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#11
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w_tom wrote:
AC electric cannot vary wildly because all grid generators must match up perfectly with all others - or the entire grid collapses. To find a clock that varies wildly, visit a PC clock. That clock is way too inaccurate to control the 50 Hz grid - where timing accuracy is essential. Power grid frequency varies automatically with the difference between supply and demand and is controlled by opening and closing the steam valves of the turbines that drive the generators. In the continental European grid (UCTE), the spec says that the time-scale defined by the rotating grid phase vector must remain within +/- 30 seconds of UTC. The UCTE control center in Laufenbourg, Switzerland sends out instructions every 24 hours to shift the nominal frequency of 50.0 Hz, if necessary, to either 49.99 or 50.01 Hz (+/- 10 mHz) if the network clock starts to drift away from UTC too far. For comparison: the measured frequency deviation from the nominal frequency is not supposed to be more than 40 mHz for more than 10% of the time and not more than 60 mHz for more than 1% of time. As such, at any moment in time the AC grid can 'vary wildly' (subjective) compared to the tolerances of a PC clock (typical RTC 32.768 kHz crystal being 20ppm @ 25C, which can accumulate to being off 52s after a month). The difference is that the power grid is routinely (24 hours) 'corrected' to bring it back so, over the long term, it remains within tolerance whereas a PC clock's error just keeps accumulating, unless one does a similar thing and syncs it with a more accurate time reference. Use a PC as the timing source. Therefore you only need write special software to write to displays- ie driven through parallel ports. But then why bother with fancy hardware displays? You can put up clock displays in Windows on the monitor. Previously posted was using an alarm clock 'kludge' - every other display is independent of the actual clock - except when the display number is first latched from a master display. Appreciate everything posted up to now is perfectly matching your request - because details are missing. What are the details? For example, the grid 50 Hz remains a most accurate source of timing which is why AC electric clocks remain on time every day - without resetting every week. If 50 Hz AC is not accurate enough, then anything in a PC or in the embedded processor is even worse. IOW you did not provide a necessary detail such as the number for timing accuracy. You did not say where those other displays get their numbers. Are you trying to display a time of day, or time of event. For that matter, what is wrong with a controller from a defective microwave oven? How to connect to a display? First select the display. Then find the driver chip for that type of display. IOW download datasheets. Read some 3 or eight pages chock full of information (assume something like 15+ minutes per page). If that example (rentron.com) is what you want, then why not buy it? I remain completely confused by what you are trying to do. If 50 Hz AC power is not accurate enough, then this example is even worse. Just more little contradictory details that completely confused me. It is possible that you have not yet learned an engineer's mentality. This project is how you learn so much. Same reason why to teach why the military want people educated in the temperament of reality. Notice what you may have perceived as a simple project might take weeks. Something that would become obvious once you first list those details. The military teaches their future officers to be engineers. To learn that reality is not as simplistic as taught in secondary school. To learn temperament that comes from putting initial ideas into reality. Making ideas into reality is rarely taught in general education. We learn it by doing what you are trying to accomplish here. Currently, you have starve your request for information with insufficient details. Daniel J Beardsall wrote: The problem with an alarm clock is that the chip is completely contained. The only information coming out would be seven segment displays - multiplexed. You would have to convert that back to some usable form OR build a memory device to store those multiplexed seven segment outputs and then drive the other displays. Four digits times seven segments plus AM/PM is 29 D flipflop latches and LED display drivers. The other displays would be independent of the actual clock, if that's what you mean. And yes, when I mentioned butchering an alarm clock that was mostly as a last-resort kind of option. What type of time do you want to store? For example, if time of day, then the alarm clock solution is better. AC powered clocks synchronize with the 60 Hz power line - whose accuracy is traceable to the National Bureau of Standards. To avoid the complexity of same, the alarm clock would be a good foundation to start construction. You would probably have to beef up or supplement the alarm clock's power supply to power your latching registers and two additional displays. The problem there is that I'm not in the US, I'm in the UK, and our electrical supply isn't quite so simple. It's mostly 50Hz but can fluctuate wildly, which often interferes with clocks. Besides, I'm not looking to run it off direct mains, I'm looking to run it off computer power, so a battery-powered alarm clock was more what I had in mind. Another alternative is to start from scrap with a single chip computer. Again, your timing crystal accuracy is just another consideration. But a single chip computer affords you great flexibility to enhance your design. Now you're talking. But I wouldn't know where to start hooking something like that into something capable of driving LED segment displays. Which I suppose is ultimately the help I'm after. |
#12
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On Fri, 03 Dec 2004 21:01:16 -0600, David Maynard wrote:
w_tom wrote: AC electric cannot vary wildly because all grid generators must match up perfectly with all others - or the entire grid collapses. To find a clock that varies wildly, visit a PC clock. That clock is way too inaccurate to control the 50 Hz grid - where timing accuracy is essential. Power grid frequency varies automatically with the difference between supply and demand and is controlled by opening and closing the steam valves of the turbines that drive the generators. In the continental European grid (UCTE), the spec says that the time-scale defined by the rotating grid phase vector must remain within +/- 30 seconds of UTC. The UCTE control center in Laufenbourg, Switzerland sends out instructions every 24 hours to shift the nominal frequency of 50.0 Hz, if necessary, to either 49.99 or 50.01 Hz (+/- 10 mHz) if the network clock starts to drift away from UTC too far. For comparison: the measured frequency deviation from the nominal frequency is not supposed to be more than 40 mHz for more than 10% of the time and not more than 60 mHz for more than 1% of time. As such, at any moment in time the AC grid can 'vary wildly' (subjective) compared to the tolerances of a PC clock (typical RTC 32.768 kHz crystal being 20ppm @ 25C, which can accumulate to being off 52s after a month). The difference is that the power grid is routinely (24 hours) 'corrected' to bring it back so, over the long term, it remains within tolerance whereas a PC clock's error just keeps accumulating, unless one does a similar thing and syncs it with a more accurate time reference. I believe I know pretty much what you're after. However I have no real hard fast wiring recommendations. There are many software programs out there to link with atomic clocks to fix your PC's inaccuracy. I run AtomTime myself and it's nice to be able to set the PC to within a fraction of a second without relying on your power main's fluctuating frequency(heh). If you buy one of the programs, several have options to check/reset the PC time against the atomic clock on a schedule every so many hours, when the PC boots up, etc. I fix people's computers (which is more spyware removal and Winders settings screwups anymore than strictly hardware upgrades) and am horrified by some of the times (and dates!) I see on some of the systems. Alternatively you might buy a standalone clock that syncs to the radio signals transmitted by atomic clocks so it would be totally independant from the PC. I am assuming they do that in England or Europe. The clock constantly monitors the broadcasts and updates itself no matter how badly the mains freq screws with it. ~~~~~~ Bait for spammers: root@localhost postmaster@localhost admin@localhost abuse@localhost ] ~~~~~~ Remove "spamless" to email me. |
#13
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Overlord wrote:
On Fri, 03 Dec 2004 21:01:16 -0600, David Maynard wrote: w_tom wrote: AC electric cannot vary wildly because all grid generators must match up perfectly with all others - or the entire grid collapses. To find a clock that varies wildly, visit a PC clock. That clock is way too inaccurate to control the 50 Hz grid - where timing accuracy is essential. Power grid frequency varies automatically with the difference between supply and demand and is controlled by opening and closing the steam valves of the turbines that drive the generators. In the continental European grid (UCTE), the spec says that the time-scale defined by the rotating grid phase vector must remain within +/- 30 seconds of UTC. The UCTE control center in Laufenbourg, Switzerland sends out instructions every 24 hours to shift the nominal frequency of 50.0 Hz, if necessary, to either 49.99 or 50.01 Hz (+/- 10 mHz) if the network clock starts to drift away from UTC too far. For comparison: the measured frequency deviation from the nominal frequency is not supposed to be more than 40 mHz for more than 10% of the time and not more than 60 mHz for more than 1% of time. As such, at any moment in time the AC grid can 'vary wildly' (subjective) compared to the tolerances of a PC clock (typical RTC 32.768 kHz crystal being 20ppm @ 25C, which can accumulate to being off 52s after a month). The difference is that the power grid is routinely (24 hours) 'corrected' to bring it back so, over the long term, it remains within tolerance whereas a PC clock's error just keeps accumulating, unless one does a similar thing and syncs it with a more accurate time reference. I believe I know pretty much what you're after. However I have no real hard fast wiring recommendations. There are many software programs out there to link with atomic clocks to fix your PC's inaccuracy. I run AtomTime myself and it's nice to be able to set the PC to within a fraction of a second without relying on your power main's fluctuating frequency(heh). If you buy one of the programs, several have options to check/reset the PC time against the atomic clock on a schedule every so many hours, when the PC boots up, etc. I fix people's computers (which is more spyware removal and Winders settings screwups anymore than strictly hardware upgrades) and am horrified by some of the times (and dates!) I see on some of the systems. Alternatively you might buy a standalone clock that syncs to the radio signals transmitted by atomic clocks so it would be totally independant from the PC. I am assuming they do that in England or Europe. The clock constantly monitors the broadcasts and updates itself no matter how badly the mains freq screws with it. Thanks. That's good information but I was just commenting on the power line frequency and am not the one looking to make the external clock, for what reason I don't know. I already sync all my local machines to tick.usno.navy.mil. Or, rather, my domain controller does and the rest sync with the DC. |
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