[time-nuts] Designing an embedded precision GPS time server
denny at cococafe.com
Fri Oct 27 22:09:57 UTC 2017
> On Oct 26, 2017, at 19:29, Chris Caudle <chris at chriscaudle.org> wrote:
> On Thu, October 26, 2017 7:38 pm, Denny Page wrote:
>> If you are going to do PTP with ptp4l, or NTP with Chrony, you are going
>> to want hardware timestamping support on the ethernet phy.
> Or the MAC. The processor used on BeagleBone Black has timestamping in
> the MAC. Not quite as accurate as stamping in the phy, but should be a
> relatively consistent fixed offset.
Yes, but it might not be as consistent as you would like. The Intel i210 is a pretty good reference. It does mac timestamping and has a built-in phy. At 100Mb, Intel advertises a timestamp latency range of 984-1024ns for outbound packets, and 2148-2228ns for inbound packets. The ranges are a result of mac clock issues, unassociated with phy communication.
External measurement shows that the mean values are actually outside the ranges given, 1044ns for outbound, and 2133ns for inbound. [I suspect, but don’t know for sure, that the offset is a result of a fixed 20ns delay between the phy and mac.] Anyway, on a loopback between two i210 instances, you will see an average total timestamp latency of 3177ns, with a standard deviation of around 100ns. Pretty good, but not that great.
What chip does the BeagleBone Black use? Do they publish specs on the ethernet timestamp latency?
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