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

On Wed, 26 Oct 2005 23:25:49 GMT, Don Bruder put finger to keyboard and composed:

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.

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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. mike

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You seem to be saying curiosity is a worthless trait.

I bet you're a republican.

I'm not absolutely certain on that myself, though many explanations have been put forth over the years, with the one I think is probably "the real situation" being that Macs (A) Don't cut the battery out of the circuit when powered up and (B) the battery isn't just keeping the RTC running, but also keeping a chunk of memory (which we call "PRAM" here in Mac-land - holds various fairly-to-really critical information) alive.

I don't know for certain about measured voltage, but I've only actually

*NEEDED* (as opposed to "shotgunning" a startup issue) to replace one battery in my Hmmm... I guess that would be about 8 Macs over the last 15 years or so. That was in a Performa 637CD that I picked up at a thrift store for ten bucks. The machine I'm typing on, a PowerMac 7500, came to me secondhand also, and as far as I have any way to know, it's still running on the factory-installed battery - 10+ years since it came off the line.

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Nope, I'm saying insisting on technical explanations for economic decisions is a worthless trait. Anybody with a wrist likely understands that it's possible to keep accurate time. If computer users rated it high in their purchase decision, you'd see very accurate clocks. It's all about the Benjamins...

I'll check my voter registration and get back to you. mike

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File that one under D for DUH.

The OP was asking for a technical explanation, and so far the responses have been enlightening. Well, except for yours.

There is no technical explanation except that the technology that is being used does not guarantee accurate clocks.

If you do the math, you'll uncover the fact that a wristwatch is phenomenally accurate compared to a RTC crystal.

I haven't been responsible for a computer design since 1989. Back in the day, the philosophy was, "design for the center of the statistical distribution and fix it in software." Fortunately, UINX was smart enough to do time correction.

I haven't been responsible for a frequency counter design group since

1975. Back in the day, the philosophy was, "use the cheapest timebase that guaranteed the specified accuracy."

I've had motherboards where they saved a nickel by leaving off the two caps on the Xtal. Adding the caps helped, but "net time" fixed it in software.

Are we seeing a trend yet? You can get any accuracy you're willing to pay for. Computer users have voted with their wallets for "lousy". I don't remember ever seeing a specification for real time clock accuracy on a motherboard. So if the clock ticks, it's in spec. Statistically, you'll sometimes get one that's unacceptable and some of those will get bitched about on the internet. It's the same reason that sometimes your Ford won't run right.

You're the Chinese engineer. Go tell the bean counter that you want to add 20 cents worth of parts to adjust the clock frequency, add $4000 worth of capital equipment to each production station, a week of additional production line time to setup and program the equipment,

30 seconds of operator time to each board test and decrease the overall yield.

It really is all about the Benjamins.

Let me restate it in technical terms. You get what you pay for, if you're lucky.

And yes, my motherboard keeps very good time (but still not anywhere near as good as my wristwatch). It's not because the design is different from any other motherboard design. It's because all the variables conspired to keep good time. I got lucky.

Benjamins!!!

mike

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Not one single person has mentioned that their computer keeps great time. My first personal computer was back in '81 and it didn't even sport a clock. Today I have collected about 16 computers (most of them are in the closet). But the majority of them keeps very good time. Some haven't been fired up in years, and it is really rare to find one off more than 5 minutes. So I am at least one user who has been quite happy with my computer clocks.

______________________________________________ Bill (using a Toshiba 2595XDVD & Windows 2000)

-- written and edited within Word 2000

Hi!

Hmmm...not all of them do. I've got a Panasonic VCR that will hold the time for a couple of hours after a power failure and a Sony that seems to be good for a few days. (Of course, both are 'mid-range' machines and the Sony might qualify as lower high end equipment.)

I also have a Panasonic time-lapse VCR that has a NiCad battery in it. I know it will the clock for at least two weeks. The manual says it could last for a month.

William

Hi!

Is that a DS1387 or something else?

I've got some microchannel-based computers that use that module. None have died yet but I know it is a matter of time.

How did you get the module open? Do you have pictures?

William

They call themselves NIST these days, and they also still have a dialup service.

Hi!

Yep, I've seen the same thing here. I have a number of old Macs (6100,

6300, LC, 9600, SE/30) that still keep decent time on the factory-installed batteries. Some of these have gotten to the point where the machines need to be powered up somewhat often to keep the settings intact.

By compare I have many a G3 CRT-type iMac around (the 2001 models,

350~500MHz) and almost all of them have had to have their batteries replaced.

Looking at things I can see one difference. Most PCs new and old power their CMOS RAM chips from the power supply when they're running. A great many ATX systems seem to keep the CMOS RAM and clock running from the ATX standby supply as long as it is running. Some older PCs also have NiCad or NiMH batteries onboard along with the circuitry to charge them when powered up.

I've investigated the Macintosh a little bit and it looks like the clock/NVRAM battery is constantly pulled upon, even when the computer is on. This could also shorten battery life.

William

Hi!

Okay then, I'll jump in here. I was just waiting for someone to say it. :-)

I am a collector and operator of the IBM PS/2 line of computers. I have a lot of them and most are powered up and running fairly often. Some run 24/7 as servers.

The clocks on these things are--for the most part--extremely accurate. I synchronize the clocks on my computers by way of the 'net to one of the many network time protocol servers in the world. The synchronization happens at least once a week, sometimes more often. I don't think I've ever seen one drift more than a second or three between synchronizations. Of course, it does depend upon the health of the clock battery. Most people never changed them during the lives of these computers...so they still run, but are rather weak. I usually replace them immediately just to avoid a leaking episode.

You can also use these computers as rather large and heavy digital clocks--they have a flourescent display panel near the power switch that is normally used for power on self test codes. Fortunately, it is is user/software-addressable and can be misused in a variety of fun ways.

The only bad thing about these is that these PS/2s make the clocks in almost all of my other machines look like a sad joke. :-)

William

I wanted to add, my alarm clock has a nicad battery backup, which is why I bought it. Changing batteries is rediculous and expensive. Why take the time and expense of changing batteries when a built in nicad is all you need. I have also seen various VCR's that seem to hold time for a certain period.

greg

I have seem many computers loose time, and a low battery seems to increase the loss. An online computer can also be reset. There are programs which sync to Universal Time, and networks, and even mail programs can perform syncronization automatically.

greg

Making a accurate oscillator requires two key things

1. An accurate crystal (note stability is not the same as accuracy)
2. An oscillator circuit design that does not "pull" or otherwise change the basic frequency of the crystal.

Clearly if you design a watch these are primary concerns, so even a cheap watch is done well

For a computer this not a primary concern and I am sure there is a wide range of accuracy in different models of computers depending on the amount of attention paid to these issues. My current computer with an intel mother board does keep could time.

Dan

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 wrote in message 
news:1130364184.236610.90350@g47g2000cwa.googlegroups.com...
> 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.
>

That second means nothing in itself. "IBM PC" type computers have a tiny bit of static RAM to hold the bios settings. If it wasn't there, and kept alive, you'd always have to set those things every time you turn the computer on.

Now, it may be that one uses more current than the other, but I can't really see that being a significant difference.

Michael

There are probably more PC with good clocks than cheap inaccurate clocks. The reason we rarely saw such posts are because people usually complain loudly if their product is a bit "off" for any reason and not many post positive comment on their product.

If everyone were to post every reviews, the bad one would get buried in a hurry.

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