Xtal controlled clocks : What does Toyota know that PC makers don't ??

Because they are designed to withstand MIL-temp ranges and the ones in PCs probably don't even meet COM-temp specs?

Don't imagine that your two data points define the whole genre, however. I have examples of computer and auto clocks that are just the opposite of your anecdotes.

Both of my vehicle (Q45 & Frontier PU) clocks are dead-on. I'm suspicious that there may be synchronization sent via the AM radio just like there is clock synchronization transmitted on TV by the PBS stations.

...Jim Thompson

That would be very useful, but I've never heard of such a thing in 35 years of electronics and broadcast engineering. (For broadcast radio, that is.)

I once heard that at least the old, mechanical clocks they OEMed into vehicles had some mechanism that woluld slightly adjust the timebase whenever the time was changed (corrected). So if you were regularly turing the time back, it would gradually slow down the reference.

Of course there in AZ, you don't change your clocks back and forth for the "DST" semi-annual madness. It still causes havoc with all our corporate cross-site meetings dealing with those sites that do/don't observe DST.

Because the PC clocks are *GENERALLY* driven by a periodic interrupt generated by the processor or related circuitry, rather than working directly from a dedicated physical time-base. Commonly, this is a sub-function of the PC's bus-timing signal, which often isn't anywhere near a number that's conveniently turned into a "one second has elapsed" signal. So when your computer does something that "shuts down", "locks out", or otherwise disturbs the normal interrupt generation/handling process, it loses track of what time it is. Newer systems generally include a "correction" mechanism - something along the lines of "If interrupts were disabled for disk I/O activity, adjust the clock forward by X amount". If they were disabled for keyboard activity, adjust the clock forward by Y amount", and so on. Some of the "X"s and "Y"s are more accurate than others - some are dead on - "A VBL interrupt locks out the interrupt generation/handling mechanism for precisely "N" thousandths of a second. Before returning from this interrupt, advance the RTC (Realtime Clock) by exactly that amount." - or might be inherently inaccurate estimations - "Disk I/O will never disable interrupts for less than "W" nor more than "Z" ticks of the clock, so adjust the clock forward "Z-W/2" ticks every time Disk I/O happens" (AKA "Take the average").

Other factors include whether the machine has been overclocked (which will change the base rate at which the clock interrupt is generated, among other timing alterations), whether your usage requires more or less adjustment than the algorithm (if any) for adjusting was planned to compensate for, and similar.

Meanwhile, the digital clock in your toyota is a dedicated device. It doesn't count on interrupts to generate a "tick" of the clock. It sits there counting pulses coming off a crystal dedicated to the purpose of doing nothing but vibrating at the right frequency, doing some math to convert pulses to clock-time, and displays the result, all without ever being interrupted, blocked, locked out, or whatever.

Long and short of it: A PC's realtime clock is a compromise design, and runs on pulses that come at a frequency that may or may not be an accurate timebase, depending on many factors that the clock has no (and by its very nature, cannot get) knowledge of, while a "real" clock - one that does nothing but tell time - operates on pulses that come at unvarying intervals, and never loses track of time - at least in relation to itself... It's entirely within the realm of possible to have two near-identical clocks "drift" with respect to each other because although they're close to being identical, there's no such thing as two bits of crystal that resonate at *EXACTLY* the same rate. The difference in rates may be 38 digits to the right of the decimal, but over the long haul, that tiny error adds up to a lost or gained second, and sooner or later, the two clocks drift in comparison to each other.

In the Europe we get time signals sent via RDS (Radio Data System). That's over broadcast FM radio though, not AM. Isn't RDS used in the US?

Tim

That's true. Correcting the time also tweaked the spring tension in the escapement. I remember those well.

What do we need with DST? We have enough daylight already ;-)

...Jim Thompson

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What a load of rubbish!

PC clocks run by themselves in a dedicated battery circuit that is used to reset the Software clock at each boot up. If your theory was true my PC would lose about 8 hours a day because it is turned offf for that time and no amount of interutps or wait states happen then.

And PCs have had a built in battery clock since the PC AT in about 19Eighty3 .

Thirty years ago, digital watches and clocks were a new thing, and expensive. They were treated as precise time pieces, so into the design went concern for temperature stability, and for adjustment. If you paid a few hundred dollars for a digital clock or watch, the cost of adjusting the time base added minimally to the cost.

And thirty years ago, you'd have one of these digital clocks around, and maybe a digital watch.

But that was on the cusp. A few years later, computers were starting to go into everything, which meant they could easily add a clock function. After all, so much of the circuitry was in place, and so was the readout. But the clocks were an added feature, rather than a precision time piece.

Suddenly, they want to cut out the cost of a trimmer across the crystal, because that adds money. Besides, the factory doesn't want to pay the cost of calibration, which suddenly does add significantly to the cost when the clock is merely a minor part of a piece of equipment, so no sense paying the cost of components that won't be needed.

Likewise, don't temperature compensate because that will add cost.

Since the clocks are usually part of an existing piece of equipment, they may be stuck with a crystal frequency that is outright off frequency, becuase the rest of the circuit needs it. Too minor a difference to be worth the cost of extra circuitry, but enough to make a difference over the long run.

Then, the mere fact that we have them all over the place adds problems. Right in this room, I've got the clock in my computer, a small LCD clock, a couple of radios that have built in clocks, an LED alarm clock, a vcr and of course my watch. I don't need all of them, but most of them are simply part of other things. Yet, because they are all there, which one is right and which are wrong? There is no way of synchronizing them to the same time. I set one, and may be off a bit on the seconds. That one over there gets set at a different time. Plus they may all have slight variations in their crystal frequency error.

Now, I don't need all those clocks, and I don't need to read down to the second on all of them. Yet the mere fact that they can't easily be set to read noon on all of them at the same time helps to create an illusion that they all keep different times.

So now we have cheap "atomic clocks" controlled by some standard time station (which one depending on where in the world you are). Virtually nobody needs that sort of accuracy, yet they are now cheap, I bought one for twenty dollars last week, so people must be buying them. But they aren't bought for the accuracy, they are bought because they are self-setting. One doesn't have to fiddle with them, and one can assume they are at least an exact time. They are a workaround for the lack of adjustment put into the average clock.

Michael

But his point is valid.

The "PC" only reads the hardware clock upon booting up, or if one runs a program to specifically do so. The rest of the time, it's a real time clock, counting ticks on an interrupt. And as mentioned, there are times when one cannot count ticks, because that interrupt has to be turned off.

Leave the computer on long enough, and accumulated error can add up. Again, this is a matter of how accurately the clock doing the interrupts is set, how many ticks get missed, and how long the computer is run before the hardware clock is read again.

The point is that if someone is not familiar with this scheme, they will note that there is a hardware clock in the computer and then wonder why the time is wrong.

Michael

Sorry, but you are wrong. Yes, the real-time-clock (RTC) chip in the machine is battery backed, and keeps track of things when powered down. The actual "this is the one the computer reads" clock that drifts is a "pseudo clock", consisting almost exclusively of software. Once the machine has booted up and taken the initial setting for the "pseudo-clock" (really just a "seconds elapsed since boot" counter) from the RTC chip, the RTC is (barring use of a program designed to go directly to the RTC for the time, instead of computing what time it's supposed to be from the seconds-since-startup counterk) ignored until the next bootup.

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Yes I will agree with that. If you had mentioned the battery clock first up I do not suppose I would have written anyting.