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Why aren't computer clocks as accurate as cheap quartz watches?

Discussion in 'Electronic Basics' started by [email protected], Oct 26, 2005.

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  1. Guest

    Why do the battery powered clocks in personal computers tend to keep
    worse time than quartz watches, even the $1 ones?

    The computer batteries measure fine, at least 3.15V.

    I thought that the problem was temperature swings in the computers
    (25-38C), but a couple of cheapo watches taped inside the computers
    kept better time.
  2. John Doe

    John Doe Guest

    Maybe the same reason we didn't have built-in clocks until after there
    were five dollar LCD wristwatches. I don't know.

    My current clock is very accurate (MSI mainboard).
  3. mike

    mike Guest

    Supply and demand.
    If your watch kept bad time, you'd send it back.
    If your computer keeps bad time, you'll reset the clock
    and bitch about it on the internet.
    Vendors care if you send it back. They don't care if you
    bitch on the internet.

    Wanted, Serial cable for Dell Axim X5 PDA.
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  4. jakdedert

    jakdedert Guest

    I've often wondered the same thing...also why a $9 alarm clock will have
    provision for battery backup, but $200 VCR (back when VCRs were $200)
    needed to be rweset with each blip in the mains voltage.

    FWIW, there are utilities which will update your computer clock from the
    National Bureau of Standards over the web.....

  5. Impmon

    Impmon Guest

    That question have been around for more than 20 years and probably
    longer. Even one PC magazine reported a major branded PC that used to
    cost $2,000 couldn't keep time as well as a $5 watch from Kmart (not
    exact quote but similiar to that)

    The OS could check via internet at regular intrevials to make
    correction to the clock but that is if the PC does have internet
    connection at all.
  6. somebody wrote
    Who is the manufacturer? I only use Gigabyte(mostly) or MSI (sometimes)
    motherboards. I find the clocks to be reasonably accurate. My guess
    would be that the OEM didn't ground the crystal case or use the proper
    guard ring techniques for noise reduction. Do you find that they run
    fast or slow?
  7. James Sweet

    James Sweet Guest

    I have one PC that will lose or gain hours at a time if the machine goes
    into standby, another that does ok but still drifts enough that I have
    to use a utility to keep it in sync with the atomic clock. On the other
    hand a Sun workstation that I fire up occasionally to play with keeps
    excellent time, even after sitting unplugged for 6 months it's usually
    within a few seconds but then it was originally $25K.
  8. Don Bruder

    Don Bruder Guest

    Dunno if it's still true in PC-land - I've been living in a Mac world
    for a LONG time now - but when I was playing with them years ago, the
    battery-backed real-time clock was read once at startup to set the
    computer's software clock, which then kept time by counting clock
    interrupts generated by the motherboard timing circuitry. That
    interrupt, like any other "not non-maskable" interrupt, can be blocked
    out for various reasons by various things, causing the software clock to
    lose time. Usually, the amount of "lost" time isn't really noticable
    except on "continuously on" machines. But with long periods between
    restarts, heavy use of software (or firmware... The blame may not be in
    your clock, but in your ROM code) that disables interrupts often or for
    extended periods, and/or no intervention (be it human or software)
    happening, it can grow to substantial amounts of time surprisingly quick.

    Immediately after startup, the clock SHOULD be reasonably close to
    right, since it will have been freshly set from the battery powered
    clock, but after a while, it *WILL* go wonky unless steps are taken to
    correct the drift - It's just the nature of the beast.
  9. Guest

    I'm looking for a techical explanation.
  10. Guest

    The only PC motherboards I've seen that consistently kept accurate time
    all had Dallas clock chips in them, and some of those chips were still
    running properly from their internal lithium cells (permanently
    encapsulated) 10 years later.
  11. Guest

    Most older motherboards used the Dallas real-time clock chip, which had
    a cheapo oscillator built in, and could use an external crystal for
    precision timekeeping. A good crystal will add a couple of dollars to
    the price of the motherboard, but the clock ic itself was over ten
    bucks. I use these chips in some other pieces of equipment and they are
    very accurate when given a good crystal for a timebase.

    I don't know what they put on modern motherboards. I throw computers
    out when they stop working now.

    However, the datasheet for the Dallas chip gives some hints that may be
    useful here. If the chip is put in an electrically noisy device like a
    computer, it can be accelerated by stray signals picked up by the clock
    circuit. I believe that NTP machines (usually) set the RTC on shutdown,
    as the cpu is more likely to be on time if it's been syncing itself
    with an atomic standard.
  12. James Sweet

    James Sweet Guest

    Yeah the Sun has a Dallas in it, in fact I had to hack in a new battery
    in one of them as the original went dead and modern chips won't work in
    it. Come to think of it I had a 486 PC with a Dallas, I think that was
    the most recent though.
  13. w_tom

    w_tom Guest

    Watches have an internal capacitor to adjust for each
    crystal. I have never seen that capacitor on motherboards
    since (I believe it was) the IBM AT. Furthermore, the PC
    clock operates at two significantly different voltages that
    will change crystal frequency. Battery voltage and voltage
    when PC is powered will cause additional fluctuation. Which
    voltage should they adjust the capacitor to? Just easier to
    not install and adjust the capacitor.
  14. Jim Adney

    Jim Adney Guest

    I recently discovered that early Sony Beta VCRs (at least) actually
    had back-up NiCad batteries which would keep the clock going for
    seveeral minutes of blackout. I just replaced one today, with a NiMH,
    and will replace a couple more in the next few days.

    I'm sure I knew about these at one time, but I just forgot about them
    until other things got me lnside that VCR again.

  15. Jim Adney

    Jim Adney Guest

    There are 2 causes.

    The first cause is that the internal clock (real time clock) on the
    motherboard is a device which the motherboard manufacturer much buy
    from someone else. They can buy this item in various degrees of
    precision, but the cost goes up as you request more precision. Since
    there is lots of pressure to keep cost down, but not so much pressure
    to keep good time, the motherboard manufacturers don't press for
    highly accurate real time clocks.

    The second reason was explained by someone else. It's the fact that
    the OS clock can fall behind the real time clock due to the way the OS
    is written. This error is minimal when your computer is first booted
    up, but it can become significant if it is left running for days. This
    error is reset to zero each time you reboot.

  16. Guest

    Well I can't answer your question but for those who don't know, XP has
    an automtic time synchronization feature which operates weekly. You
    have to be on line of course. The synchronization can also be done
    manually. See: Start>Control Panel>Date and Time. My pc keeps time
    very well.

  17. Andy Cuffe

    Andy Cuffe Guest

    I've always wondered why the batteries in macs run down so quickly. I
    rarely see a PC newer than 10 years old with a bad battery, but I
    consistently see 3-5 year old macs with totally dead clock batteries.
    Macs even have a much larger (and more expensive) lithium cell than
    most PCs. I've seen a few 15 year old 486's with the same type and
    brand battery used by apple that still measures full voltage.

    I think the answer to the original question is just no quality control
    for clock accuracy. My current PC is pretty accurate, but it's still
    not as good a cheap watch.
    Andy Cuffe

    <-- Use this address until 12/31/2005

    <-- Use this address after 12/31/2005
  18. DevilsPGD

    DevilsPGD Guest

    In message <> Jim Adney
    And at the end of the day, most users have internet access these days,
    so it's less critical since the clock will be updated by most modern
    operating systems automatically.
    It's worth noting that an OS can correct for this too, either by using
    an external time source when available, or periodically resynchronizing
    from the BIOS clock if no reliable external source is available.
  19. Franc Zabkar

    Franc Zabkar Guest

    It's true for Win98, but I don't know about XP.

    Just for fun, here's something interesting that I discovered recently:

    I could make time run backwards on a Win98SE machine by doing
    something innocuous.

    -- Franc Zabkar

    Please remove one 'i' from my address when replying by email.
  20. mike

    mike Guest

    Very often, the economic or political considerations
    dwarf any technical consideration.
    The technical explanation is that they do what's cheap.
    Wishing for a grand technical reason won't make it so.

    Wanted, Serial cable for Dell Axim X5 PDA.
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Bunch of stuff For Sale and Wanted at the link below.
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