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'[pic] Weak pullups'
2006\06\03@013718 by Palaniappan C

picon face
Hi all,

What is weak pullup ? The strength 'strong' or 'weak' related
to voltage level or current level ?

I am using PIC18f2525 and datasheet gives only 'weak' pullup current
and not resistor value,  Why it is ?.

My another doubt is , Weak pullups are done by resistors or active
components  ? ( Because, In academics, i have read somewhere , it is
difficult to fabricate passive components on IC than active components).

regards,
palani.

2006\06\03@040917 by Bernd Rüter

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This "weak"-pullups are maybe aquivalent to a 10k resistor.

Palaniappan C schrieb:
{Quote hidden}


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2006\06\03@043140 by Robert Rolf

picon face
Palaniappan C wrote:

> What is weak pullup ? The strength 'strong' or 'weak' related
> to voltage level or current level ?

You answer your own question below.
Strong or weak is relative to what is needed, based on transition
frequency. What is adequate at low frequencies is not sufficient
at high transition rates.

> I am using PIC18f2525 and datasheet gives only 'weak' pullup current
> and not resistor value,  Why it is ?.

Because it's a CURRENT.

> My another doubt is , Weak pullups are done by resistors or active
> components  ? ( Because, In academics, i have read somewhere , it is
> difficult to fabricate passive components on IC than active components).

Correct.
So they fabricate a current source that gets turned on to enable
'weak pullup'.

R

2006\06\03@071709 by Bob Axtell

face picon face
Bernd Rüter wrote:
> This "weak"-pullups are maybe aquivalent to a 10k resistor.
>
> Palaniappan C schrieb:
>> Hi all,
>> What is weak pullup ? The strength 'strong' or 'weak' related to
>> voltage level or current level ?
>> I am using PIC18f2525 and datasheet gives only 'weak' pullup current
>> and not resistor value,  Why it is ?.
>> My another doubt is , Weak pullups are done by resistors or active
>> components  ? ( Because, In academics, i have read somewhere , it is
>> difficult to fabricate passive components on IC than active components).
>>
>> regards,
>> palani.
>>
My measurements, done several years ago, simply setting the port I/O as
input and pulled down through a  10.0K 1% resistor and using scope
measurements, gave me a value of approx 53K. But since they don't
guarantee these values, they could have easily changed. I can't imagine
it being very low, however.


--Bob

2006\06\03@080046 by Spehro Pefhany

picon face
At 11:07 AM 6/3/2006 +0530, you wrote:
>Hi all,
>
>What is weak pullup ? The strength 'strong' or 'weak' related
>to voltage level or current level ?
>
>I am using PIC18f2525 and datasheet gives only 'weak' pullup current
>and not resistor value,  Why it is ?.

They are not resistors. They are small p-channel MOSFETs typically
operating in the pinch-off region when conducting, so they are
(relatively) constant currrent sources for small values of output
voltage relative to ground. As the voltage on the output approaches
Vdd-Vth, the current will drop in a nonlinear fashion. The gate of
the MOSFET is switched to ground to turn the weak pullup on, and to
Vdd to turn it off (see port pin schematics in your datasheet).

They are very loosely specified, since process parameters can vary
over a wide range and Microchip doesn't want to throw any chips out
because of pullup current out of spec, so the specs are made loose.

The current is relatively large, partly because of Microchip's loose
port pin leakage specification (as much as 1uA leakage is possible),
and for EMC considerations, but perhaps smaller at the low end than
you'd like in an electrically hostile environment. You can always
add parts outside if you don't like it, it's basically free if you
can use it.

>My another doubt is , Weak pullups are done by resistors or active
>components  ? ( Because, In academics, i have read somewhere , it is
>difficult to fabricate passive components on IC than active components).

Yes, the typical current is determined by the geometry of the MOSFET
for a given process.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


2006\06\03@085620 by olin piclist

face picon face
Robert Rolf wrote:
> So they fabricate a current source that gets turned on to enable
> 'weak pullup'.

I don't think so.  Most likely it's just a P channel high side FET
deliberately made small to have a high on resistance.  In that case it looks
more like a resistor than a current source.

The main distinction of "weak" is that its current is intended to be small
enough to be within the normal sink capability of external components
driving the pin.  The pullup is also designed to tolerate the output
externally held low indefinitely, which is something the standard strong
pullup is not rated for.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\06\03@104015 by kravnus wolf

picon face
Olin,

  In another words, weak pullup saves power since it
does not require that much current to move between the
states since MOSFET required voltage and not current
thanks to it's high impedence? Should we just design
preferable around weak pullup pins for I/O?

Thanks
John

--- Olin Lathrop <olin_piclistspamKILLspamembedinc.com> wrote:

{Quote hidden}

******************************************************************
> Embed Inc, Littleton Massachusetts, (978) 742-9014.
> #1 PIC
> consultant in 2004 program year.
> http://www.embedinc.com/products
> --

2006\06\03@142232 by Peter

picon face

On Sat, 3 Jun 2006, Palaniappan C wrote:

> What is weak pullup ? The strength 'strong' or 'weak' related
> to voltage level or current level ?
>
> I am using PIC18f2525 and datasheet gives only 'weak' pullup current
> and not resistor value,  Why it is ?.
>
> My another doubt is , Weak pullups are done by resistors or active
> components  ? ( Because, In academics, i have read somewhere , it is
> difficult to fabricate passive components on IC than active components).

They are small area PMOS transistors. 'weak' is marketingspeak for the
more accurate engineering term 'we cannot guarantee much about this,
take is as it is'. You have to admit that 'weak' sounds better.

Peter

2006\06\03@142650 by Peter

picon face

On Sat, 3 Jun 2006, Bob Axtell wrote:

> My measurements, done several years ago, simply setting the port I/O as input
> and pulled down through a  10.0K 1% resistor and using scope measurements,
> gave me a value of approx 53K. But since they don't guarantee these values,
> they could have easily changed. I can't imagine it being very low, however.

It is better to plot the value with a resistor box. Since the pullups
are actually PFETs they  almost stop sourcing current when the pin is
near enough Vdd. I got bitten by this when trying to use such a pin to
drive reset on another cmos device. Not good.

Peter

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