pierce oscillator not oscillating

Discussion in 'General Electronics Chat' started by LeonidElect, Jul 8, 2015.

  1. LeonidElect

    Thread Starter New Member

    Jul 8, 2015
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    Hello,
    Recently I have created PCB with Pierce oscillator. Unfortunately it is not oscillating at all.
    Adding the schematics below.

    XTAL is - LFXTAL002996BULK - frequency of 32.7680kHz and load capacitance - 12.50pF
    I have tried to work with capacitors : 15pF and 30pF
    I have tried to remove R1 and to put it to value of 10Mohm
    I have tried to set R2 to 2Kohm or to 158Kohm.
    input of my u2 (A) is always near 1.2v and the output is the VDD (2.5V)
    As I understand the inverter understand the 1.2v as logical '0' so the output is always logical '1'.
    I have already tried manually to set A to 0v and to 2.5v to make the oscilator awake but no luck there.

    Waiting for your help


    upload_2015-7-8_15-41-39.png
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
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    I'm guessing that the inverter isn't performing at a Vdd of only 2.5V. Temporarily run it from 5V to see if that starts oscillations.
     
    DickCappels likes this.
  3. DickCappels

    Moderator

    Aug 21, 2008
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    You are using a Schmitt trigger, and an HC version at that. You need something that can be biased into its linear, or at least analog region, such as a CD4009A, CD4007A or equivalent unbuffered device. A-series gates can be used too, but be sure to tie the unused inputs to Vdd or Vss as appropriate.
     
  4. LeonidElect

    Thread Starter New Member

    Jul 8, 2015
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    I will try it asap. Thanks
     
  5. DickCappels

    Moderator

    Aug 21, 2008
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    Toward MikeML's comment about the 2.5 volt supply, the A-series CMOS parts are rated to run down to 3V; 2.5V might not work.

    I didn't notice the low power supply voltage. Good catch, MikeML.
     
  6. LeonidElect

    Thread Starter New Member

    Jul 8, 2015
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    I have tried now the 3.3v and it seems like the 1.2v in the input of inverter results with 3.3v on output. Is it worth to try with 5v or the Shmid-Trigger is bad for this oscillator anyway?
     
  7. pwdixon

    Member

    Oct 11, 2012
    488
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    If you short U2/2 to GND/VDD does the output of U2 toggle?
    Just to see the circuit oscillate you could try shorting the crystal perhaps with the addition of a larger cap across C2 to get a non-stabilised oscillation just as a test, the frequency would obviously be a function of C2 (total) and R2.
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    What kind of device did you try, and how was it connected?

    No, you need an analog amplifier to handle the small signal through the crystal, and Schmitt triggers are designed so their output is always saturated in one binary state or the other, or rapidly switching between states.

    By the way, a bypass capacitor on the power supply is a very good idea with this circuit, and you should probably limit the power to the crystal according to the example of R2 in the schematic below.


    [​IMG]
    Note: The circuit above should have a capacitor like 0.1 uf or so from Vdd to Vss.
     
  9. LeonidElect

    Thread Starter New Member

    Jul 8, 2015
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    Can try the Inverter below? just because it matches my PCB sizes :
    TEXAS INSTRUMENTS - SN74LVC1G04DBVR - IC, INVERTER, SINGLE, SMD, SOT-23-5
     
  10. DickCappels

    Moderator

    Aug 21, 2008
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    The one you mentioned has high output current capabilities, suggesting that it has a buffered output and might not be the best choice because of higher power dissipation than you might otherwise need. Silicon Labs recommends using the SN74LVC1G04 for this kind of oscillator, so if it fits your requirements, that one would have a much better chance of working satisfactorily.
     
  11. LeonidElect

    Thread Starter New Member

    Jul 8, 2015
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    0
    Thanks a lot. I will try it :)
     
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