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#1
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RTC accuracy
My computer RTC is quite accurate as computer clocks go, being
off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? |
#2
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RTC accuracy
pawihte wrote:
My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? In timing circuits a high frequency oscillator is desired. If there are occasional voltage spikes (either negative or positive) and an occasional "cycle" is missed...it's not critical. |
#3
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RTC accuracy
pawihte wrote:
My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? In my experience, the RTC varies a bit depending on the quality and frequency response of the oscillator they choose to use. It also sometimes varies depending on the quality of power supplied to the system. I had thought that since it draws from a battery it wouldn't be an issue, but the only system I've ever had with a timing issue seemed to be fixed with a PSU swap. |
#4
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RTC accuracy
"MCheu" wrote in message ... pawihte wrote: My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? In my experience, the RTC varies a bit depending on the quality and frequency response of the oscillator they choose to use. It also sometimes varies depending on the quality of power supplied to the system. I had thought that since it draws from a battery it wouldn't be an issue, but the only system I've ever had with a timing issue seemed to be fixed with a PSU swap. It seems the RTC draws power from the PSU even in the presence of a battery since it keeps time and BIOS settings without a battery as long as the computer is powered on. Seems strange though that it should be noticeably affected by voltage variations, even spikes, in the output of a regulated PSU, especially as it's not seriously affected by a change of several hundred millivolts in battery voltage. |
#5
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RTC accuracy
pawihte wrote:
My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? The real-time clock runs constantly, but is used only to preset the date/time when the operating system boots. At this point, an operating system task begins counting out time. Too many interrupts may cause the cpu to skip the time-keeping task now and then, allowing operating system time to get behind. ATX power supplies include a 5 vdc output that remains energized as long as the power supply is connected to AC power. The RTC runs from this power, falling back to the button battery only if the power supply source is lost. |
#6
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RTC accuracy
MCheu wrote:
pawihte wrote: My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? In my experience, the RTC varies a bit depending on the quality and frequency response of the oscillator they choose to use. It also sometimes varies depending on the quality of power supplied to the system. I had thought that since it draws from a battery it wouldn't be an issue, but the only system I've ever had with a timing issue seemed to be fixed with a PSU swap. To see how it is powered, have a look at 25281202.pdf reference schematic here. It contains a relatively modern motherboard schematic (some bits are not realistic enough for my tastes - they should have used a real commercial device for the Super I/O). http://www.intel.com/design/chipsets...ics/252812.htm PDF page 82 has the diode-OR of CMOS battery, and onboard regulator output PDF page 85 has +5VSB to V_3P3_STBY onboard regulator (three terminal) PDF page 79, shows V_5PO_STBY is the +5VSB coming from the power supply. The voltage regulator on page 85, should reduce the impact a PSU would have on timekeeping. But if +5VSB drops low enough, I supposed there could be a less stable voltage feeding the RTC (which lives inside the Southbridge). The voltage really shouldn't step radically out of bounds. On page 85, the circuit is a dual footprint, allowing two different devices to be used. If you look at Figure 12 here, you can see the equation for the adjustable regulator. They're trying to set the thing to around 3.32 volts. The CMOS battery is probably closer to ~3.0 or so. http://www.onsemi.com/pub_link/Collateral/MC33269-D.PDF It the power supply is ON, then one leg of the diode OR gets 3.32 volts. If the power supply is OFF at the back, the CMOS battery delivers around 3.0 volts. The operating frequency of the RTC 32768Hz crystal might not be exactly the same, for those two cases. If the power supply is weak, the voltage could be anywhere between 3.32 and 3.0 volts. If the regulator output drops below 3.0 volts, then the CMOS battery takes over. Based on that example, I don't see a mechanism for the timekeeping to be grossly affected. ******* The timekeeping while the OS is running, is AFAIK, done with clock tick interrupt counting. That is traceable to a different crystal than the one used by the RTC. The RTC is there mainly for timekeeping, when the OS is not running. The characteristics of the OS maintained time, will be different than the RTC, both due to the different crystal used (the one on the clockgen), and due to the possibility of problems with the clock tick interrupt servicing. http://www.maxim-ic.com/appnotes.cfm/an_pk/632 "Each PC contains two clocks. Although they are known by several different names, we will call them the "hardware clock" and the "software clock." The software clock runs when the PC is turned on and stops when the PC is turned off. The hardware clock uses a backup battery and continues to run even when the PC is turned off." The CK409 clockgen on the Intel reference schematic, is on PDF page 21. The quartz crystal used is 14.318MHz (4 x color burst 3.579545MHz). The OS maintained clock will be traceable to the properties of that 14.318MHz quartz crystal. The RTC time, on the other hand, is traceable to its 32768Hz quartz crystal. I don't think the computer designers care too much, about refining either of these. My digital watch, for example, has a trimmer capacitor inside, to adjust + or - on the frequency. Computers don't have that. Paul |
#7
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RTC accuracy
The OS and software will affect the clock time. Most programs will not affect clock time, but some older programs would cause the clock to lose time when they were running. pawihte wrote: My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? -- Mike Walsh |
#8
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RTC accuracy
"Paul" wrote in message ... MCheu wrote: pawihte wrote: My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? In my experience, the RTC varies a bit depending on the quality and frequency response of the oscillator they choose to use. It also sometimes varies depending on the quality of power supplied to the system. I had thought that since it draws from a battery it wouldn't be an issue, but the only system I've ever had with a timing issue seemed to be fixed with a PSU swap. To see how it is powered, have a look at 25281202.pdf reference schematic here. It contains a relatively modern motherboard schematic (some bits are not realistic enough for my tastes - they should have used a real commercial device for the Super I/O). http://www.intel.com/design/chipsets...ics/252812.htm PDF page 82 has the diode-OR of CMOS battery, and onboard regulator output PDF page 85 has +5VSB to V_3P3_STBY onboard regulator (three terminal) PDF page 79, shows V_5PO_STBY is the +5VSB coming from the power supply. The diode-OR switchover circuit is very basic and indicates that they don't consider stringent regulation necessary at that point. The voltage regulator on page 85, should reduce the impact a PSU would have on timekeeping. But if +5VSB drops low enough, I supposed there could be a less stable voltage feeding the RTC (which lives inside the Southbridge). The voltage really shouldn't step radically out of bounds. And of course, if the +5V rail fell below 4.3V which would provide less than the 1V dropout limit of the onboard regulator IC, that would probably affect other circuits more vital than the RTC. On page 85, the circuit is a dual footprint, allowing two different devices to be used. If you look at Figure 12 here, you can see the equation for the adjustable regulator. They're trying to set the thing to around 3.32 volts. The CMOS battery is probably closer to ~3.0 or so. I wonder why they don't just use the fixed 3.3V version of the MC33269. It would provide a tighter (1%) tolerance on the output voltage than would be guaranteed by using two 1% resistors (possible 2% error) plus the tolerance of the bandgap on-chip reference. http://www.onsemi.com/pub_link/Collateral/MC33269-D.PDF It the power supply is ON, then one leg of the diode OR gets 3.32 volts. If the power supply is OFF at the back, the CMOS battery delivers around 3.0 volts. The operating frequency of the RTC 32768Hz crystal might not be exactly the same, for those two cases. If the power supply is weak, the voltage could be anywhere between 3.32 and 3.0 volts. If the regulator output drops below 3.0 volts, then the CMOS battery takes over. Based on that example, I don't see a mechanism for the timekeeping to be grossly affected. The in-circuit voltage of my CMOS battery is currently about 3.0V vs. about 3.3V when new. I normally turn off power at the plug for 8-10 hours a day, sometimes up to 20 hrs, but my clock is still accurate to within a few seconds in between weekly online synchronisations. ******* The timekeeping while the OS is running, is AFAIK, done with clock tick interrupt counting. That is traceable to a different crystal than the one used by the RTC. The RTC is there mainly for timekeeping, when the OS is not running. The characteristics of the OS maintained time, will be different than the RTC, both due to the different crystal used (the one on the clockgen), and due to the possibility of problems with the clock tick interrupt servicing. This is where my limited knowledge of computer technology lets me down. If OS timekeeping depends on the base CPU clock, won't factors like spectrum spreading also cause inaccuracies in the RTC? http://www.maxim-ic.com/appnotes.cfm/an_pk/632 "Each PC contains two clocks. Although they are known by several different names, we will call them the "hardware clock" and the "software clock." The software clock runs when the PC is turned on and stops when the PC is turned off. The hardware clock uses a backup battery and continues to run even when the PC is turned off." The CK409 clockgen on the Intel reference schematic, is on PDF page 21. The quartz crystal used is 14.318MHz (4 x color burst 3.579545MHz). The OS maintained clock will be traceable to the properties of that 14.318MHz quartz crystal. The RTC time, on the other hand, is traceable to its 32768Hz quartz crystal. I don't think the computer designers care too much, about refining either of these. My digital watch, for example, has a trimmer capacitor inside, to adjust + or - on the frequency. Computers don't have that. That's probably the root of it all. |
#9
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RTC accuracy
Bryce wrote:
pawihte wrote: My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? The real-time clock runs constantly, but is used only to preset the date/time when the operating system boots. At this point, an operating system task begins counting out time. Too many interrupts may cause the cpu to skip the time-keeping task now and then, allowing operating system time to get behind. You can test this experimentally by rebooting. See if the time error corrects itself from the RTC. Don't know anything about current technology, but back in the old days, I had one system where a few microamps of capacitor leakage drug the RTC voltage down to the point that time was unstable. Was easy to find with "freeze spray"...but, today, a can of freon costs more than a new motherboard. ATX power supplies include a 5 vdc output that remains energized as long as the power supply is connected to AC power. The RTC runs from this power, falling back to the button battery only if the power supply source is lost. |
#10
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RTC accuracy
pawihte wrote:
My computer RTC is quite accurate as computer clocks go, being off by no more than a few seconds over a period of several days without syncing, whereas I've noticed some computers to be off by minutes in 24 hours even with a healthy battery. I've heard that the accuracy of a computer clock not only depends on the oscillator in the RTC hardware, but is also influenced by interrupt calls (or something like that) in OS and app environment. If this is true, do I just happen to have a good RTC hardware or does my OS and software installation also have something to do with it? Saying you have an RTC component in your system says nothing about what devices it actually uses or the design of its circuit (outside the chip and what crystal is used). http://en.wikipedia.org/wiki/Real-time_clock http://www.intel.com/Assets/PDF/appnote/292276.pdf Sections 3.3 and 3.4. They don't give actual accuracy measurements because so much depends on the circuit design and environmental conditions. A lot depends on the quality of the crystal used and its variation over temperature and its drift (seconds of drift per month). http://www.maxim-ic.com/appnotes.cfm/an_pk/504 "An error of 23ppm is about 1 minute per month." http://www.maxim-ic.com/appnotes.cfm/an_pk/632 "PC clocks are not particularly good at keeping accurate time. Simple clocks like a wris****ch and most of the clocks in your home keep better time than a standard PC clock." http://www.maxim-ic.com/view_press_r...elease_id/1101 Depends on how old is your motherboard and its design as to whether it utilizes a much more accurate (i.e., lower PPM error rate) clock. The one described here (published in 2005) has a very low error rate that results in a discrepancy of 2 minutes per year. I use an NTP client to sync my clock once per hour. |
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