Pierce oscillator not oscillating

Discussion in 'General Electronics Chat' started by hspalm, Oct 21, 2011.

  1. hspalm

    Thread Starter Active Member

    Feb 17, 2010
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    Hi,
    so I'm trying to make a pierce oscillator circuit. It's said it's the easiest and most common oscillator circuit for crystals, but how screwed am I without the crystal datasheet?

    [​IMG]

    My crystal is a 32.768 khz watch crystal. R1 is 2.2K, RF is 10M, inverter is SN74LS04.

    I have tried capacitors 19pf, 22pf and 68pf.

    As you may know about know, I'm not much into these analog circuits... Hence the trial and error thing. All I want is to clock my AtTiny13 to make a stepper motor clock. The thread from where I got the picture (but not the values I've been trying) suggested another inverter gate as a buffer for the signal, which answered one of my wuestions but left me two:
    What type of signal should I expect to see on the first inverter output? And could the circuit be so sensitive to external capacitance and that's why I don't see anything on my scope (probe capacitance?)? What I'm seeing is a somewhat noisy 50mv DC.

    I appreciate any hints and help, as always. I appreciate it very much!
     
  2. #12

    Expert

    Nov 30, 2010
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    You are expecting square waves almost as high as the power supply voltage. The buffer amp will keep the probes from upsetting the oscillator. You can NOT build this on a plug-in type of hobby board. Too much capacitance wandering around inside it.
    That's all I have for you.

    Next!

    Edit: Correcting myself: I don't see how you can make this work with a plug-in board when 10 or 20 pf is the right value for the capacitors.
     
    Last edited: Oct 21, 2011
  3. Wendy

    Moderator

    Mar 24, 2008
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    Actually, I think your crystal data sheet is probably the least of your problem, not that it is a big problem.

    Recently I needed to design a similar circuit using the same crystal and a 4060. I made an interesting discovery, there are about 6 different 4060 data sheets out there and they are all different. I went with the easiest to implement with the parts I had, and it worked 1st time.

    BTW, it was on a protoboard, it is still working and holding within 4 seconds per day.

    If you want to see the lay out...

    Digital clocks

    Post 12
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
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    What is the input current loading of a 74LS04? I'm no expert in these matters - going to try to look it up, but 10Mohm Rf sounds way to big if that thing is LS TTL.

    Or this may be utter hogwash, but those component values look CMOS ish to me.

    Edit: Here is the datasheet: the input currents look far too big to me.

    http://www.ti.com/lit/ds/symlink/sn74ls04.pdf
     
    Last edited: Oct 21, 2011
  5. Wendy

    Moderator

    Mar 24, 2008
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    There was a similar thread just under a year ago using a CMOS 555.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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  7. hspalm

    Thread Starter Active Member

    Feb 17, 2010
    201
    8
    Thank you for your effort in helping me out on this.
    I've decided to try the NE555 version from this thread http://forum.allaboutcircuits.com/showthread.php?t=51574&highlight=32.768&page=3. Thanks for the tip Bill Marsden.

    My only remaining question before I try it out, what goes to pin 5 and 7 on the NE555?

    Edit: I left them floating just to test, and it worked! Unfortunately not at 5 volts (I got about 5KHz). But for good measure, my above question still stands.

    One more thing. You guys are really awesome. You go out on the internet, search for applicable application notes, data sheets etc. You analyze my circuits and really go about your way to understand every single wrong I've made. And then on top of it, you never treat a dumb question as a dumb question. I've seen people here writing essays of information directly to people who clearly have NO intention of trying anything else then free circuits and schematics. Thumbs up!!
     
    Last edited: Oct 22, 2011
  8. #12

    Expert

    Nov 30, 2010
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    Pin 5 is an internally generated voltage. It is one third of the distance from Vcc to ground. It is the reference for the "threshold" comparator. It is called the "control voltage". It can be controlled externally to change the switching voltage of the threshold comparator. A capacitor is usually applied to pin 5 to hold the voltage steady during switching.

    Pin 7 is the discharge pin. It is connected internally to a transistor that conducts to ground at the right time. That transistor discharges the timing capacitor in most applications.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    If you look at the 555 Hysteretic Oscillator it also doesn't use 5 or 7. This is because int his configuration the 555 is a inverting Schmitt Trigger.
     
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