Electronics > Microcontrollers

CPU crystal "pull" range and how critical are the caps, for e.g. USB FS

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JPortici:

--- Quote from: T3sl4co1l on February 26, 2024, 07:07:59 pm ---Well, how do the automotive folks do it?  I haven't had a car lose more than minutes over the time I've owned it.  In all weather year round.

Well, plus or minus exactly one hour technically, because we still live under the hell of daylight savings, but besides that...

Tim

--- End quote ---

Depnding on brand, GPS (15yo VAGs did that) or internet.
My almost 10 years old octavia has the option to syncronize the clock for time keeping via GPS, the much older Fiat Punto i had before didn't have anything but i don't remember losing more than one minute every six months, good enough if you have to adjust it for DST anyway >:D
Wouldn't be surprised if radio clock was used back then

IME: crystals (or MEMS) to be used if FRC doesn't meet expectations (and unless CAN or ethernet are involved, FRC is usually much more than good enough)

peter-h:

--- Quote ---Well, how do the automotive folks do it?  I haven't had a car lose more than minutes over the time I've owned it.  In all weather year round.
--- End quote ---

I had a quick read of some appnotes a while ago.

In production, it's not hard to do a proper cal. Modern RTCs have a cal feature, whereby a counter adds or subtracts a millisecond or so every so often. Or you can do it in software; car clocks usually show just hh:mm so you just mess with the seconds. This kind of cal can be done in an instant, and anybody in that business must be doing it because the usual ~20ppm watch xtal is way too crap. In the old days you had a trimmer cap on the xtal, adjusted with a servo screwdriver.

Ideally you do a two point cal but I don't know how that can be done quickly because you need to elevate the temperature a bit. You would then use a temp sensor to regulate the adjustment; most CPUs now have a temp sensor.

The other thing is that in a car you have loads of battery capacity :) So you can use a decent xtal - say 4MHz. These are much better over temp. This is nothing new; in the 1960s Bulova were promoting "more accurate" quartz watches which used such an xtal. May have been 2MHz. I developed a product many years ago which used a special Swiss xtal, 2MHz, for a specific reason. 1MHz was not good, 4MHz could not be used for some other reason. The 2MHz one (I still have a bag of them - SMT in a glass case) was perfect for an accurate and fairly low power RTC, implemented in a mixed-signal ASIC I did the design for.

I think every mfg does a 1 point cal which is instant, and this is probably good enough. I suspect a 2 point cal is of the same order as xtal ageing. This is sort of related
https://www.eevblog.com/forum/chat/cheapest-way-to-get-datetime-from-gps/


--- Quote ---Depnding on brand, GPS (15yo VAGs did that) or internet.
My almost 10 years old octavia has the option to syncronize the clock for time keeping via GPS, the much older Fiat Punto i had before didn't have anything but i don't remember losing more than one minute every six months, good enough if you have to adjust it for DST anyway >:D
Wouldn't be surprised if radio clock was used back then
--- End quote ---

It would be zero if they used GPS or timecode :)

JPortici:

--- Quote from: peter-h on February 26, 2024, 07:58:39 pm ---It would be zero if they used GPS or timecode :)

--- End quote ---

as a matter of fact, my current old car has zero :)

PCB.Wiz:

--- Quote from: faststoff on February 26, 2024, 08:38:55 am --- I have had to deal with a product that was, on average, 40 ppm off

--- End quote ---
40ppm was not a large error, years ago, when 100ppm and 50ppm crystal were common / cheapest.
These days, you can usually find 10ppm @ 25'C spec crystals, or oscillators, so at least the initial setup can be tighter.

Of course, if you want wide temperature tolerance, the crystal ppm/T curves then start to matter.

faststoff:

--- Quote from: PCB.Wiz on February 27, 2024, 03:52:17 am ---
--- Quote from: faststoff on February 26, 2024, 08:38:55 am --- I have had to deal with a product that was, on average, 40 ppm off

--- End quote ---
40ppm was not a large error, years ago, when 100ppm and 50ppm crystal were common / cheapest.
These days, you can usually find 10ppm @ 25'C spec crystals, or oscillators, so at least the initial setup can be tighter.

Of course, if you want wide temperature tolerance, the crystal ppm/T curves then start to matter.

--- End quote ---
I agree, the 40 ppm is by itself not that big, and any system relying on a >=10ppm crystals for timekeeping should have some idea of how to handle clock drift if accurate timekeeping is important IMHO. In this case, it created secondary problems as the system around the product was not rigged well to handle clock correction easily once the product was out in the field, and thus we would start to get datalogs with quite unpredictable timestamps as units accumulated months - years worth of 40 ppm drift.

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