[time-nuts] PPS for NTP Server - How Close Is "Good Enough"?

Attila Kinali attila at kinali.ch
Mon Jun 15 18:22:50 EDT 2015


On Fri, 12 Jun 2015 18:29:29 -0400
Ed Armstrong <eds_equipment at verizon.net> wrote:

> > The Ethernet on the Raspberry Pi is on the USB bus.  That adds about a 1/2 ms
> > of jitter.
> Is it possible to modify the kernel so the USB is polled more often, and 
> would that significantly reduce the jitter?

Yes. You can add more interrupt slots per frame. IIRC there was an
upper limit on how many you can add (sorry, I don't remember any
specifics, it's been some time since I read the USB standard).
What you need to ensure as well is, that the device itself does nothing
weird with the data, aka that any pipelining and collation is turned off.

			Attila Kinali

-- 
I must not become metastable. 
Metastability is the mind-killer.
Metastability is the little-death that brings total obliteration.
I will face my metastability. 
I will permit it to pass over me and through me. 
And when it has gone past I will turn the inner eye to see its path. 
Where the metastability has gone there will be nothing. Only I will remain.




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