[time-nuts] Net4501

[Mike S]

On 6/5/2013 6:51 PM, Didier Juges wrote:
> without using the high stability CPU oscillator option, just the PPS
> from one of my Thunderbolts.
>
> Is that even worthwhile? Will the high resolution built-in counters
> of the Elan processor be useful without the external oscillator
> compared to a plain vanilla ARM SBC?

It may actually be worse. I've got three running with the ELAN timer 
tweak, but only the standard CPU crystal. Two with Oncore 12+, one with 
Garmin 18x. All produce loopstat offsets in the 10's of us, very closely 
linked to temperature. A tbolt feeding a PC keeps the offset much 
tighter than any of the three, but has much more jitter. One 4501 will 
do better than the PC if the temperature is fairly stable (slowly 
changing 3C diurnal change).

I think that's because when NTP sees low jitter, it increases the time 
constant, so it takes much longer to correct for temperature related 
excursions.

I haven't really looked into it further, there may be an NTP tweak which 
forces a shorter time constant.