[ntp:questions] Accuracy of NTP - Advice Needed

unruh unruh at invalid.ca
Sat Dec 24 06:06:19 UTC 2011


On 2011-12-24, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Fri, Dec 23, 2011 at 5:51 PM, Mike S <mikes at flatsurface.com> wrote:
>> At 01:58 PM 12/23/2011, Chris Albertson wrote...
>>
>>> But there is no hop of doing uS
>>> level over the gigabit either net.
>>
>>
>> IEEE 1588.
>
> Seem 1588 can work at the sub uS level but actual real-work software
> on Linux works at the "tens of uS" level.  The limiting factor is the
> hardware's ability to time stamp eithernet packets.   If you have
> special hardware you can get to sub-uS but not using PTPd on Linux or
> BSD.

If you really go to  stamping the interrupt directly as it comes in on
the kernel level, rather
than waiting for a driver (eg the serial driver) to report that the
itnerrupt has occured to userland, you can get it down to 1-2us.


>
> Linux's and BSD's time stamper for the PPS interface works better
> (less jitter) then its time stamper for Ethernet.
>
> In a standard Cat-5 cable there are 8 wires in 4 pair.  Ethernet only
> uses 2 pair.  I simply used a "spare" pair for a pulse.   The parts
> required cost under $5 assuming you have tools to build cables already
> or old cables you can hack.
>
 >



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