3 ms·

Crystal errors tend to be around 20 ppm (parts per million) After a week, 20 ppm would drift 12 * 10^-6 * 7 * 24 * 60 *60 = 12 seconds. Your motherboard proba

by peteey 3y ago

Crystal errors tend to be around 20 ppm (parts per million) After a week, 20 ppm would drift 12 * 10^-6 * 7 * 24 * 60 *60 = 12 seconds. Your motherboard probably has a cr2032 keeping it powered when unplugged. Crystals: https://www.digikey.com/en/products/filter/crystals/171?s=N4IgjCBcoExgrABiqAxlAZgQwDYGcBTAGhAHsoBtEAZhgDoB2ANgA4ACAawAkAvEAXRIAHAC5QQAVQB2ASxEB5DAFkCWPAFcATgRABffUA https://www.digikey.com/en/products/filter/crystals/171?s=N4...

crote 3y ago
It kinda makes you wonder why desktop computers don't use the AC frequency as a stable-ish time source. Short-term accuracy is pretty poor, but it can definitely do better than 12 seconds over a week!
- moffkalast 3y ago
  I suppose it's because no AC ever gets to the motherboard in your typical ATX setup? It's all just DC 12/5/3 volts and could be coming from a battery for all it knows. There would need to be an optional standard way of getting time from the PSU and have the AC time keeping there.
  - crote 3y ago
    Of course, but there's no reason why a 50/60Hz signal couldn't have been included in the ATX power connector back when it was established a few decades ago. In an alternate universe it would've been put in there, together with all the weird -12V / -5V rails nobody uses these days. Getting it these days would indeed be pretty much impossible.
    - labcomputer 3y ago
      Sure, but it costs extra and nobody (to a first approximation) cares. So why bother?
- brohee 3y ago
  That's a very optimistic assumption, the target is 50Hz but if it is below or over for a long period of time (e.g. high load in winter making it hard to sustain the nominal frequency) there are no provision to make it run faster or slower unless the time drifted by more than 30s (that's possibly only valid for Europe). More at https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html
  - calfuris 3y ago
    The standards are tighter in the US, with corrections being triggered by 10/3/2s differences and stopping at 6/0.5/0.5s (Eastern/Texas/Western interconnections). Src: https://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf https://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf
fanf2 3y ago
There’s a fun thing about quartz wristwatches: one of the biggest contributions to frequency fluctuations in a quartz oscillator is temperature. But if it is strapped to your wrist, it is coupled to your body’s temperature homeostasis. So a quartz watch can easily be more accurate than a quartz clock! Really good watches allow you to adjust their rate, so if it runs slightly fast or slow at your wrist temperature, you can correct it. One of the key insights of John Harrison, who won the Longitude prize, was that it doesn’t matter so much if a clock runs slightly fast or slightly slow, so long as it ticks at a very steady rate. Then you can characterise its frequency offset, and use that as a correction factor to get the correct GMT after weeks at sea.
- lxgr 3y ago
  That would require tuning it to the average body temperature though, right? Or are you saying that what makes quartz crystals drift is the change in temperature?
  - fanf2 3y ago
    Both are true :-)
    - lxgr 3y ago
      Oh, I missed your comment about being able to tune some wristwatches quartz! I wasn't aware that was a thing. Still, wouldn't the temperature of a watch while being worn vary as least as much as when sitting in a drawer (unless you live in a region blessed with t-shirt weather year around)? One of my favorite wristwatches I used to wear as a teenager had a thermometer, but I don't remember how exactly that varied over the year, just that it always showed neither quite my body temperature, nor quite the ambient one :)
      - labcomputer 3y ago
        The crystals used in watches are usually cut and selected so that a local minimum or local maximum of the tempco is near the temperature of your wrist. Thus, the tempco is near zero, so human-to-human differences don’t matter much. One thing to notice is that quartz watches almost always have a metal backplate touching your wrist so that the crystal can have good thermal contact. Presumably, the thermometer in your watch was decoupled from that plate.
throw0101a 3y ago
> After a week, 20 ppm would drift 12 * 10^-6 * 7 * 24 * 60 *60 = 12 seconds. Where are you getting that 12 from?
- dmoy 3y ago
  It should read 20*, not 12. The end result is 12 seconds.

by peteey 3y ago

crote 3y ago

It kinda makes you wonder why desktop computers don't use the AC frequency as a stable-ish time source. Short-term accuracy is pretty poor, but it can definitely do better than 12 seconds over a week!

moffkalast 3y ago
I suppose it's because no AC ever gets to the motherboard in your typical ATX setup? It's all just DC 12/5/3 volts and could be coming from a battery for all it knows. There would need to be an optional standard way of getting time from the PSU and have the AC time keeping there.
- crote 3y ago
  Of course, but there's no reason why a 50/60Hz signal couldn't have been included in the ATX power connector back when it was established a few decades ago. In an alternate universe it would've been put in there, together with all the weird -12V / -5V rails nobody uses these days. Getting it these days would indeed be pretty much impossible.
  - labcomputer 3y ago
    Sure, but it costs extra and nobody (to a first approximation) cares. So why bother?
brohee 3y ago
That's a very optimistic assumption, the target is 50Hz but if it is below or over for a long period of time (e.g. high load in winter making it hard to sustain the nominal frequency) there are no provision to make it run faster or slower unless the time drifted by more than 30s (that's possibly only valid for Europe). More at https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html https://wwwhome.ewi.utwente.nl/~ptdeboer/misc/mains.html
- calfuris 3y ago
  The standards are tighter in the US, with corrections being triggered by 10/3/2s differences and stopping at 6/0.5/0.5s (Eastern/Texas/Western interconnections). Src: https://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf https://www.naesb.org/pdf2/weq_bklet_011505_tec_mc.pdf

fanf2 3y ago

There’s a fun thing about quartz wristwatches: one of the biggest contributions to frequency fluctuations in a quartz oscillator is temperature. But if it is strapped to your wrist, it is coupled to your body’s temperature homeostasis. So a quartz watch can easily be more accurate than a quartz clock! Really good watches allow you to adjust their rate, so if it runs slightly fast or slow at your wrist temperature, you can correct it. One of the key insights of John Harrison, who won the Longitude prize, was that it doesn’t matter so much if a clock runs slightly fast or slightly slow, so long as it ticks at a very steady rate. Then you can characterise its frequency offset, and use that as a correction factor to get the correct GMT after weeks at sea.

lxgr 3y ago
That would require tuning it to the average body temperature though, right? Or are you saying that what makes quartz crystals drift is the change in temperature?
- fanf2 3y ago
  Both are true :-)
  - lxgr 3y ago
    Oh, I missed your comment about being able to tune some wristwatches quartz! I wasn't aware that was a thing. Still, wouldn't the temperature of a watch while being worn vary as least as much as when sitting in a drawer (unless you live in a region blessed with t-shirt weather year around)? One of my favorite wristwatches I used to wear as a teenager had a thermometer, but I don't remember how exactly that varied over the year, just that it always showed neither quite my body temperature, nor quite the ambient one :)
    - labcomputer 3y ago
      The crystals used in watches are usually cut and selected so that a local minimum or local maximum of the tempco is near the temperature of your wrist. Thus, the tempco is near zero, so human-to-human differences don’t matter much. One thing to notice is that quartz watches almost always have a metal backplate touching your wrist so that the crystal can have good thermal contact. Presumably, the thermometer in your watch was decoupled from that plate.

throw0101a 3y ago

> After a week, 20 ppm would drift 12 * 10^-6 * 7 * 24 * 60 *60 = 12 seconds. Where are you getting that 12 from?

dmoy 3y ago
It should read 20*, not 12. The end result is 12 seconds.