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Getting accurate frequency with a 555 — Parallax Forums

Getting accurate frequency with a 555

inakiinaki Posts: 262
edited 2005-11-30 04:07 in General Discussion
I need to generate a·constant square wave of 32768Hz with a maximum error of 50Hz.
I am trying to get that·frequency with a 555.
I get about 32768Hz·at start time·but the frequency changes with time, probably as the components heat, and some time after the frequency is wrong by more than 50Hz which is the maximum allowed error.

Do you think the 555 is suitable for a project like this or should I go straight to a crystal oscillator ?

·

Comments

  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-11-28 12:54
    555s use RC circuits to set the timing, which are the least accurate oscillator. I would get a TTL full or half can oscillator at the frequency.

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    ·1+1=10
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2005-11-28 15:04
    If you do the math, you can see why it really is nearly impossible. Your capacitors and resistors have fairly wide tolerances to begin with.
    1% is 1 out of 100. You need a scope to properly calibrate.

    You are calling for 50 out of 32768. Which is .0015 or .15% You have to consider temperature stability as well.

    A crystal oscillator without temperature control can easily do it. So can the TTL oscillator.

    The TTL oscillator is the simplest because without a scope you can get into trouble with attaching the correct drive circuit, capacitors and resistor to a crystal, but the TTL consumes more power if you already have the drive circuit available.

    I have not seen any resonators for that specific frequency. I believe that is a clock crystal frequency, right?

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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • David BDavid B Posts: 592
    edited 2005-11-28 16:29
    Here's a circuit for oscillating one of those tiny watch crystals at 32768 Hz (I haven't tried it).

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page11.htm

    David
  • Paul BakerPaul Baker Posts: 6,351
    edited 2005-11-28 17:39
    Im thinking something like this: http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2940
    fairly expensive, but you can request a sample.

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    ·1+1=10
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2005-11-29 14:16
    Paul. That is a very nice temperature compensated crsytal, a bit pricey though.

    I was wondering where to get one and for how much as they are 10 times more accurate than the regular crystals.·Timekeeping is about the only place that you need that tight control.

    Regular crystals can be easily bought for under $1, but this is $3.75 if you buy a thousand.· Maybe they would send a sample.


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    "When all think alike, no one is thinking very much.' - Walter Lippmann (1889-1974)

    ······································································ Warm regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

    Post Edited (Kramer) : 11/29/2005 2:22:41 PM GMT
  • BeanBean Posts: 8,129
    edited 2005-11-29 16:17
    Parallax sells the xtal here: http://www.parallax.com/detail.asp?product_id=251-03230

    Bean.

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    "SX-Video·Module" Now available from Parallax for only $28.95

    http://www.parallax.com/detail.asp?product_id=30012

    "SX-Video OSD module" Now available from Parallax for only·$49.95
    http://www.parallax.com/detail.asp?product_id=30015

    Product web site: www.sxvm.com

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    ·
  • pjvpjv Posts: 1,903
    edited 2005-11-29 21:24
    Hi Inaki;

    If 20 to 100 ppm is sufficiently accurate, then I like the Epson SG3030 from Digikey; around a buck, and an operating current of 2 uAmps.

    For a slightly higher price Epson also has better than 5 ppm at 5 uA.

    Cheers from Maui, Hawaii,

    Peter (pjv)
  • BeanBean Posts: 8,129
    edited 2005-11-29 22:08
    Peter,
    I just looked that part up on digikey, and it's over $20 and you have to buy 25 of them.
    Is SD3030 the correct part # ?
    Bean.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "SX-Video·Module" Now available from Parallax for only $28.95

    http://www.parallax.com/detail.asp?product_id=30012

    "SX-Video OSD module" Now available from Parallax for only·$49.95
    http://www.parallax.com/detail.asp?product_id=30015

    Product web site: www.sxvm.com

    Those that would give up freedom for security will have neither.
    ·
  • pjvpjv Posts: 1,903
    edited 2005-11-30 04:07
    Hi Bean;

    Your post really shocked me as I was quite sure last year I paid something a bit over a dollar, so the $28 didn't make any sense to me. Looking at the Digikey site, I now find that in singles the price is $3.12. It seems my memory was incorrect. Also there was a listing for $28 and tape and reel proces of $1.48 for 250. Perhaps it was this latter price I was remembering, anyhow sorry for the mis-communication.

    I really like the low current as a battery will last a very long time.

    Cheers from Maui, Hawaii,

    Peter (pjv)
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