# Pull up resistor

• posted

Will someone tell me if I am doing this right? I am using the calculations on this page

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.
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Chris W

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Connect all the unused inputs which are supposed to be tied high
directly to the positive supply and all the inputs which are supposed
• posted

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

Thanks!

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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!

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

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

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

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Sounds like mosaic ala Marshall McLuhan.

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.

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gotta

it.

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!

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