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pull up and pull downs

Discussion in 'Electronic Basics' started by [email protected], Apr 17, 2006.

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  1. Guest

    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
     
  2. Dan H

    Dan H Guest



    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
     
  3. Brian

    Brian Guest

    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
     
  4. Guest

    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
     
  5. Brian

    Brian Guest

    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
    http://www.doctronics.co.uk/ldr_sensors.htm and
    http://www.interq.or.jp/japan/se-inoue/e_pic6_5.htm.
    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
     
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