[time-nuts] Making a Raspberry Pi NTP server without soldering

[Chris Albertson]

Yes it is polled.  But polled at what rate?  It adds jitter but the figure
of merit is the jitter in the system clock at the client side.  How much
does this USB polling degrade the client's clock?


On Tue, Oct 22, 2013 at 11:48 AM, Hal Murray <hmurray at megapathdsl.net>wrote:

>
> albertson.chris at gmail.com said:
> > How the MAC connects is kind of like worrying if the chrome platting in
> that
> > little hook on your tape measure has a well controlled plating thickness.
> > Yes of course it matters, just not enough to worry about because all the
> > other error sources are much larger.
>
> I think that's misleading.  USB is polled.  That adds a layer of jitter.
>
> There are probably some systematic errors too.  When you send a packet, it
> leaves synchronized with the USB clock.  The return packet will also be
> synchronized with that clock so the return network time will be rounded up
> to
> the next clock tick with no jitter.
>
> It would be interesting to see if we can measure it.
>
> Besides, this is time-nuts.  We pay attention to that sort of detail.
>
> But I agree that if all you are interested in is having reasonable time on
> nearby PCs a few ms won't matter.
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
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-- 

Chris Albertson
Redondo Beach, California