Subject: Re: ntp and pps
To: None <email@example.com>
From: Wolfgang Rupprecht <firstname.lastname@example.org>
Date: 05/12/2000 11:52:48
email@example.com (Andreas Wrede) writes:
> Is anyone using ntp refclocks that make use of kernel pps?
Kernel pps yes, ntp no. I see the same problems with ntp that you do.
I did get a small PPS program (essentially lifted from "RFC 2783,
Pulse-Per-Second API, March 2000" running under -current.
One thing I noticed was that the CLEAR/ASSERT settings were reversed.
One I inverted the test, I do get a stream of PPS reports as such:
Clear timestamp: 958156930.999696000, sequence: 419
Clear timestamp: 958156931.999696000, sequence: 420
Clear timestamp: 958156932.999697000, sequence: 421
Clear timestamp: 958156933.999698000, sequence: 422
Clear timestamp: 958156934.999695000, sequence: 423
Clear timestamp: 958156935.999695000, sequence: 424
Clear timestamp: 958156936.999696000, sequence: 425
Clear timestamp: 958156937.999696000, sequence: 426
Clear timestamp: 958156938.999695000, sequence: 427
Clear timestamp: 958156939.999696000, sequence: 428
Clear timestamp: 958156940.999695000, sequence: 429
Clear timestamp: 958156941.999695000, sequence: 430
Clear timestamp: 958156942.999697000, sequence: 431
Clear timestamp: 958156943.999697000, sequence: 432
Clear timestamp: 958156944.999698000, sequence: 433
Clear timestamp: 958156945.999695000, sequence: 434
Clear timestamp: 958156946.999695000, sequence: 435
Clear timestamp: 958156947.999695000, sequence: 436
Clear timestamp: 958156948.999695000, sequence: 437
Notice that the nS part of the timestamp is always 0.
I'm concerned that most of the advantage of using a GPS with a few
10's of nS of noise on it PPS line will be lost if the kernel proceeds
to timestamp things with only 1uS of resolution.
Anyone know how the other open source folks do this? Do they grab the
higher frequency information from the CPU's tick-counter register?
Wolfgang Rupprecht <firstname.lastname@example.org>
DGPS signals via the Internet http://www.wsrcc.com/wolfgang/gps/dgps-ip.html