why pullup resistor and why the capacitors in the mC oscillator ciruit ?

Discussion in 'Embedded Systems and Microcontrollers' started by Embedded Lover, Feb 23, 2009.

  1. Embedded Lover

    Thread Starter Member

    Dec 22, 2008
    26
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    Hi All ,

    I have 2 questions here ,

    I am using 8051 mC and when using porto , it says I have to connect it to pullup resistor . I know pullup resistor is not for mC work only it has a general function I want to know it well .

    Second question ,

    [​IMG]

    For clock generation a crsytal oscillator is used as picture above.when connecting the crystal oscillator , they also connect 2* 30pF capacitors I want to know their function , why they are here ?

    also the reset pin has the capacitor and resistance. why they are here and what determines their values ?.


    Thanks in advance .
     
  2. silvrstring

    Active Member

    Mar 27, 2008
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  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    And for the capacitors with the crystal, take a look at the attached PDF.

    Greetings,
    Bertus
     
  4. Embedded Lover

    Thread Starter Member

    Dec 22, 2008
    26
    0
    Thanks very much Silvrstring and Bertus

    but I didn't get the capacitors Idea yet .
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    The resistor and capacitor on the RESET line exist so that when power is first applied, the RESET line is pulled high until the 8.2k resistor drains enough charge from the capacitor until the RESET line falls to a logical low level. This helps to ensure that the mC starts up in a known state.
     
  7. ahwaheed

    New Member

    Feb 25, 2009
    4
    0
    these capacitors for noise immunity and to get a stable oscillation from the crystal oscilator.
     
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