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

Ed Armstrong eds_equipment at verizon.net
Fri Jun 12 18:29:29 EDT 2015

On 6/10/2015 5:00 AM, Hal Murray wrote:
> eds_equipment at verizon.net said:
>> OK, here is the actual question. Do you think it is OK to consider a  pulse
>> which arise 250 ns early to be close enough? And no, I am not  forgetting
>> about that 3 ns, there is about 3 ns of delay added by the  output
>> circuitry.
> If you want to earn your time-nut merit badge, you have to measure it.
> 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?

> If you are using ntpd, there is a fudge factor that lets you correct for any
> offset.  (and another one that lets you use the other edge, but I think the
> Pi doesn't capture it)

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