[ntp:questions] LOCL clock reachability not 377?

mike cook michael.cook at sfr.fr
Fri Aug 1 08:07:15 UTC 2014


Le 1 août 2014 à 00:43, Rob a écrit :

> William Unruh <unruh at invalid.ca> wrote:
>> I think you need to read up on the cmos clock. As I said, it reports
>> only the seconds, but is settable and "readable" to microseconds. 
> 
> The CMOS clock is running off a 32768Hz crystal, so no way it can be
> more accurately set than 30us.

  Some, such as the venerable MC146818A can use higher clock speeds. 

> 
> Even it could be possible in theory to set and read it accurately to
> that value, apparently Linux does not do that.  That makes it questionable
> to me if it can be done.  I could understand when Windows would not
> exploit such a capability, when there is no monetary gain to be made.
> But the Linux developers are too proud and too nerdy to skip such an
> opportunity.

  Inclined to agree. None of the TC data sheets I have looked at show them holding microsecond resolution data. All have just one byte recording integral  seconds. Quite a few RTCs have either a clock out, or programmable square wave outputs, which when used could give access to higher resolution, but as you point out , access to that is not implemented in general purpose computers, or OS's.

> 
> The fact that there is a microsecond-accurate API to set and read the
> clock does not indicate anything.  Remember Linux can run on any platform,
> and there may be other platforms, now or in the future, that can use
> this accuracy.
> 
> In my experience, the clock always jumps a few hundred milliseconds
> when rebooting a PC.
> 
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