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Title: How accurate is your computer clock? Post by: k4kyv on May 26, 2005, 12:42:28 PM My computer is set to automatically check internet time every day, and to reset the clock. Otherwise it gets off a few seconds a day. My previous Windows computer was much worse than this one- it lost about 5 minutes a week. At work, we use Apple computers, and I have noticed the same thing- a couple of seconds a day.
I have two cheap Timex wrist watches that cost < $25 each. Each one will stay within 5 seconds of WWV for a couple of months without resetting. Why is it that a computer that costs > $1000 won't keep as good time as a cheap electronic wristwatch? Unless I am missing something, the computer clock is ultimately based on a xtal-controlled time base oscillator, as is a "quartz" timepiece. Why is it so difficult to maintain comparable precision in frequency on a xtal oscillator that happens to be mounted on a computer motherboard? Title: How accurate is your computer clock? Post by: WA1GFZ on May 26, 2005, 01:24:28 PM internal reference is drifting around because it isn't temperature compensated like a watch reference. BTW most of the time your watch is sitting at room temp or body temp.
My new dvd is great it gets the latest time off the cable so I don't have to look at flashing 12:00 for 6 months at a time. Title: How accurate is your computer clock? Post by: WD8BIL on May 26, 2005, 01:36:48 PM Mine is close enough to get me to the dinner table on time !!!!
Title: How accurate is your computer clock? Post by: Paul, K2ORC on May 26, 2005, 01:45:13 PM Quote from: WD8BIL Mine is close enough to get me to the dinner table on time !!!! Bud knows! Only two times of the day you really need to know: when its time to eat and when it ain't. And I'm never late for either. Rancid http://www.lares.dti.ne.jp/~yugo/storage/monocrafts_ver3/03/index.html Title: How accurate is your computer clock? Post by: KE1GF on May 26, 2005, 01:47:55 PM Don when you're talking about modern computers there are actually two clocks involved.
There is the hardware clock the "RTC" Real Time Clock and the system clock. When the computer isn't running the hardware clock keeps time for the computer running off the battery installed on the mother-board. When the system is booted the RTC is basically ignored and is used to throw interrupts to the operating system to perform involuntary context switches for processes back to the operating system's task scheduler, among other things too. So usually when a system is booted the system clock of the operating system is synched with the hardware RTC and when the system is shutdown the RTC is synched back with the system clock. ie: On boot RTC -> system clock on shutdown system clock -> RTC Inevitably the arrival of this interrupt (IRQ) which returns control from processes back to the operating system and also updates the "system clock" from the hardware RTC is not perfect. The system clock either may race or lag behind what the actual "real time" is. There is a ton of reasons including but not limited to CPU stalls, integer errors in calculating the MIPS of the CPU (under Linux this is called bogoMIPS) or other amounts of time where the CPU is losing control of the system like DMA from hardware etc etc etc.... To drift from the main subject for a second, IRQ arrivals to the CPU are the basis for the entropy in a modern systems pseudo random number generator, ie Keyboard, Mouse, Harddisks, Network Interface etc. If this gives you any insight. Just like the "RTC"/"System Clock" The seed from the pseudo random number generator (enumerator) is loaded/stored on boot/shutdown. IRQ's from these devices cause context switches too... It's a complex machine running under the hood of your box. The answer to solve this problem is to use a program that implements a version of the christian algorithim and use your internet connection to keep your machine synched to an atomic clock. The "NIST" National Institute of Standards and Technology http://www.nist.gov/ provides a time server on the internet and programs to keep your system in check. http://tf.nist.gov/service/its.htm Christian's algorithim does a very good job keeping your computer's "System clock" accurate in which the explanation of how it does this is beyond the scope of this discussion. Time drift is a real world problem for all of us in the computer industry, thus we run programs that keep all of our computers synched to a time server so that they're all running accurate time. This is a key necessity to team software development when using concurrent repositories for versions source code. And the list goes on. I hope this is sufficient explanation of why your computer clock is lousy and what to do about it. I remember this question was very similar to an essay question that I had to answer for my distriubuted systems course, but I had to explain how Christian's algorithm actually worked. Title: How accurate is your computer clock? Post by: Glenn K2KL on May 26, 2005, 02:59:20 PM very cool link Paul!!!... what a fast wrist! 8) ... Bet he was a MelBay student! :lol: :lol:
http://www.lares.dti.ne.jp/~yugo/storage/monocrafts_ver3/03/index.html Title: How accurate is your computer clock? Post by: VA3ES - Piss-Weak Ed on May 30, 2005, 04:45:18 PM I use a little app that automatically looks up NIST every 15 minutes and synchs my clock. I can choose from dozens of official time sources but have settled on NIST at Ft. Collins CO. The app I use is called "D4" but there are lot's of similar apps available.
Title: How accurate is your computer clock? Post by: Jeff 'OGM on May 30, 2005, 05:41:28 PM Bill, I never knew that there was a system clock and a hardware clock that were separate, or that this trade-off took place at bootup. Tnx for the detailed description of the problem.
...and how about when BSOD's or lockups occur? Does that throw the clock(s) even further off? Jeff KA1OGM Title: ntp Post by: Ian VK3KRI on June 01, 2005, 08:19:51 AM Not sure about windows but on unix the 'ntp' (network time protocol) daemon will discipline your local machine clock to external reference sources. Typically these sources will be in turn synced to Cesuim, GPS CDMA or even WWV reference clocks. ntpd actually calculates and applies a continuos offset to the local clock using PLL type algorithms etc -and the main man of ntp is a ham W3HCF.
Pentuim class processors inclue the 'TSC' - Time Stamp Counter Register which increments each clock cycle. This is used (in i586 Linux at least) as the master reference as to real time. ntp calculates a PPM offset to this and applies it multiple times a second. Personally, at home, I run ntp to external reference sources and a local GPS as a local reference clock so I can keep the offset down to 10s of nano seconds . (Of course youre asking why bother and the answer is - Because I can! ) The GPS also provides gating for my freq counter. At work I have a number of geographically distributed machines whose target is <100uS difference between them - ntp to the rescue again. I know simple NTP clients are available for windows that will tweak the time relative to an external ntp source periodially but iI'm not sure if a full impelmentations is available that will contunually discipline the local clock Ian VK3KRI Title: How accurate is your computer clock? Post by: Jim, W5JO on June 01, 2005, 10:23:05 AM With Windows XP one can choose to sync the clock to either the microsoft server, or NIST. Click on the time down in bottom right corner of the toobar and the time setting window opens. The last tab will allow you to choose. My time seems to track the atomic clock setting by the computer pretty close.
Title: HARRUMPH! Post by: Powell on June 02, 2005, 10:06:19 AM I don't use the computer clock for telling time. I leave that to a REAL clock...a Telechron!!!!! AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
:D :) :o 8) Powell |