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'Unused PIC Pins tied to GROUND?'
1998\11\04@193340 by enpassant

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I read in a book somewhere that unused microchip pins should be grounded
to prevent them from "floating".  In the case of the PIC (16F84) should
unused I/O pins and any other unused pins be tied to ground???  Being
new with this stuff I'm not sure.  Thanks.

1998\11\04@200858 by James Cameron

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enpassant wrote:
> I read in a book somewhere that unused microchip pins should be
> grounded to prevent them from "floating".

I've seen that to.  Unused inputs I thought it said.  Outputs don't
float, but are driven.

> In the case of the PIC (16F84) should unused I/O pins and any other
> unused pins be tied to ground???

On the PIC 16F84, the only possible unused pins are I/O pins.  Set them
as outputs, and you will not need to attach them to anything.

The problem with grounding them is that your software may attempt to
raise them to a high level, in which case current may flow.

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1998\11\04@200913 by Harold Hallikainen

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On Wed, 4 Nov 1998 19:33:31 -0500 enpassant <.....enpassantKILLspamspam@spam@AMERITECH.NET>
writes:
>I read in a book somewhere that unused microchip pins should be
>grounded
>to prevent them from "floating".  In the case of the PIC (16F84)
>should
>unused I/O pins and any other unused pins be tied to ground???  Being
>new with this stuff I'm not sure.  Thanks.
>

Unused input pins should not be allowed to float  (at mid-voltage the
chip draws more current).  For maximum flexibility, I program unused pins
as outputs, which can then be left unconnected.  I can then later decide
to use the pins without cutting wires or traces.

Harold



Harold Hallikainen
haroldspamKILLspamhallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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1998\11\04@201305 by Dmitry Kiryashov

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enpassant wrote:
>
> I read in a book somewhere that unused microchip pins should be grounded
> to prevent them from "floating".  In the case of the PIC (16F84) should
> unused I/O pins and any other unused pins be tied to ground???  Being
> new with this stuff I'm not sure.  Thanks.

I think you may leave it untied but you should program this pins
as output to avoid current leakage. (in input state this pins are
able to floating and this result to current leakage ability)

You may damage the PIC if you'll tied pins _directly_ to ground and
after that trying to set it output=high (for instance due to mistake
in code)

WBR Dmitry.

1998\11\06@123228 by John Payson

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|> I read in a book somewhere that unused microchip pins should be
|> grounded to prevent them from "floating".

|I've seen that to.  Unused inputs I thought it said.  Outputs don't
|float, but are driven.

True.  One caveat here is that until the PIC manages to start up its outputs
will all be floating.  In most cases this shouldn't be a problem, but it may
cause the PIC to draw extra current while held in reset.

|> In the case of the PIC (16F84) should unused I/O pins and any other
|> unused pins be tied to ground???

|On the PIC 16F84, the only possible unused pins are I/O pins.  Set them
|as outputs, and you will not need to attach them to anything.

|The problem with grounding them is that your software may attempt to
|raise them to a high level, in which case current may flow.

Port pin RA4 may safely be connected to ground, since it's an open-collector
output.  Other port pins may be tied to either VDD or VSS through INDIVIDUAL
pullup or pulldown resistors, but in case of errant program execution pins which
are tied directly to VDD, ground, or each other could (in theory) be damaged or
(in practice) cause extremely high levels of current to be drawn (50+mA).

Note also that on the 16C5x, there's no way to set the RTCC pin as an output
and it's almost useless as an input, but it may be very conveniently tied to the
/MClr pin.

ObTrivialQuestion: In what fraction of 16C5x applications is the RTCC pin used
for any purpose?  If there were any way to use the pin without giving up the onl
y
timer it'd be handy, but there isn't.  I wonder how hard it would be to allow th
e pin
to be read as an input at address [5.4] and whether any existing software would
have a problem with that (the pin is spec'd to read as zero, unconditionally).



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