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_ -- This communication is intended for the use of the recipient to whom it is a= ddressed, and may contain confidential, personal, and or privileged informa= tion. Please contact us immediately if you are not the intended recipient o= f this communication, and do not copy, distribute, or take action relying o= n it. Any communications received in error, or subsequent reply, should be = deleted or destroyed. ---
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