[time-nuts] Can Lady Heather Keep Computer Clock On Time?

[Bob Camp]

Hi

There is a standard NTP driver that "talks Thunderbolt". The full blown NTP package is pretty easy to set up. It will hold ms accuracy slaved to a GPS.

Bob



On Jul 29, 2010, at 7:29 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> 
>> The telescope can point to within arc seconds of a star and that implies
>> that the computer clock needs to be within 50 ms.
> 
>> If he does a Windows NTP sync first thing in the evening after a few hours
>> there's too big an error.
> 
> That looks like the classic time vs frequency problem.
> 
> What is the primary goal?  Pointing or tracking?  Pointing requires time.  
> Tracking requires frequency.
> 
> If you are otherwise happy with "Windows NTP sync", you may be able to solve 
> your problem by doing a sync before pointing at another object if it's been 
> more than N hours since the last sync.
> 
> 
> Long song and dance....  [Remember, I don't run Windows so I may screwup 
> anything that's Windows specific.]
> 
> 
> The typical PC has 2 crystals.  One runs at 32 KHz.  The other is usually 
> 14.xxx (from early PC days) that gets PLLed up to make clocks for the CPU and 
> PCI and USB and ...
> 
> The 32KHz crystal runs the battery backed RTC/TOY/CMOS clock.  It's a watch 
> crystal so it should be pretty good.  But it's not very convenient for 
> keeping time at the microsecond level.
> 
> The 14 MHz crystal is stable, but typically not very accurate.  (Remember low 
> cost.)  That's accurate at the PPM level, it will be fine if you just put a 
> scope on it.  It may be off by 50  PPM.  Even if the hardware is good, the 
> software can screw things up.  (Linux is good at this.  Current kernels don't 
> get a consistent answer on the same hardware.  Jumps by 200 PPM from boot to 
> boot are not uncommon.)
> 
> [Network and audio and ??? cards typically have a separate crystal.  They are 
> usually not convenient for timekeeping but if you do serious audio work you 
> can measure it's actual frequency.]
> 
> Let's see if I can do the math right...
> 
> 3 hours is 10,000 seconds.  50 PPM times 10,000 seconds is 500,000 
> microseconds.  So if the clock is off by 50 PPM, it will drift 1/2 second in 
> 3 hours.  Even 5 PPM will drift 50 ms in 3 hours.
> 
> 
> The main reason for running real ntpd rather than just setting the time 
> occasionally is that ntpd will figure out how far off the frequency is and 
> correct for it.  ntpd calls that fudge factor "drift" and prints it out in 
> PPM with 3 digits to the right of the decimal point.
> 
> If all you need is 50 ms, you should be able to get that most of the time by 
> just running ntpd over the net.  It's sure worth a try.  It may not be good 
> enough if you have a crappy net connection or change from no-load to 
> uploading tons of data from observations earlier in the evening.  (Contact me 
> off list if you want help in monitoring a ntp server and/or setting up and 
> interpreting its log files.)
> 
> 
> 
> Odds and ends to keep in mind...
> 
> Modern PCs use spread spectrum clocking.  That fudges things by 1 or 1/2 % or 
> so which is huge in terms of PPM.  The point is that you have to measure it.  
> Just doing the math from the nominal CPU frequency isn't good enough.
> 
> The actual frequency is temperature dependent, so things will change if you 
> open the roof and let the cold air in or the CPU changes from idle (or off) 
> to working hard.  The ballpark is 1 PPM per 10 F.
> 
> One of the classic ways to screwup timekeeping is to miss interrupts, 
> typically because some other interrupt routine is running too long.  This was 
> easy to tickle on (very) old Linux systems that used PIO rather than DMA for 
> disk transfers.  I only mention it because you might have some strange 
> hardware with buggy interrupt routines.
> 
> Normal Windows clocks tick every 10 ms.  Windows has a multimedia mode that 
> does much better.  There is a switch in the Registry or something.  It may 
> help to use that mode.  I think you want to leave it on.  (The Meinberg 
> ntpd-installer package turns it on.)
> 
> ntpd is both a client and server.  A system will act as a client to get time 
> from lower stratum servers and act as a server to provide time to higher 
> stratum servers.
> 
> 
> 
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
> 
> 
> 
> 
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