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'[EE]: unused pins'
2004\06\21@020330 by rad0

picon face
When making a pcb, should you tie the unused pins directly to
ground, or should it be through a resistor?

Memory is failing...thanks.

Or what is the proper procedure or good practice?

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2004\06\21@024146 by Jinx

face picon face
> When making a pcb, should you tie the unused pins directly to
> ground, or should it be through a resistor?

My preference on PCBs with an uncertain future is to set unused
pins as low outputs and have pads for a pull-down and pull-up and
a spare pad in case the s/w requires an added input at a later date

However, there would be trouble in the event of a short of Vcc to
a pin set as o/p 0V. If you think there's a possibility of such a short
of opposite polarity to the pin, setting the pin as an input with a
p/u or p/d would be safer, bearing in mind that tying them straight
to a rail means you might not remember to include pads for the
p/u or p/d

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2004\06\21@044251 by Jinx

face picon face
> When making a pcb, should you tie the unused pins directly to
> ground, or should it be through a resistor?

Further reading (ooops sorry JamesN, not thinking)

http://www.piclist.com/techref/logic/xtrapins.htm

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2004\06\21@091627 by Byron A Jeff

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On Mon, Jun 21, 2004 at 01:03:35AM -0500, rad0 wrote:
> When making a pcb, should you tie the unused pins directly to
> ground,

Absolutely not! Ouch! If for whatever reason the software ever sets that pin
to a high output, then you've just blown your output buffer.

> or should it be through a resistor?

If necessary use a resistor. And then one that will draw less than 20ma of
current. In this case you probably want to leave this I/O pin as an input.

>
> Memory is failing...thanks.
>
> Or what is the proper procedure or good practice?

Good practice is to leave the pin open, preferably wired to a short open
pad just in case you ever need it. Then in your software you set the pin to
an output. That's all that's required.

BAJ

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2004\06\21@115732 by David VanHorn

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face
At 09:16 AM 6/21/2004 -0400, Byron A Jeff wrote:

>On Mon, Jun 21, 2004 at 01:03:35AM -0500, rad0 wrote:
>> When making a pcb, should you tie the unused pins directly to
>> ground,
>
>Absolutely not! Ouch! If for whatever reason the software ever sets that pin to a high output, then you've just blown your output buffer.

While this borders on a religious flamewar, I've never seen a micro that wasn't capable of withstanding a short to VCC or GND on an output pin.

That being said, I would generally agree, set them as outputs, and leave them untied, or add a series resistor to ground, and make them output low.

Input with a pullup would be second choice.

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2004\06\21@121010 by Spehro Pefhany

picon face
At 10:57 AM 6/21/2004 -0500, you wrote:
>At 09:16 AM 6/21/2004 -0400, Byron A Jeff wrote:
>
> >On Mon, Jun 21, 2004 at 01:03:35AM -0500, rad0 wrote:
> >> When making a pcb, should you tie the unused pins directly to
> >> ground,
> >
> >Absolutely not! Ouch! If for whatever reason the software ever sets that
> pin to a high output, then you've just blown your output buffer.
>
>While this borders on a religious flamewar, I've never seen a micro that
>wasn't capable of withstanding a short to VCC or GND on an output pin.

I agree with this! Someone, hire this man!

>That being said, I would generally agree, set them as outputs, and leave
>them untied, or add a series resistor to ground, and make them output low.

Yes, it can cause other problems if the power supply is weak- perhaps
adding nasty states where the unit doesn't start up properly or half-dies.

>Input with a pullup would be second choice.

Yup.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2004\06\24@183427 by Andrew Warren

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David VanHorn <.....PICLISTKILLspamspam@spam@mitvma.mit.edu> wrote:

> this borders on a religious flamewar, ....

   Yeah, but I'm going to jump in and stir the pot, anyway.

> I would generally agree, set [unused IO pins] as outputs, and leave
> them untied, or add a series resistor to ground, and make them
> output low.

   The problem with making them open outputs is that they'll be
   FLOATING INPUTS during power-on reset and before your firmware
   configures them as outputs.

   Anyone who refuses to tie unused pins to ground should be even more
   worried about leaving the pins open; it seems much more likely that a
   floating input pin will latchup than a tied-low input pin will
   magically become a high-driving output.

   The safest way, of course -- and the most expensive -- is the
   other one you suggested:  Tie the unused pins to ground through
   a resistor.

   -Andy

=== Andrew Warren -- aiwspamKILLspamcypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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2004\06\25@102305 by David VanHorn

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>
>    The problem with making them open outputs is that they'll be
>    FLOATING INPUTS during power-on reset and before your firmware
>    configures them as outputs.

For a couple of uS, yes. That's harmless.
I don't wait till a week after boot, to init my I/O.


>    Anyone who refuses to tie unused pins to ground should be even more
>    worried about leaving the pins open; it seems much more likely that a
>    floating input pin will latchup than a tied-low input pin will
>    magically become a high-driving output.

Before they latch up, they have to get hot.
Temperature is heat * time / mass.
I can't control the mass, but I keep the time as short as practical.
Typically 10uS after reset goes up, my I/O is initted.

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