r3 - 2004-10-12 - 02:59:46 - SteveKosteckeYou are here: NTP >  Support Web > HowAccurateIsTheCmosClock
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Question

How accurate is the CMOS clock?

Answer

Some people say the CMOS RTC is much more accurate than the timer interrupt in terms of frequency error. Actually I don't know, but I have one concrete example:

A PC used a stratum 1 server with PPS had had a hardware fault, and it had been powered off for about 18 days. When running the typical frequency error was 15 to 17 PPM and when the system was rebooted the RTC clock was off by 18 seconds. That would be an error of roughly 12 PPM.

-- SteveKostecke - 02 May 2003

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