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PICList Thread
'[PIC] Unused inputs left to float - was PIC looses'
2007\11\17@041412 by Robin Abbott

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Wouter's email gave me pause for thought. On all of our PIC applications we
have left unused pins to float and normally set to input without any
apparent ill effect. However in the old days of CMOS this was bad news as
the high impedance could lead to oscillating inputs and high current drain.

Does anyone know if this applies to PIC's or has the input technology long
moved on?

Robin Abbott
Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front
end
spam_OUTrobin.abbottTakeThisOuTspamfored.co.uk
http://www.fored.co.uk


{Original Message removed}

2007\11\17@052404 by Dario Greggio

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Robin Abbott wrote:

> Does anyone know if this applies to PIC's or has the input technology long
> moved on?

Definitely it still applies!


--
Ciao, Dario

2007\11\17@054144 by Robin Abbott

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So this implies unused inputs should be set to outputs and held low or high
I guess.

Thanks for that, although a quick look at some app notes seems to show the
MC don't necessarily do this !


Robin Abbott
Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front
end
.....robin.abbottKILLspamspam@spam@fored.co.uk
http://www.fored.co.uk


-----Original Message-----
From: piclist-bouncesspamKILLspammit.edu [.....piclist-bouncesKILLspamspam.....mit.edu] On Behalf Of
Dario Greggio
Sent: 17 November 2007 10:24
To: Microcontroller discussion list - Public.
Subject: Re: [PIC] Unused inputs left to float - was PIC looses it's
programmemory

Robin Abbott wrote:

> Does anyone know if this applies to PIC's or has the input technology
> long moved on?

Definitely it still applies!


--
Ciao, Dario

2007\11\17@061158 by Xiaofan Chen

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On Nov 17, 2007 5:14 PM, Robin Abbott <EraseMErobin.abbottspam_OUTspamTakeThisOuTfored.co.uk> wrote:
> Wouter's email gave me pause for thought. On all of our PIC applications we
> have left unused pins to float and normally set to input without any
> apparent ill effect. However in the old days of CMOS this was bad news as
> the high impedance could lead to oscillating inputs and high current drain.
>
> Does anyone know if this applies to PIC's or has the input technology long
> moved on?
>

If you are developing real application and not demo boards, you should
add a pull-up or pull down to set the input to a defined state. Or you can
set it to output and output either low or high. All in all, to set the pin to
a defined state.

As for why Microchip ANs have some unused pin not following the
recommendations, there are some bad ANs from Microchip. Anyway,
they are meant for demo usage, not real product...

Xiaofan

2007\11\17@062020 by Jinx
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> Does anyone know if this applies to PIC's or has the input technology
> long moved on?

I don't recall any mention by Microchip of what to do with unused
inputs. There may be an AN for it, but AFAIK it's not in any PIC
manual. The focus is on pins that are being used, not aren't

It's definitely A Thing To Be Aware Of

"What should I do with pins that I am not using?"

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

2007\11\17@132644 by Bob Axtell

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Robin Abbott wrote:
> Wouter's email gave me pause for thought. On all of our PIC applications we
> have left unused pins to float and normally set to input without any
> apparent ill effect. However in the old days of CMOS this was bad news as
> the high impedance could lead to oscillating inputs and high current drain.
>
> Does anyone know if this applies to PIC's or has the input technology long
> moved on?
>
> Robin Abbott
> Forest Electronics - Home of WIZ-C ANSI C Compiler for PIC's with RAD Front
> end
> robin.abbottspamspam_OUTfored.co.uk
> http://www.fored.co.uk
>  
>
> {Original Message removed}

2007\11\17@133021 by Bob Axtell

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MC writes poorly-thought-out app notes. Many are wrong, and use bad
engineering practice as well.
Don't pay too much attention to what MC does.

--Bob A

Robin Abbott wrote:
{Quote hidden}

> {Original Message removed}

2007\11\17@174924 by David Meiklejohn

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> If you are developing real application and not demo boards, you should
> add a pull-up or pull down to set the input to a defined state.

Is it considered ok to connect inputs directly to Vdd or Vss, or should a
pull up/down resistor always be used?  I can see that using a resistor is
a good idea in case the PIC somehow ends up with what is supposed to be an
input, set to output, in which case the resistor protects against e.g.
grounding a high output, but is there any other reason to use them?  E.g.
I note that MCLR is often held high through a resistor, instead of
directly to Vdd.  What's the danger in connecting it (an input-only pin)
to Vdd?


David Meiklejohn
http://www.gooligum.com.au

2007\11\17@184107 by Jan-Erik Soderholm

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Using a resistor makes in-field changes (such as adding
an extra output pin) easier then if the pin was hard
wired to Vdd or Vss on the PCB. The resistor can be
de-soldered and make the pin available for other uses.

Jan-Erik.

David Meiklejohn wrote:
{Quote hidden}

2007\11\19@091912 by M. Adam Davis

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CMOS technology has generally improved to the point where a floating
pin in very unlikely to damage the chip (a very real problem in the
early days of CMOS - the chip could fry itself).

However, a floating input is still an input.  It can oscillate due to
a variety of inputs (EM, static, nearby traces via capacitance, etc),
or it may sit at a particular voltage.

If you measure the current of a CMOS device, and drive its inputs in
the linear region (rather than digital) you'll find that it draws more
current because the high transistor and low transistor are both
conducting, drain VCC straight to ground.  The current I've seen
(atmel part) was in the range of 1-3mA, which is enough to cause
erratic operation on a watch battery type application.

Also note that if the input is oscillating you end up with a nice transmitter.

If you have room, put in the resistors and leave it an input - this is
the safest option.  If you don't have room, set it as an output and
leave it unconnected, but it's possible that this will cause you
problems if/when the code goes off into lala land for some other
reason.

-Adam

On 11/17/07, Robin Abbott <KILLspamrobin.abbottKILLspamspamfored.co.uk> wrote:
{Quote hidden}

> {Original Message removed}

2007\11\19@163611 by James Newton

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As Jinx pointed out:

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

I believe all the answer to ever question asked in this thread (as well as
the 500 or so versions of this thread that came before it) are answered on
that page.

{Original Message removed}

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