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'[PIC]: Pull-down resistor value'
2003\02\24@043010 by Andreas Nyholm

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Hello.

I found out that 10k is a good value for a pull-down resistor on a PIC-input
but what are the limits? Is it also ok with 100k?

/Andreas

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2003\02\24@044458 by Russell McMahon

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> I found out that 10k is a good value for a pull-down resistor on a
PIC-input
> but what are the limits? Is it also ok with 100k?

You really need to explain the question in a little more detail to be sure
that the answer suits your applications.
But, yes 100K is almost always OK.
1 megohm is often OK and
10 megohm is getting somewhat risky.

Assuming you are using the pin as an input which is pulled high by a
mechanical single pole switch connected to Vdd and which is pulled low by
the pull down then:

- The limits occur when input leakage either from the PIC or external
sources cause the pin to go high when the switch is open.
- Induced voltages in the input line do the same.


At the other extreme, the smallest value usable is governed by how much
current you are willing to draw through your switch contact. A 10k resistor
draws about 0.5mA and this is almost always entirely reasonable. 1K or even
100r is entirely OK but seldom necessary. Most switch contacts require a
small "wetting current" - this varies with contact material, and in
specialist cases you may wish to ensure that this current is provided by the
resistor that you use. If your switch is normally closed and the equipment
is battery operated this may be an significant addition to overall current
drain.

If you used a 10M resistor then an induced voltage in a long input lead
might cause the pin to be pulled high unexpectedly. Within a piece if
equipment even 10M is probably safe. Leakage resistance SHOULD be much
higher than 10M but for practical purposes it should not be necessary to
exceed say 1M. Exceptions might occur in special cases such as when you use
the pin as a touch switch input.



       Russell McMahon

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2003\02\24@045327 by David Duffy

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Andreas wrote:
>I found out that 10k is a good value for a pull-down resistor on a PIC-input
>but what are the limits? Is it also ok with 100k?

I usually use 4K7, 10K, 47K or 100K depending on what other resistors
are already required on the board. High values may give trouble in noise
pickup especially if it's for a switch and the wiring is long. Low values
cause higher current draw but if it's for a switch input, won't impact too
much if the switch is open most of the time anyway. There's no *one*
correct answer for this - it does depend on the application.
David...

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2003\02\24@065854 by Andreas Nyholm

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>>
>> ~12V in-|=2k=|--|----|-----|------PIC pin
>>                 |    |     |
>>                 Z    C     R
>>                 |    |     |
>>                 |----|-----|------ GND
>>
>> Z= 5,1V
>> C= 100n
>> R= 100k

>OK.
>I assume there is a switch there somewhere or that the AC turns off and on.
>The 2K is far lower than it needs to be if you use a 100K pullup.
pulldown, right? :)
>Not a problem - just doesn't need to be that low.

Oops :)
I just found out that once again I was very unclear! :)
What I meant was about 12V NOT AC 12V. My mistake! Sorry about that!!!

The reason I have only 2k is becourse the 12V is comming from an
serial-port, and is not always 12V and I noticed that with a laptop and 10k
instead of 2k the Zener voltage was below 4.5V.

Actually a do have a diode before the 2k resistor in case of negative
voltage but I didn't think that was relevant here.

Anyway, thanks for your help!
/Andreas

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2003\02\24@071525 by Russell McMahon

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> I just found out that once again I was very unclear! :)
> What I meant was about 12V NOT AC 12V. My mistake! Sorry about that!!!
>
> The reason I have only 2k is becourse the 12V is comming from an
> serial-port,

So that is effectively AC as the serial port is at +V or -V.
In the case of an "enthuiastic" RS232 driver my cautions still apply. With
the laptop driver there may not be enough drive to hurt.

> Actually a do have a diode before the 2k resistor in case of negative
> voltage but I didn't think that was relevant here.

You were extremely wise to add the diode!
Too many people have tried to do this and rely on the PIC internal diodes.
Using the external series diode as you have you completely avoid that
problem.



       Russell McMahon











>
> Anyway, thanks for your help!
> /Andreas
>
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2003\02\24@073356 by Olin Lathrop

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Andreas Nyholm wrote:
> I found out that 10k is a good value for a pull-down resistor on a
> PIC-input but what are the limits? Is it also ok with 100k?

The high end limit comes from the PIC pin leakage.  However, the value
based on that is so high that it the input may pick up noise.  The low end
limit is 0 if the pin is always an input.  If the pin must also be able to
drive to logic high level sometimes, then the limit comes from the maximum
current the PIC pin can source, and possibly the minimum logic 1 level of
the receiving device.


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

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2003\02\24@073812 by David Duffy

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Andreas wrote:
{Quote hidden}

Is this being fed from the serial port TX line?
Have you measured the voltage on the that pin or is the 12V a guess?
The 100n capacitor seems large unless the baud rate is very low.
David...

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2003\02\24@084223 by Mike Singer

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Russell McMahon wrote:
...
[long-long Russell's reply]
...

Hi, Russell.

I suspect, you've spent 10 times more your time to
answer the elementary post then the guy to think about
it before asking.

The best answer could be Olin's, but he keeps him
very shy, after he was severely criticized for teaching
newbies in a right way.

What is better: to trigger the guy use his own brain;
or write long answers that happened to be pretty
irrelevant to his vague goals?

Mike.
--------------
I do admit your right to post whatever you want,
the idea of this post is to discuss side-effects of the
pressure against Olin.

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2003\02\24@132555 by Herbert Graf

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> I found out that 10k is a good value for a pull-down resistor on
> a PIC-input
> but what are the limits? Is it also ok with 100k?

       Well it depends on what you are doing. The lower limit is governed by the
max current that can go into a pin. Since you are going the other way we
won't worry about that limit. The upper limit depends on your application.
If for example you're connecting your output to the gate of a transistor the
upper limit is determined by the parasitic current flowing into the gate and
the Vthreshold of the transistor (assuming static operation, if you want
quick dynamic operation the limit is much lower). Since the current going
into a gate is so tiny this limit is probably in the realm of hundreds of
mega ohms, at least. To find out what is your limit we need to know what you
are connecting the output to. TTYL

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2003\02\24@151129 by Peter L. Peres

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On Mon, 24 Feb 2003, Andreas Nyholm wrote:

*>Hello.
*>
*>I found out that 10k is a good value for a pull-down resistor on a PIC-input
*>but what are the limits? Is it also ok with 100k?

Input leakage is specified at 100nA (?) worst case and Vil at 0.9V so you
can use a resistor at most 0.9V/100nA Ohms ( = 9Meg). In practice do not
use anything larger than half that value (4.7Meg say), and preferrably
less.

Peter

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