Date: Tue, 27 Jul 1999 01:26:45 -0700 From: Mike Smith <mike@smith.net.au> To: "David Schwartz" <davids@webmaster.com> Cc: "Mike Smith" <mike@smith.net.au>, stable@freebsd.org Subject: Re: Tuning the system's clock Message-ID: <199907270826.BAA01383@dingo.cdrom.com> In-Reply-To: Your message of "Thu, 22 Jul 1999 12:40:31 PDT." <001801bed47a$0efc31a0$021d85d1@youwant.to>
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> > The TSC is calibrated at boot time using the RTC; that calibration is > > inherently limited to the accuracy of the RTC. > > Exactly. The RTC is accurate to maybe 150 ppm. The other problem is that > this calibration occurs right after power up, when the system is 'cold'. > It's not going to be representative of the system's 'warm' rate either. That assumes that you don't pause the system to allow it to warm up first. 8) > For one thing, the TSC oscillator can be replaced with a high-precision > OCXO. But that won't help you if you can't tune the TSC divisor, will it? > (Of course, XNTP would be the right tool for this purpose.) Actually, I don't think you'd have any luck replacing the TSC oscillator without a lot of nasty boardwork; many applications these days are using programmable frequency synthesisers using embedded crystals in the 10-40MHz range; I've seen others using cheap NTSC colourburst crystals and even a couple using 32kHz watch crystals. > In any event, tuning the TSC divisor via sysctl causes my FreeBSD-STABLE > machine to reboot instantly. I've just found that NTP seems to work the best > when the system is already keeping pretty good time. This is definitely a bug. Have you spoken to Poul about it yet? The "instant reboot" sounds particularly unpleasant; it should be fairly straightforward to track it down though. -- \\ The mind's the standard \\ Mike Smith \\ of the man. \\ msmith@freebsd.org \\ -- Joseph Merrick \\ msmith@cdrom.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
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