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'[PIC]: When to use internal weak pull-up resistors'
2001\05\28@080432 by Leslie Lowe

picon face
Hello all

Are the internal weak pull-up resistors good for anything or should they be
avoided at all cost. There is no info in the datasheet on what to use or not
use them for. Is something as simple as a button ok? My major concern is
pcboard space.

My inputs will be from external sources but powered by the same battery, so
they may or may not be connected, but most of the time connected.

But now you have programming, my first instruction would be of course to
enable the GPPU weak pull-ups. So could things get screwed up before that?
But before that would be an internal reset.

Oh well I hope someone can enlighten me.

Leslie Lowe
Hawaii, USA
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2001\05\28@095453 by Byron A Jeff

face picon face
>
> Hello all
>
> Are the internal weak pull-up resistors good for anything or should they be
> avoided at all cost. There is no info in the datasheet on what to use or not
> use them for. Is something as simple as a button ok? My major concern is
> pcboard space.

That's exactly what they are supposed to be used for. They eliminate the
need for a external pullup resistor.

>
> My inputs will be from external sources but powered by the same battery, so
> they may or may not be connected, but most of the time connected.

Probably doesn't matter. The pullup will pull the input pin up if nothing
is connected. Now I wouldn't advise having something hot pluggable, because
you could get spikes when plugging something in...

>
> But now you have programming, my first instruction would be of course to
> enable the GPPU weak pull-ups. So could things get screwed up before that?
> But before that would be an internal reset.

They'll just be normal inputs without pullups until they are enabled, which
will float. But it should be for long.


>
> Oh well I hope someone can enlighten me.

Hope this helps.

BAJ

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2001\05\28@110905 by Drew Vassallo

picon face
> > use them for. Is something as simple as a button ok? My major concern is
> > pcboard space.
>
>That's exactly what they are supposed to be used for. They eliminate the
>need for a external pullup resistor.

This is not entirely true.  They don't "eliminate" the need for external
pullups.  You have to be aware of your requirements for rise/fall times.
The weak pullups are exactly that... "weak."  If you need faster response,
you're better off using 10K, 4.7K, or even 2.2K.  I don't know what the weak
pullups are, but they're probably something in the 100K range.  Maybe the
datasheet gives more information; I've never had to use the pullups.

--Andrew
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2001\05\29@020955 by Leslie Lowe

picon face
Ok so I've thoroughly read the datasheet i think, it says that each of the
PORTB pins has a weak internal pull-up (+/-200 uA typical @ 5V). So correct
me on my math

R = 5V / 0.0002A
Pull-ups = 25Kohm  ?? did I get it right ??

So this should be ok for buttons right?

Ok so how about measuring a pulse width that varies from 1ms to 2ms with an
accuracy of 2us, is this a problem for rise and fall times?

Leslie Lowe
Hawaii, USA

>This is not entirely true.  They don't "eliminate" the need for external
>pullups.  You have to be aware of your requirements for rise/fall times.
>The weak pullups are exactly that... "weak."  If you need faster response,
>you're better off using 10K, 4.7K, or even 2.2K.  I don't know what the
>weak
>pullups are, but they're probably something in the 100K range.  Maybe the
>datasheet gives more information; I've never had to use the pullups.
>
>--Andrew
>
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2001\05\29@022235 by Roman Black

flavicon
face
Leslie Lowe wrote:
>
> Ok so I've thoroughly read the datasheet i think, it says that each of the
> PORTB pins has a weak internal pull-up (+/-200 uA typical @ 5V). So correct
> me on my math
>
> R = 5V / 0.0002A
> Pull-ups = 25Kohm  ?? did I get it right ??
>
> So this should be ok for buttons right?
>
> Ok so how about measuring a pulse width that varies from 1ms to 2ms with an
> accuracy of 2us, is this a problem for rise and fall times?

Yes. I would use a much lower pull-up resistor
for fast stuff. Maybe 1k. Resistors are cheap and
small.

The internal pullups are good for slow and close
things. Like on-board jumpers and dipswitches.
Once the track gets too far from the PIC the
weak pullups might not be enough to kill noise
from the line.

Maybe you could use them for pushbuttons if you
keep the buttons very close and don't mind doing
the extra work with good software debouncing.
:o)
-Roman

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2001\05\29@094229 by Olin Lathrop

face picon face
> Are the internal weak pull-up resistors good for anything or should they
be
> avoided at all cost. There is no info in the datasheet on what to use or
not
> use them for. Is something as simple as a button ok? My major concern is
> pcboard space.

The weak pullups are very handy for connecting switches without additional
parts, particularly normally open pushbuttons.  Yes, I have done this many
times.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, spam_OUTolinTakeThisOuTspamembedinc.com, http://www.embedinc.com

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