Date: Thu, 15 Aug 2019 16:02:56 -0600 (MDT) From: Ross Alexander <rwa@athabascau.ca> To: freebsd-arm@freebsd.org Subject: Re: Is it a good idea to use a usb-serial adapter for PPS? Yes, it is. Message-ID: <alpine.BSF.2.21.99999.352.1908151522260.30858@autopsy.pc.athabascau.ca>
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In <24b0eaf25b64d6098b390df092866c69e352d859.camel@freebsd.org>,
Ian Lepore writes:
> [... ed.] I arranged to use a very stable nearly-drift-free
> frequency source instead of a cheap crystal for counting time in the
> kernel.
You have my complete and focussed attention. Say on.
> [WRT USB 2, ed.] the polling happens in time slots that are 125
> microseconds wide. There's just no reason for a lot of jitter or
> latency.
125 microseconds is a lot of jitter. Latency is a don't care, you can
fudge that out. Looking at a Pi 1b+, running some consumer grade
Ublocks GPS module, a five year old Linux, and with a view of only
half the sky (but using PPS on a GPIO pin):
> autopsy:/u0/rwa > ntpq chime
>
> ntpq> lpee
> remote refid st t when poll reach delay offset j=
itter
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D
> oPPS(0) .PPS. 0 l 8 16 377 0.000 0.001 =
0.002
> *SHM(0) .GPS. 5 l 6 16 377 0.000 419.464 31=
0.013
>
> ntpq> rl &1
> associd=3D10146 status=3D911a conf, reach, sel_falsetick, 1 event, sys_pe=
er,
> srcadr=3DPPS(0), srcport=3D123, dstadr=3D127.0.0.1, dstport=3D123, leap=
=3D00,
> stratum=3D0, precision=3D-20, rootdelay=3D0.000, rootdisp=3D0.000, refid=
=3DPPS,
> reftime=3De1005453.fffff7a5 Thu, Aug 15 2019 15:59:47.999,
> rec=3De1005454.debc0ef4 Thu, Aug 15 2019 15:59:48.870, reach=3D377,
> unreach=3D0, hmode=3D3, pmode=3D4, hpoll=3D4, ppoll=3D4, headway=3D0, fla=
sh=3D00 ok,
> keyid=3D0, ttl=3D0, offset=3D0.001, delay=3D0.000, dispersion=3D0.233,
> jitter=3D0.002,
> filtdelay=3D 0.00 0.00 0.00 0.00 0.00 0.00 0.00 =
0.00,
> filtoffset=3D 0.00 0.00 0.00 0.00 0.00 0.00 0.00 =
0.00,
> filtdisp=3D 0.00 0.24 0.48 0.72 0.96 1.20 1.44 =
1.68
>
> ntpq> rl
> associd=3D0 status=3D0413 leap_none, sync_uhf_radio, 1 event, spike_detec=
t,
> version=3D"ntpd 4.2.7p397@1.2483-o Sun May 3 05:32:19 UTC 2015 (1)",
> processor=3D"armv7l", system=3D"Linux/4.1.12-v7+", leap=3D00, stratum=3D6=
,
> precision=3D-19, rootdelay=3D0.000, rootdisp=3D733.955, refid=3DSHM(0),
> reftime=3De1005456.debbfb5d Thu, Aug 15 2019 15:59:50.870,
> clock=3De100545c.084cd64c Thu, Aug 15 2019 15:59:56.032, peer=3D10147, t=
c=3D4,
> mintc=3D3, offset=3D0.000921, frequency=3D0.047, sys_jitter=3D310.013202,
> clk_jitter=3D0.000, clk_wander=3D0.000
The jitter is expressed in units of 1 millisecond, unless I am badly
mistaken; for which possibility I apologize in advance.
(as an aside, has editing quotation text gone utterly out of style?
Present company excepted, of course.)
regards,
Ross
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D
Ross Alexander, (780) 675-6823 desk / (780) 689-0749 cell, rwa@athabascau.c=
a
54.71593 N 113.30835 W
Order is simply a thin, perilous condition
we try to impose on the basic reality of chaos.
-- William Gaddis, _J R_
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