a pretty fair question to ponder.

Port Pins as INPUTS
If as INPUTS, set either to ground or Vdd .  Floating INPUT is a NO NO
in CMOS.  Floating the input gates of a CMOS devices will allow them to
charge to the surrounding fields.  This could  cause the floating input
to hanging in between a logic 1 or 0 (1/2 Vdd) where the CMOS device
drains its highest current and heats it up.   (I guess you know that)

Port Pins as OUTPUTS
Well if you have your unused Pins set as OUTPUTS, and floating, a few
things can happen.  Accidental shorts to these pins can happen.  You'll
have to check the data sheets on what kind of protection there is
against this.  Shorts from a voltage source higher then the device's Vdd
may damage them.  Out side of shorts there is no real problem setting
them to OUTPUTS.  If I may add, as OUTPUT, the programming if it sends
logic changes to this unused pin, current during transit still normally
occur internally to the output gate.  Also if your supply source is from
a batt, then you might take this current into consideration, and not set
your unused pins to output.

On the other hand, you can still get away with the OUTPUT setting while
using a batt, just as long as there is no logic changes take place on
these outputs.

I personally I like to set the I/O Pins to INPUT and ground them.  Yet
there are times I may make them OUTPUTS and let them float and chance
the problems of shorts.  Mainly if I make the unused Pins as output,
generally this will occur in my projects because the USED PINS on their
common port is also outputs (it is less of a pain in the neck to program
around).

In the real world, unused and used OUTPUTS are prone to taking a short,
and can be damaged, no more differently.

Dan M.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2002\09\01@111917 by Wouter van Ooijen

face picon face
> Well if you have your unused Pins set as OUTPUTS, and floating, a few
> things can happen.

A basic course on logic learned me (long ago) that when a particular
line of reasoning states that a contradiction is true, you are allowed
to take *any* proposition as beging true. So once you have achieved the
above effect (both floating and outputs) I guess a lot more than just 'a
few things' can happen..... (like the S/N ratio on the PICList suddenly
reaching 1.0).

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2002\09\01@195834 by David Duffy

flavicon
face
Dan M wrote:
>Hi Brendan,
>That is a pretty fair question to ponder.
>
>Port Pins as INPUTS
>If as INPUTS, set either to ground or Vdd .  Floating INPUT is a NO NO
>in CMOS.  Floating the input gates of a CMOS devices will allow them to
>charge to the surrounding fields.  This could  cause the floating input
>to hanging in between a logic 1 or 0 (1/2 Vdd) where the CMOS device
>drains its highest current and heats it up.   (I guess you know that)

Yes, this is correct.

{Quote hidden}

I guess you and I will have to disagree on this Dan. Newbies just don't need
the added complication of having port pins grounded and accidently setting
them to output killing the chip in the process. The ideal way for them is to
use pull-up/down resistors I guess but not many will actually do it.  :-)

>In the real world, unused and used OUTPUTS are prone to taking a short,
>and can be damaged, no more differently.

In the real world, your unused pins won't get shorted because they don't
go anywhere near the customer interface. (usually not connected at all)
Regards...
___________________________________________
David Duffy        Audio Visual Devices P/L
U8, 9-11 Trade St, Cleveland 4163 Australia
Ph: +61 7 38210362   Fax: +61 7 38210281
New Web: http://www.audiovisualdevices.com.au
___________________________________________

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.


2002\09\02@075832 by Olin Lathrop

face picon face
> >In the real world, unused and used OUTPUTS are prone to taking a short,
> >and can be damaged, no more differently.
>
> In the real world, your unused pins won't get shorted because they don't
> go anywhere near the customer interface. (usually not connected at all)

Not to mention that you've got much bigger problems if something is shorting
against traces on the board.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email RemoveMElistservspamspamBeGonemitvma.mit.edu with SET PICList DIGEST in the body


2002\09\02@094948 by Dan Marchesani

picon face
Hi Olin Latrop

In addition having an input held at 1/2 Logic the voltage between 0 and Vdd will cause the CMOS Device's internal gate to heat up.  Floating inputs is certainly the worse thing anyone can do with CMOS.

I recall the early days of CMOS where many engineers that had little practices in design using this new family.  I seen many products that actually got placed on the Market, only to come back and have to be reworked.  

Most had experience with TTL.  In TTL it was OK to float an input and get away with it.  But CMOS Device Inputs being Hi Z, allow them to charge up from pick up of the surrounding field (noise).  

In a CMOS INVERTER as an example, once the input gate charges enough to reach UNKNOWN LOGIC a voltage of 1.5 v above Vss (with a 5 V supply), the output of this gate starts to conduct between the two internal output FET.  Once your input reaches Mid Voltage of 2.5 Volts,  Both these FET's are on. allowing  current is flowing between the Source and Ground, using the two internal FET as a load.  After some time they might heat up enough to be damaged.

Well I am glad you asked this question.  Many people getting involved with CMOS today need to understand this major point.  


Good Luck, you just took another step up.

Dan Marchesani


Olin Lathrop wrote:
Brendan, I simply ignore unused pins.

Bad idea.

CMOS inputs are very high impedence and can pick up noise if not driven.
Noise on an input will increase overall current draw from the extra clock
edges, and can be worse if the input is held in the mid region.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu




-- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body

2002\09\02@102553 by Dan Marchesani

picon face
Dan Marchesani wrote:

>
> In a CMOS INVERTER as an example, once the input gate charges enough
> to reach UNKNOWN LOGIC a voltage of 1.5 v above Vss (with a 5 V
> supply), the output of this gate starts to conduct between the two
> internal output FET.  Once your input reaches Mid Voltage of 2.5
> Volts,  Both these FET's are on. allowing  current is flowing between
> the Source and Ground, using the two internal FET as a load.  After
> some time they might heat up enough to be damaged.
>
Additionally:

We all kind of view the devices we work with as logical.  We tend to
over look the analog aspect.  After reading many responses to the
subject I noticed the over view was pointing mainly to the logic level
being either 1 or 0.  Clocking, Oscillating, and transit between these
two logic levels.  The current drain verses frequency etc.  (Some did
address the 1/2 Logic voltage problem -  they are the really experienced. )

Floating Input Gates as I described can damage the device, which I think
is the most paramount design problem over all.  In normal operation,
current drain only occurs during transit between the two logic levels,
but once the input is held at MID POINT, a condition that is NOT normal
for logic devices.  Chip Manufacturers design these devices to with
stand TRANSIT CURRENT.

Why we don't see our designs failing to damage due to floating inputs,
is because the surrounding fields (noise) are analog in  nature.  They
never hold the voltage at 1/2 logic ( Mid point between Vss and Vdd).
So I guess they don't heat up to a point of getting damaged right away.
 Over time they could and will fail.

Danny Marchesani

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email spamBeGonelistserv@spam@spamspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body


2002\09\02@104512 by Dan Marchesani

picon face
Olin Lathrop wrote:

>Sooner or later you'll get bitten.  Yes, you can get away with floating
>inputs most of the time.  If you don't want to learn from everyone here that
>you shouldn't leave inputs floating, that's your business, but please stop
>advising others to do so.
>

I TOTALLY AGREE............  FLOATING INOUTS IS THE WORST THING ANYONE
CAN HAVE IN A DESIGN "THE WORST"

Dan Marchesani

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email TakeThisOuTlistservspamspammitvma.mit.edu with SET PICList DIGEST in the body


2002\09\02@111649 by Dan Marchesani

picon face
Hi Olin,

I always pondered the best way to handle them.  Of all I read, your
notes are worth following....

Only problem with partially used ports where some pins are used as
outputs, the output logic from the program needs to be masked to
maintain  the unused Hardware Pin Logic LOW.  Its a little bit of a
pain, but I guess it is worth it, if future prototyping / board changes
are kept in mind.  I guess this one of my sticking points I could never
resolve in my thinking.

Yes, your NOTE Number 2 is certainly a good reason to follow this
method.  Crazy problems can and do occur.  Sometimes the processor can
take off on it's own, changing what we set the I/O pins for.  If an
unused pin is tied to ground, and some how is forced into being an
output, with a logic HI....  It doesn't take a rocket  scientist to
figure the rest, ha ha..

Thanks
Dan Marchesani

Olin Lathrop wrote:

{Quote hidden}

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listservEraseMEspammitvma.mit.edu with SET PICList DIGEST in the body


2002\09\02@114914 by Dan Marchesani

picon face


Wouter van Ooijen wrote:
Well if you have your unused Pins set as OUTPUTS, and floating, a few
things can happen.

A basic course on logic learned me (long ago) that when a particular
line of reasoning states that a contradiction is true, you are allowed
to take *any* proposition as beging true. So once you have achieved the
above effect (both floating and outputs) I guess a lot more than just 'a
few things' can happen..... (like the S/N ratio on the PICList suddenly
reaching 1.0).

Wouter van Ooijen
Hi Wouter van Ooijen
-------------------------------------------------------------------------
In the following I (Dan Marchesani)wrote

Well if you have your unused Pins set as OUTPUTS, and floating, a few
things can happen.  Accidental shorts to these pins can happen.  You'll
have to check the data sheets on what kind of protection there is
against this.  Shorts from a voltage source higher then the device's Vdd
may damage them.  Out side of shorts there is no real problem setting
them to OUTPUTS.  If I may add, as OUTPUT, the programming if it sends
logic changes to this unused pin, current during transit still normally
occur internally to the output gate.  Also if your supply source is from
a batt, then you might take this current into consideration, and not set
your unused pins to output.
-------------------------------------------------------------------

It is tough to try to determine what is the best way to handle so many possibilities.
Like many others here on the list, we are trying to sort the best method.  

Mr. Olin Lathrop wrote some really good points on the matter.  In my thinking of the many possibilities, I had to many sticking points.  Olin made a very good point of one thing I gave little thought about.  What will happen if we decided to tie our unused pins to ground and set them to INPUTS.

Well taking the possibility such as a processor going crazy.  Changing the I/O configuration from INPUT to OUTPUT.  Gee we are going to be  left with a short to ground.  

Yes there are many things to consider, but once the path is reached, there is a final solution that will best fit all possibilities.  

UNUSED PINS - should be set as OUTPUTS, and not connected to anything.  At some point in time later in the development of a project, these unused pins can simply be set as needed in F/W and hardware can be attached to the floating pin(s) without having to undo connections made if tied to something.

If  in the final development you may still have unused pins, there is no extra cost in the production model finally being  released.
 
Dan Marchesani
-- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body

2002\09\02@120408 by James Newton, webmaster

face picon face
source=
http://www.piclist.com/postbot.asp?id=piclist\2002\08\28\192537a

This subject has been discussed at:
http://www.piclist.com/techref/logic/xtrapins.htm

---
James Newton: PICList.com webmaster, former Admin #3
RemoveMEjamesnewtonEraseMEspamspam_OUTpiclist.com  1-619-652-0593 phone
http://www.piclist.com/member/JMN-EFP-786
PIC/PICList FAQ: http://www.piclist.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email @spam@listservRemoveMEspamEraseMEmitvma.mit.edu with SET PICList DIGEST in the body


2002\09\02@132323 by Dan Marchesani

picon face
Hi David Duffy

(I wrote Dan Marchesani)

> In the real world, unused and used OUTPUTS are prone to taking a short,
> and can be damaged, no more differently.


In the real world, your unused pins won't get shorted because they don't
go anywhere near the customer interface. (usually not connected at all)
Regards...
___________________________________________

I was not overly to misleading in my views, just pointing out some of
the craziness....

In my notes there, I was just pointing out a possiblity and what can
happen.  Yes it is going to be a confusing issue for the newbies, but I
guess some of the crazy things that can happen they should get exposure
to.

Yes your right in the real real world where the final product goes to
the consumer...  Gee in my real world I got wires all over the place and
at times it is a mess...

Forgive me on taking the term to literal...( I was picturing my real
world, ha ha)

Yes, your right we will disagree on IN or OUT.  Your for input and I am
for output (but before this writing I was undecided-ha ha..).  Either
way (yours or mine -  if things works OK (Hardware and F/W ) with the
processor, it doesn't matter any more..  I think these we both can agree
on this....

This subject did get a great response, and I also learnt a few things.
I finally worked out how I will best configurate unused pin next time.
Yes there is many possibilities to what can and will happen.. Olin
Lathrop gave a good example that I think is best to follow.

Certainly UNUSED as INPUT and FLOATING is a NO NO...

I like the idea of setting them to Output and floating with out any load
with a state of LOW.

Others thinking it is ok to set to input and use the internal resistors
to hold the logic state.

After reading Olin's notes.  He took in to concideration the processor
taking off on it's own and changing the I/O configuration.

I see it like Olin does, and after considering as many of the
possiblities that can happen, the configurated as OUTPUT place in a LOW
STATE. with the pin floating  is the best for ME...

Finally I think I have come to decide the best way for me.  It is a
really tough decision as you can see from all that took the time to try
to make their points on the subject.

For the newbies all I can say is, take some caution and try to
understand why there is so many choices.  Sort out what many of us are
saying, make your final determination.  There are many sticking points
in this issue.  That is why many are stating it is really not to much of
a big thing going with either IN or OUT configurations.  But in the end
the Pin must be set to either one - ha ha...

That going to be your choice.......

Dan Marchesani

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email EraseMElistservspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body


2002\09\02@163031 by Dan Marchesani

picon face
Hi All

In trying to decide on a configuration, we take into consideration, what can happens that is not in our control.  Generally, once a design that goes out into the field.

We can make a list of these problems we know that can happen.  (This is a
bit out of scope of unused pins per say.)  Make a list of these problem.
 Look at it and then select either IN or OUT.  We'll find there is no perfect
way.  In all our attempt to come up with a solution, we'll find, none that
can fit all problems.  But some are better then others.  

======================================================
Wouter van Ooijen wrote to me and stated the following:

A basic course on logic learned me (long ago) that when a particular line of reasoning states that a contradiction is true, you are allowed to take *any* proposition as beging true. So once you have achieved the above effect (both floating and outputs) I guess a lot more than just 'a few things' can happen..... (like the S/N ratio on the PICList suddenly reaching 1.0). 

======================================================
For the NEWBIES, there is no doubt in regard to selecting unused pins as INPUT that floats.  Its a NO NO.  This is really basic.

Outside the scope where other things can and do go wrong it is a different story.  If they go wrong it is not the intention in general of the design to address all problems. We should try to address them, but there are times it can not be address until it has been field tested.

It is the intention of the project to work as designed, and if it does then that is good.  We as engineers should address as much protection into our designs as best we can.  Mainly to "protect" our self from the flack later....  But something will go wrong no matter what.  Even the best minds got flack in their faces......  You just have to take it the best you can, and go on.

Dan Marchesani
=====================
Olin Lathrop wrote:
In the real world, unused and used OUTPUTS are prone to taking a short,
and can be damaged, no more differently.
In the real world, your unused pins won't get shorted because they don't
go anywhere near the customer interface. (usually not connected at all)

Not to mention that you've got much bigger problems if something is shorting

-- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body

More... (looser matching)
- Last day of these posts
- In 2002 , 2003 only
- Today
- New search...