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#1
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Digital Clocks
Why is it that the multitude of electronic clocks that now inhabit nearly
every device and appliance in most households are so often fast or slow, often by 15 minutes or more? Oddly enough, the most accurate clock I own is a mechanical wall clock that I haven't touched for over 6 months, and still doesn't need setting. The answer is that mechanical clocks had achieved a high degree of accuracy before they were mostly replaced by digital clocks. One way they achieved this was to accomplish a way to capture the information gained when a clock is reset to the correct time. When such a mechanical clock is advanced 3 minutes in a 24 hour period, it speeds up the mechanism by 3 minutes in a 24 hour period. A very significant feat when accomplished mechanically, but it would be much easier to accomplish in a the circuitry of a digital clock. For some reason this wasn't done, at least not in any devices I'm familiar with. I know that the clock in Windows is now reset from a central time server, but that's a crude solution to what should have been a simple design feature. It would have been a simple thing to calculate the time between clock settings, and adjust the speed of the clock accordingly by correcting the factors that are applied to the quartz crystal frequency. Wide variances in time setting differences such as moving to a different time zone or daylight savings adjustments could easily be filtered out of the correction factor. I guess it's the old American way of building something. As soon as the thing works, the job is done and it's Miller time. It's nice when they can budget a little extra time for enhancements. That's the difference between "good enough" and "excellence." |
#2
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Digital Clocks
Part of the answer That's the difference between "good enough" and
"excellence." is cost. Consumer devices are very cost sensitive, and how the clock is generated is part of that. A typical 32.768kHz resonator or crystal will have a nominal error of up to 200ppm (parts per million) but which will get worse depending on temperature and ageing. That's by far the most common frequency used for this function, although many processors simply use their main clock and divide it to the closest approximation - that, in particular, will give larger errors. (32.768kHz divided through a 15 bit binary counter yields exactly 1Hz). The circuitry needed to auto-adjust would add unnecessary costs (at least in the view of the manufacturers) for a non-essential feature (absolute accuracy over time). Typically we would just buy a simple RTC that may even have the crystal internally - see the list at Maxim for a typical selection: http://www.maxim-ic.com/products/tim...ime_clocks.cfm (Most of the clocks are actually Dallas Semi parts) In higher end and truly embedded equipment (where there are no user adjustable controls), much depends on whether there is a method of externally synchronising the clock so the error between external sync is negligible. If so, there is no point in using (relatively) expensive techniques. Can we auto adjust? Sure. Do we? Only if we must. Cheers PeteS |
#3
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Digital Clocks
"PeteS" schreef in bericht oups.com... Part of the answer That's the difference between "good enough" and "excellence." is cost. Consumer devices are very cost sensitive, and how the clock is generated is part of that. A typical 32.768kHz resonator or crystal will have a nominal error of up to 200ppm (parts per million) but which will get worse depending on temperature and ageing. That's by far the most common frequency used for this function, although many processors simply use their main clock and divide it to the closest approximation - that, in particular, will give larger errors. (32.768kHz divided through a 15 bit binary counter yields exactly 1Hz). The circuitry needed to auto-adjust would add unnecessary costs (at least in the view of the manufacturers) for a non-essential feature (absolute accuracy over time). Typically we would just buy a simple RTC that may even have the crystal internally - see the list at Maxim for a typical selection: http://www.maxim-ic.com/products/tim...ime_clocks.cfm (Most of the clocks are actually Dallas Semi parts) In higher end and truly embedded equipment (where there are no user adjustable controls), much depends on whether there is a method of externally synchronising the clock so the error between external sync is negligible. If so, there is no point in using (relatively) expensive techniques. Can we auto adjust? Sure. Do we? Only if we must. Cheers PeteS One remark about the nice tubes the 32.768kHz crystals are usualy built in is that they are extremly fragile. One bump and the frequency is not accurate anymore, the crystals may start making noise or stop working all together. The equipment with these crystals should be carefully handled. If I buy a new product and know it has a crystal I will always check the accuracy. Way too often it's out of spec. because of bumps. If you make a system yourself, always fasten these crystals either by solder or by wire. Don't just use the leads it's not enough! Alexander |
#4
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Digital Clocks
A lot of digital clocks use ICs that rely on the 60Hz or 50Hz signal
from the mains to keep time. Crack open any cheap alarm clock. They're extremely unreliable and susceptible to EM interference from other devices, but they're also very cheap to build. |
#5
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Digital Clocks
On 2005-11-11, Aaron Mavrinac wrote:
A lot of digital clocks use ICs that rely on the 60Hz or 50Hz signal from the mains to keep time. Crack open any cheap alarm clock. They're extremely unreliable and susceptible to EM interference from other devices, but they're also very cheap to build. EMI... so that's the problem... (they prolly don't like the ripple contol signals either) the mains cycles keep pretty good time from day to day, a little active filter or resonant circuit could presumably make all the difference.. (and probably cost an extra 50c) -- Bye. Jasen japun umop - pueleaZ maN |
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