Pull up resistor

C

Chris W

Guest
Will someone tell me if I am doing this right? I am using the
calculations on this page
http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html
for a 74HCT74. The data sheet doesn't list an Iih, just Ii of +/- 1uA
so I guess that is all the current I need to make an input go high? Vcc
[min] is 4.5V Vih is listed as 2v min so I will use 2.5v for a margin.
Solving the formula on that page in step 5 for R, I get . . .

R = (Vcc[min] - Vih)/Ii or
R = (4.5 - 2.5)/.000001 = 2,000,000

So I can use as a 2m ohm resistor to pull up an input high, right? What
kind of current draw might there be if there were no resistor, just a
direct connection to Vcc? Finally what if I want to pull up 8 inputs on
2 different 74HCT74s do I just use 2,000,000/8 or 250,000 ohms? Right
now I have a 100k ohm resistor pulling up all 8 inputs and it seems to
work fine.

--
Chris W

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give the gifts they want
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Why is it called a pull-up resistor and what are its uses? I'm sorry to
hijack the thread.

Thanks!

"John Fields" <jfields@austininstruments.com> wrote in message
news:tilg415ovimm4ppjguq2dqvqerhiets12i@4ax.com...
On Mon, 28 Mar 2005 12:33:09 -0600, Chris W <1qazse4@cox.net> wrote:

Will someone tell me if I am doing this right? I am using the
calculations on this page
http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html
for a 74HCT74. The data sheet doesn't list an Iih, just Ii of +/- 1uA
so I guess that is all the current I need to make an input go high? Vcc
[min] is 4.5V Vih is listed as 2v min so I will use 2.5v for a margin.
Solving the formula on that page in step 5 for R, I get . . .

R = (Vcc[min] - Vih)/Ii or
R = (4.5 - 2.5)/.000001 = 2,000,000

So I can use as a 2m ohm resistor to pull up an input high, right? What
kind of current draw might there be if there were no resistor, just a
direct connection to Vcc? Finally what if I want to pull up 8 inputs on
2 different 74HCT74s do I just use 2,000,000/8 or 250,000 ohms? Right
now I have a 100k ohm resistor pulling up all 8 inputs and it seems to
work fine.

---
Connect all the unused inputs which are supposed to be tied high
directly to the positive supply and all the inputs which are supposed
to be tied low to ground.

--
John Fields
Click to expand...
 
John wrote:

Why is it called a pull-up resistor and what are its uses? I'm sorry to
hijack the thread.
Click to expand...
You can read about it here:

http://www.seattlerobotics.org/encoder/mar97/basics.html



--
Rikard Bosnjakovic http://bos.hack.org/cv/

Anyone sending unwanted advertising e-mail to my address will be
charged $250 for network traffic and computing time. By extracting
address from this message or its header, you agree to these terms.
 
Yeah, I just found this article and was about to let you know that I found
it.

It's still not basic enough for a beginner but it's good enough (high
impedance and all that) but then again learning is not linear so you gotta
jump here and there and come back to previous material until you get it.

Thanks!

"Rikard Bosnjakovic" <bos@REMOVETHIShack.org> wrote in message
news:QI42e.133515$dP1.471223@newsc.telia.net...
John wrote:

Why is it called a pull-up resistor and what are its uses? I'm sorry to
hijack the thread.

You can read about it here:

http://www.seattlerobotics.org/encoder/mar97/basics.html



--
Rikard Bosnjakovic http://bos.hack.org/cv/

Anyone sending unwanted advertising e-mail to my address will be
charged $250 for network traffic and computing time. By extracting
address from this message or its header, you agree to these terms.
Click to expand...
 
John wrote:

It's still not basic enough for a beginner but it's good enough (high
impedance and all that) but then again learning is not linear so you gotta
jump here and there and come back to previous material until you get it.
Click to expand...
If learning was linear I think that, paradoxally enough, you wouldn't
learn anything.


--
Rikard Bosnjakovic http://bos.hack.org/cv/

Anyone sending unwanted advertising e-mail to my address will be
charged $250 for network traffic and computing time. By extracting
address from this message or its header, you agree to these terms.
 
On Tue, 29 Mar 2005 06:41:40 GMT, Rikard Bosnjakovic
<bos@REMOVETHIShack.org> wrote:

John wrote:

It's still not basic enough for a beginner but it's good enough (high
impedance and all that) but then again learning is not linear so you gotta
jump here and there and come back to previous material until you get it.

Sounds like mosaic ala Marshall McLuhan.
Click to expand...

If learning was linear I think that, paradoxally enough, you wouldn't
learn anything.
Click to expand...
Possibly. Pattern recognition based on language skills is (I think) the
basis of all rational thought and therefore learning. I think you can
learn linearly if the pattern is linear or an extrapolation of previous
linear patterns.
 
"Rex" <rex@yahoo.invalid> wrote in message
news:560i41plspmcifalffdvsuq5a63i3n1h65@4ax.com...
On Tue, 29 Mar 2005 06:41:40 GMT, Rikard Bosnjakovic
bos@REMOVETHIShack.org> wrote:

John wrote:

It's still not basic enough for a beginner but it's good enough (high
impedance and all that) but then again learning is not linear so you
gotta
jump here and there and come back to previous material until you get
it.

Sounds like mosaic ala Marshall McLuhan.

If learning was linear I think that, paradoxally enough, you wouldn't
learn anything.

Possibly. Pattern recognition based on language skills is (I think) the
basis of all rational thought and therefore learning. I think you can
learn linearly if the pattern is linear or an extrapolation of previous
linear patterns.
Click to expand...
Language skills are not required for learning. You DO learn through language
skills faster but it requires you to relate to the vocabulary that's being
used.

It's like building a house. You stop from bottom and you build to top. At
the lowest level are all your experiences/observations. As you go to the
top, things start getting more language specific and abstract. At the top
levels you don't need as much experience since you have founding experiences
to build on.

Anyway... since I'm still in school, I'd like to say that best teachers are
those who relate topics to real world experiences, not those who can say 10
technical terms in a minute. The technical terms won't make sense to those
who don't know those terms (haven't related it to their knowledge...they
know the same thing you do but they haven't put a label to their knowledge).

So when you teach you explain in terms that can easily be picked up from
experience (known as layman's terms) so that people can relate their
knowledge/experiences/observations to yours and finally you put a label to
those things so that they know exactly what you mean next time you say that
term.

I hate teachers who think they are the shit when they speak in a formal
manner all the time with a bunch of buzz words when they are trying to teach
me something because they're not there to teach me but instead to stroke
their own egos or they are ignorant to how the brain works (which I think is
important to know when you become a teacher if you're to be successful).

Abstract concepts are easier learned by yourself and not at school. Unless
you have a lot of experience using/dealing with highly technical terms you
won't get much from listening to someone say it (you have to go at your own
pace until you assimilate the word...sometimes it's hard to assimilate
because you haven't been told the whole story).

Anyway, I can go on forever like this, I can even write a whole book based
on learning and other related topics but then again, I have better things to
do with my time, like learn electronics and create some sweet gadgets :)

Thanks listening to my rant!
 
On Mon, 28 Mar 2005 12:33:09 -0600, Chris W <1qazse4@cox.net> wrote:

Will someone tell me if I am doing this right? I am using the
calculations on this page
http://www.interfacebus.com/IC_Output_Input_Pullup_Resistor_Values.html
for a 74HCT74. The data sheet doesn't list an Iih, just Ii of +/- 1uA
so I guess that is all the current I need to make an input go high? Vcc
[min] is 4.5V Vih is listed as 2v min so I will use 2.5v for a margin.
Solving the formula on that page in step 5 for R, I get . . .

R = (Vcc[min] - Vih)/Ii or
R = (4.5 - 2.5)/.000001 = 2,000,000

So I can use as a 2m ohm resistor to pull up an input high, right? What
kind of current draw might there be if there were no resistor, just a
direct connection to Vcc? Finally what if I want to pull up 8 inputs on
2 different 74HCT74s do I just use 2,000,000/8 or 250,000 ohms? Right
now I have a 100k ohm resistor pulling up all 8 inputs and it seems to
work fine.
Click to expand...
---
Connect all the unused inputs which are supposed to be tied high
directly to the positive supply and all the inputs which are supposed
to be tied low to ground.

--
John Fields
 
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