[ntp:questions] Linux 11-minute mode (RTC update)

Serge Bets serge.bets at NOSPAM.laposte.invalid
Tue Jul 22 14:50:21 UTC 2008

Hello David,

 On Tuesday, July 22, 2008 at 7:45:06 +0100, David Woolley wrote:

> Serge Bets wrote:
>> Hwclock 2.33 sets the RTC with typically a maximum error of
>> 10 microseconds
> That would only be possible if the RTC's counter was updated on both
> phases of the 32kHz clock, which I rather doubt.

Excellent example, Dave! The source 32K crystal period being 30 µs, you
point the finger at a real issue. An issue that has been squashed some
time ago by hwclock's feedback mechanism. Not a problem anymore.

For simplification, I'll assume everything else is instant and perfect,
and will neglect RTC restart delay. When we set the RTC, the last crystal
pulse happened between 0 and 30 µs ago. Let's pick 18 µs for the
example. This last pulse becomes the start of this RTC second, the next
second will start 32768 pulses later, and so on. Result: the RTC is in
advance by +18 microseconds. Problem.

However the feedback mechanism measures this +18 µs offset, and corrects
future readouts by -18. Final result: nothing less than perfection. :-)

Serge point Bets arobase laposte point net

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