deciding load capacitance of 32 MHz crystal for sys clock in STM32F030C6

Discussion in 'The Projects Forum' started by Dimpi, Oct 15, 2013.

  1. Dimpi

    Thread Starter Member

    Sep 6, 2013
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    0
    Hi all,

    i need to use an external crystal for sys clock in STM32F030C6..whose value is 32MHz .in the datasheet value of load capacitance is not mentioned.then how to decide the load capacitance of the crystal??
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have found 22pf to be fairly optimal, anywhere from 15pf to 22pf should usually work.
    At least when using in conjunction with a PicMicro.
    Max.
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    5pF to 22pF are typical.
    Why are you using a 32MHz crystal?
    8MHz is a more common value to use.

    Why are you using a crystal in the first place?
    The mcu has an internal oscillator that is used on startup. No external crystal is required.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I often use the 32Khz on the T1OSC input, it makes for tighter code when using time periods centered on real time.
    Also if switching to systems with a different MCU CLK, the code remains the same.
    Max.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Misread the OP (again) I thought you had posted 32KHZ for a real time clk!!.
    Max.
     
  6. Dimpi

    Thread Starter Member

    Sep 6, 2013
    96
    0
    helo sir,
    it is mentioned in datasheet of STM32F030C6,that an external 4-32 MHz crystal is to be used for system clock and yes it is mentioned that 32KHZ is to be used for RTC.??

    so can't we use enternal crystal for sys clock and RTC at the same time??i m using it for accuracy purpose.
     
    Last edited: Oct 16, 2013
  7. Dimpi

    Thread Starter Member

    Sep 6, 2013
    96
    0
    helo sir,tnx for rply.
    for accuracy purpose m using external crystal for system clock.in STM32F030C6 datasheet it is mentioned "to use external clock of 4-32 MHz".so what should i do??what value to use....
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    What do you mean by accuracy purpose?

    If you require accurate time-of-day, use the 32768Hz crystal. This is connected to special pins.

    If you need accurate timing such as PWM output or frequency synthesis then use 8MHz crystal. Otherwise it may be ok to use the internal oscillator that requires no crystal.
     
  9. Dimpi

    Thread Starter Member

    Sep 6, 2013
    96
    0
    tnx for d rply sir.
    yes m making 3 ph energy meter project,in which the i/p signal pulse (proportional to instantaneous power {as from WT7752B IC})is applied to the micro(STM32F030C6)...
    so sir if u r saying to use 8Mhz crystal,then shud i not go with datasheet,what are likely outcomes if i use 32 Mhz crystal?Sir please guide.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If your processor allows a external clk to control one of the on board timers then connecting a 32Khz crystal on these pins allows monitoring the 1 sec roll over time.
    This timer will operate independently of the system Clk.
    You would need to consult the data sheet to confirm this.
    Max.
     
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