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pull up and pull downs

hiiii guys,

I am new to this group and have read the discussion about pull up resistor. Its really good explained. My problem is related to the same topic. First of all when to use pull up and when to use pull down? I mean how do I differenciate the applications. I know some basic things but still its not totally clear to me. Specifically talking about microcontroller/microprocessor...I can use pins as input as well as outputs. But i have seen both applications and got at some instant confused why here pull up and not pull down or why pull down and not pull up, because it seemed okay if i do exchange one with another..Second thing about the value of pull ups and pull downs. I do confuse there also sometimes...

thanks

Nishant

Reply to
NishantZala
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There are 2 general conditions where pull up or down is used:

If you have unused inputs a pull up or down is used to establish the proper input condition. For examply a 4 input gate where only three inputs are used.

On an active input is common that circuit drivning the input only forces on state at the input leaving the other state to made by the pullup or pull down resistor. This would be something like an on/off switch or an open collector output driving an input.

Dan

Reply to
Dan H

Dan covered some input conditions. There are also some output conditions. Some logic devices will use a NPN transistor as an output, where internally the emitter is grounded and the base is switched on and off, but the collector is the output (with no pull -up component). Depending on what you want the output to drive, determines what value resistor you will need (as a pull-up resistor). For example, will it be driving another logic input or maybe a LED (with a resistor in series with it). Brian

Reply to
Brian

Thanks brian and Dan first of all..Actually I know some basic things like not to float logical inputs you need pull down and to drive output devices from open collector you need pull ups. But suppose I have some analog sensors which i need to sense through micro analog channels,what should i select not to let the pins float? Because I have seen both applications in that case pull up and pull down. Or suppose I want to connect some micro output pins to a CMOS Asic inputs what can i use...because I thought i can use both in the last case where i just need to fix the input pins either to gnd or to vcc...once again I am totally new to analog world but very curious to know lot of practical things which you need in practical life....

Thanks..

Nishant

Reply to
NishantZala

There are a lot of books written on what you want to know. There is also a lot of information on the Internet. To get you started, take a look at

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and
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Read and study these. If you have some questions after taking a close looks at these, come back and ask. We will be glad to help you. Brian

Reply to
Brian

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