Searching \ for 'pulling outputs to 5 volts?' 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/ios.htm?key=output
Search entire site for: 'pulling outputs to 5 volts?'.

Truncated match.
PICList Thread
'pulling outputs to 5 volts?'
2000\04\07@125913 by Steven Rightnar

flavicon
face
I have seen it everywhere (pull all unused output lines to 5 volts). OK what
if you dont do this? I have one curcuit that seemed to work fine and I did
not pull the outputs up.

Thanks,

Steve

2000\04\07@131147 by jamesnewton

face picon face
This is FAQ: See
http://www.piclist.com/faq
under Notes, Unused pins
Or directly:
techref.massmind.org/techref/default.asp?url=logic/xtrapins
For a complete discussion. Let me know if I'm missing any points there.

---
James Newton (PICList Admin #3)
spam_OUTjamesnewtonTakeThisOuTspampiclist.com 1-619-652-0593
PIC/PICList FAQ: http://www.piclist.com or .org


-----Original Message-----
From: pic microcontroller discussion list
[.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU]On Behalf Of Steven Rightnar
Sent: Friday, April 07, 2000 09:58
To: PICLISTspamKILLspamMITVMA.MIT.EDU
Subject: pulling outputs to 5 volts?


I have seen it everywhere (pull all unused output lines to 5 volts). OK what
if you dont do this? I have one curcuit that seemed to work fine and I did
not pull the outputs up.

Thanks,

Steve

2000\04\07@131355 by Thomas C. Sefranek

face picon face
Steven Rightnar wrote:

> I have seen it everywhere (pull all unused output lines to 5 volts). OK what
> if you dont do this? I have one curcuit that seemed to work fine and I did
> not pull the outputs up.
>
> Thanks,
>
> Steve

The trick is to TERMINATE the inputs so they don't ast as antennas!

--
 *
 |  __O    Thomas C. Sefranek  .....tcsKILLspamspam.....cmcorp.com
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

ARRL Instructor, Technical Specialist, VE Contact.
hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

2000\04\07@132201 by Andy Kelley

picon face
It just draws extra current.

Andy K.

On Fri, 7 Apr 2000 09:58:15 -0700 Steven Rightnar
<EraseMErightnarspam_OUTspamTakeThisOuTDOMINION.LLUMC.EDU> writes:
> I have seen it everywhere (pull all unused output lines to 5 volts).
> OK what
> if you dont do this? I have one curcuit that seemed to work fine and
> I did
> not pull the outputs up.
>
> Thanks,
>
> Steve

________________________________________________________________
YOU'RE PAYING TOO MUCH FOR THE INTERNET!
Juno now offers FREE Internet Access!
Try it today - there's no risk!  For your FREE software, visit:
dl.http://www.juno.com/get/tagj.

2000\04\07@132208 by David VanHorn

flavicon
face
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

At 09:58 AM 4/7/00 -0700, Steven Rightnar wrote:
>I have seen it everywhere (pull all unused output lines to 5 volts). OK what
>if you dont do this? I have one curcuit that seemed to work fine and I did
>not pull the outputs up.

You don't have to pull outputs up, unless there's an input connected to
them somewhere, and it's open-collector.

Unused pins on a micro should be programmed as outputs, and there are
various cults as to wether they should be set high or low. I don't think it
really matters.

Unused Inputs should always be pulled, but which direction may matter,
check the data sheet.
-----BEGIN PGP SIGNATURE-----
Version: PGPfreeware 6.5.2 for non-commercial use <http://www.pgp.com>

iQA/AwUBOO4zQ4FlGDz1l6VWEQJdqgCfW6x6S2NHaQRXXapurF/dKvb3lqYAnREr
UDhxPMjeGFPxDwuaqXHh1x08
=q8zg
-----END PGP SIGNATURE-----

2000\04\07@144733 by M. Adam Davis

flavicon
face
Some recommend setting the micro's unused pins as inputs to lower power drain.
However, these are CMOS devices, and leaving an input to blow in the wind causes
it to oscillate erratically.  These oscillations use current as transistors in
the chain from the input go from high to low.

To keep inputs from oscillating, it is recommended to tie them high or low
through a large resister (10k-100k).  The current draw on the pin through the
resister is incredibly low.

I don't know what the case is between making a pin an input or output, and which
draws more power, but I believe I've read microchip's recommendation in their
various data sheets, so it shouldn't be too difficult to find.

-Adam

Steven Rightnar wrote:
>
> I have seen it everywhere (pull all unused output lines to 5 volts). OK what
> if you dont do this? I have one curcuit that seemed to work fine and I did
> not pull the outputs up.
>
> Thanks,
>
> Steve

2000\04\07@145140 by hgraf

picon face
> I have seen it everywhere (pull all unused output lines to 5
> volts). OK what
> if you dont do this? I have one curcuit that seemed to work fine and I did
> not pull the outputs up.

       CMOS doesn't like having floating inputs since the inputs are very high
impedance. Current flowing close to those pins could easily get the inputs
switching back and forth, creating a ton of unwanted oscillations which
could couple into other parts of the chip (and circuit) wreaking havoc with
your design. Now, having said all of this, I have also built prototypes that
worked fine with floating inputs. The biggest problem is when this problem
appears, it is very difficult to pin the symptoms to this problem. TTYL

2000\04\07@152413 by Paul Howell

flavicon
face
Well I guess I'll weigh in with my opinion...

As far is I know the reason you want to tie off inputs to *either* Vdd or
ground is to avoid excess (and unnecessary) current drain. It happens
because the NMOS transistor and the PMOS transistor have overlapping
threshold voltages. That is, there is a voltage range where *both* the NMOS
and PMOS transistors are on producing a path from Vdd to ground. Of course
with either a Vih or Vil applied, one or the other is firmly off. I am not
aware of an oscillating condition, it seems unlikely since the inputs look
to the outside world like two capacitors in parallel and to move the pin
voltage you would have to have real charge coming in or going off, and we
said at the start that they are open. Instead, I suspect that if both of the
input diodes are similar, then the leakage current across them would tend to
put an open input somewhere around Vdd/2 which is squarely in the spot that
both input transistor are turned on.

I think you can achieve the desired effect by making all unused pins outputs
(not tri-stated) and applying either a high or a low by writing to the
concerned register. What, if any difference it makes high or low is not
clear to me.
{Quote hidden}

2000\04\07@161747 by Dan Michaels

flavicon
face
Steven wrote:
>
> I have seen it everywhere (pull all unused output lines to 5 volts). OK what
> if you dont do this? I have one curcuit that seemed to work fine and I did
> not pull the outputs up.
>

Whatever else you do, don't ever pull OUTPUT lines to 5v (or 0v),
unless you do it through a resistor. Input lines can be tied
directly, but this can still fry you if you accidentally configure
the pins as outputs later.

2000\04\07@171332 by Randy A.

picon face
Yes, WHATEVER you do always use something like a 10k resistor if you need to
pull up an output, NEVER tie an output to + volts, -volts or gnd directly.
However it is common practice and in almost all cases required that any
unused inputs to gates etc. be pulled either to + V or Gnd depending on the
specific chip specs.

Hope this is some help
Randy A.

2000\04\07@184126 by David E Arnold

picon face
This is good advice, but in general, won't most of us just use the internal
pullup, which _is_
through a resistor?

-Dave





Dan Michaels <oricomspamspam_OUTLYNX.SNI.NET> on 04/07/2000 01:14:26 PM

Please respond to pic microcontroller discussion list <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>

To:   KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU
cc:    (bcc: David E Arnold/SYBASE)
Subject:  Re: pulling outputs to 5 volts?




Steven wrote:
>
> I have seen it everywhere (pull all unused output lines to 5 volts). OK what
> if you dont do this? I have one curcuit that seemed to work fine and I did
> not pull the outputs up.
>

Whatever else you do, don't ever pull OUTPUT lines to 5v (or 0v),
unless you do it through a resistor. Input lines can be tied
directly, but this can still fry you if you accidentally configure
the pins as outputs later.

2000\04\07@205426 by Dan Michaels

flavicon
face
At 03:40 PM 4/7/00 -0700, you wrote:
>This is good advice, but in general, won't most of us just use the internal
>pullup, which _is_
>through a resistor?
>
>-Dave
>

Only on port B, I believe.
=============

{Quote hidden}

2000\04\08@012600 by David E Arnold

picon face
Oh,

Only portB has pullups built in? Good to know.

-Dave





Dan Michaels <RemoveMEoricomspamTakeThisOuTLYNX.SNI.NET> on 04/07/2000 05:52:13 PM

Please respond to pic microcontroller discussion list <PICLISTEraseMEspam.....MITVMA.MIT.EDU>

To:   EraseMEPICLISTspamMITVMA.MIT.EDU
cc:    (bcc: David E Arnold/SYBASE)
Subject:  Re: pulling outputs to 5 volts?




At 03:40 PM 4/7/00 -0700, you wrote:
>This is good advice, but in general, won't most of us just use the internal
>pullup, which _is_
>through a resistor?
>
>-Dave
>

Only on port B, I believe.
=============

{Quote hidden}

2000\04\08@065626 by paulb

flavicon
face
David E Arnold wrote:

> Only portB has pullups built in? Good to know.

 That's basically why the Option register bit is described as:

 bit 7: RBPU: PORTB Pull-up Enable bit
   1 = PORTB pull-ups are disabled
   0 = PORTB pull-ups are enabled (by individual port latch values)

 .. which latter comment means that the pull-ups are only enabled for
inputs, not outputs (makes sense).
--
 Cheers,
       Paul B.

More... (looser matching)
- Last day of these posts
- In 2000 , 2001 only
- Today
- New search...