Car clock

[Bob Eager]

Our phones sync with the NTP server!

[Clive Arthur]

Comparatively. You don't put your watch in a fridge or in a low oven, it's either on your wrist or in your house. That's much better than a car. And if you do have it on during a snowball fight, that won't be for long.

IIRC watch crystals have a temperature/frequency curve where the frequency is a maximum at around 25'C and drops off either side.

[Scott]

Nissan Micra. 2008.

[Theo]

Maybe that's a 'feature'. Some people like to set their clock 5 minutes fast so they aren't late, and might complain if the clock showed accurate time with no means of adjustment.

If you cold boot the car (disconnect battery and wait a while) I wonder if it picks up the time accurately?

Theo

[Clive Page]

I've had quite a number of quartz wrist-watches over the years. The first, bought during a visit to the USA a long time ago, cost under two dollars. None have cost me more than £20. All of them have been accurate to a few seconds a month which is good enough for me.

So it is disappointing to find that car clocks, mine among them, are out by a minute or two a month. I think it's partly the car environment - wide temperature variations make it hard to get a quartz crystal to keep time. The car companies know that it's difficult to get accuracy without a lot of expense, so they don't even try, they buy the cheapest crystals then can get and don't bother to trim them.

On the other hand, most cars now have a sat-nav system so it would be easy to link them to reset the dashboard clock to GPS time at intervals, but that would cost a few pence more per car during manufacture. Since few of us decide against buying a new model of car because it has a poorly designed clock circuit, the problem will persist.

[Dave Plowman (News]

I'd say GPS not necessarily the best way on a cheap car. But most cars have a radio and that carries a time signal.

[Bob Eager]

I don't think they're in the watch business any more, but I visited these people once:

(Hoptroff, that is)

[charles]

My car (Mazda 6), now 5+ years old, gets its time from the SatNav system. It could, like clock radios I have, get its time from RDS.

[jon]

It might be keeping good time, but offset.

[Martin Brown]

Provided the load capacitors are right and the drive power appropriate the 32kHz xtals used in watches are very good indeed. Most these days are digitally tuned by setting a digital fiddle counter so that 1s on the watch/clock matches a reference second. No variable capacitors.

More likely buy in an electronics module from the cheapest supplier.

This is a nice article by TI on the typical crystal cuts used and their characteristics.

Adding GPS was how the scope makers fixed their dodgy RTC problem! Also meant the user didn't have to enter their location correctly.

[Paul]

If the clock is part of the entertainment system, it might be harder to work with on the bench, and make an adjustment.

Separate clock modules, fitted in a dash, are more likely to run on the bench with a 13.5V supply for company. If it had a ceramic trimmer, you could adjust it. When the clock is buried in an entertainment system, getting it open would be a problem.

Quartz time pieces can run fast or slow, and on either side of the "correct" value. Only if the control scheme requires a "bias", do they preferentially pull the clock to one side. This is done on Unix computers, so that time corrections by software, only happen in one direction, not both directions.

Car clocks normally work too well, to not have temperature compensation. The parabolic temperature curve for the quartz crystal, can be corrected by attaching a network for the purpose. As in the picture here. This can change a 20 to 50ppm design, into a 1 to 3ppm design. The scheme is almost as good as using an oven to keep a constant temperature.

Paul

[Andrew]

You could always dial 17070 and select the ringback test. That updates your phone or caller-id box with the correct time

[Andrew]

Having slightly fast is usually not an issue if you need to get somewhere at a particular time.

[Jim GM4DHJ ...]

what happened to 1471 ? ...

[Jim GM4DHJ ...]

no wait I was thinking about XRB then your number....

[ARW]

Keeping it on at night might cause it to overheat...

[Scott]

My father told the tale that my granny always set the clock 10 minutes fast for this reason. Everyone knew it was 10 minutes fast and made the mental adjustment so the exercise was futile.

[NY]

Are the speed limits signposted (eg an electronic sign whose numbers change at different times of day), or does "everybody just know" what/when the speed limits are?

Variable speed limits according to time of day outside a school is a very good idea: it saves the UK convention of setting a speed limit which is absurdly low for most of the day or night, except for the critical hours when schools are coming or going.

[charles]

I think you will find such variable limits in Scotland

[Martin Brown]

Clock crystals are generally cut so that their linear temperature coefficient is almost zero and there is a tiny quadratic variation with ambient temperature typically -10ppm at about 10 and 40C spot on at 25C.

Figure 6 in the TI datasheet:

It is pretty easy to get within 5ppm on average over that range. Cars in winter can get both hotter and colder though.