URGENT!!! PLZZZ need help 32.768 KHz oscillator. Need schematic, PCB and working code

Discussion in 'General Electronics Chat' started by t06afre, Apr 8, 2011.

  1. t06afre

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    May 11, 2009
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    Ok hope my title did get me some attention:D
    I am working on a digital clock constructed with a CPLD circuit. And I will be using a 32.768 KHz oscillator as the master clock source. Do any here have a working simple circuit using 74HC or 74HCT logic. I have no experience with clock crystals.
     
  2. eblc1388

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  3. R!f@@

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    Apr 2, 2009
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    Are u kidding....
     
  4. retched

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    Dec 5, 2009
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    HAHAHAAHHAHAH!!!

    Actually, I think this is no joke.

    If it were a joke, t06 would have posted it in Off-Topic.


    T06afre. are you looking for a 7400 series based circuit to divide the 32k crystal down to a 1-hz pulse?
     
  5. t06afre

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    No I am making a standard clock that shows minutes and hours inside a CPLD http://www.altera.com/literature/ds/m7000.pdf. But I need to drive it with a 32.768 Khz osc. I have heard the 32.768 Khz crystals may be more picky than crystals for higher frequecys.
    [​IMG]
    This is my normal osc setup. But I am unsure it will work with a clock crystal. So to save some time I am asking you for advice
     
  6. Wendy

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    So a 555 or a CMOS is out?
     
  7. t06afre

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    I have a good supply of 74HC04 circuits;). That is why. I would in normal cases have used a PIC. But this project will be done with none PICS. I will only connect the 32.768KHz crystal oscillator and the 7 segments to the CPLD. All other logic will be inside the CPLD.
     
  8. Wendy

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    Well, the 74C04 is CMOS, and the 555 circuit had a CMOS version. A straight inverting Schmitt trigger seems to hold promise, but not too many people have experience with a CPLD (at least I didn't).
     
  9. jpanhalt

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    Check out AN118 and AN340 from Fairchild.

    John
     
  10. SgtWookie

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    The circuit you posted will ring the dickens out of the crystal, because you don't have any limits on the drive to it, or limits on the output voltage. Might work, but I wonder how driving it that hard will affect the stability and life of the xtal, not to mention the HC04's.
     
  11. t06afre

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    So what do you suggest Sarge...a Pierce oscillator properly constructed?
     
  12. SgtWookie

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    Yes, a Pierce oscillator, preferably with a 74HC14 or other Schmitt-trigger device that has an inverted output. One of the risks is operating in overtone mode, which will give you some odd multiple of the desired frequency. You add a series resistor between the output of the inverter and the xtal to limit the drive the xtal sees. What size resistor you'll need depends on your xtal.
     
  13. THE_RB

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    I think it might be a good project to work out how to use the gates in the CPLD to drive the xtal. It sounds like this is something you may want to do on many of the CPLD projects (use a xtal!).

    It should be as easy as using the CPLD as some inverter pins and using a typical inverter xtal driver. And like SgtWookie said those little 32kHz xtals are very low power so use a resistor.
     
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