Understanding the pull up resistor

Discussion in 'Embedded Systems and Microcontrollers' started by Dragonblight, Aug 26, 2009.

  1. Dragonblight

    Thread Starter Active Member

    Aug 19, 2009
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    It seems that more often than not, documents and datasheets for microcontrollers call for pull up resistors (ex. the I2C specification sheet).

    Can anyone explain what these resistors from Vdd to the data/clock bus are for?
     
  2. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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  3. rushi53

    Member

    May 8, 2009
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    There are many reasons behind using Pullup registers
    Some of them are:

    • Current Outsourcing capacity of IC is less than required for a output device, in that case we need to source extra current using pullup register.
    • Output of IC is floating, i.e. neither 0 nor 1, in that case the voltage needs to be pulled high to VCC, i.e. logic 1

    Let me know if you need any more help


    ----------------------------------------------------------
    Thanks,
    Rushikesh
    www.projectsof8051.com
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    If you have something that just switches on & off, whether a physical switch (like a button or keyboard switch) or an open-collector transistor, with one side to ground or 0V, the state of the other side (call it the output) is not defined when the switch is off; the output is floating.

    A pull-up resistor between that output and V+ means you have two definite states: switched to ground and pulled to V+.

    Likewise, you can have a pull-down resistor to 0V when the switch is connected to V+.

    The other common reason for a pull-ups is when more than one device needs to control the same signal. That is the case with the I2C bus, each device has an open-collector output switching to ground, so any device can pull the bus line low without 'fighting' other devices.

    The actual value of the pull-up depends on the circuit. Something that needs a fast responce or has long wiring, or in an electrically noisy environment will use a fairly low value, eg. 220 Ohms.

    A short connection between two components within a circuit board may use 4K7 or 10K, it's all down to the application.
     
  5. Dragonblight

    Thread Starter Active Member

    Aug 19, 2009
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    You guys and your answers are amazing!

    Thanks!
     
  6. rushi53

    Member

    May 8, 2009
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    You are always welcome....

    Can you let us know your purpose behind using pullup register.

    I mean what is your project/circuit details ?
     
  7. Dragonblight

    Thread Starter Active Member

    Aug 19, 2009
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    There was no particular project for this one, being self taught (pretty much exclusively through datasheets) I understand the concepts without some of the basic terminology.
     
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