Is it a good idea to use a usb-serial adapter for PPS? Yes, it is.

[Ross Alexander]

In <24b0eaf25b64d6098b390df092866c69e352d859.camel at 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  jitter
> ==============================================================================
> oPPS(0)          .PPS.            0 l    8   16  377    0.000    0.001   0.002
> *SHM(0)          .GPS.            5 l    6   16  377    0.000  419.464 310.013
>
> ntpq> rl &1
> associd=10146 status=911a conf, reach, sel_falsetick, 1 event, sys_peer,
> srcadr=PPS(0), srcport=123, dstadr=127.0.0.1, dstport=123, leap=00,
> stratum=0, precision=-20, rootdelay=0.000, rootdisp=0.000, refid=PPS,
> reftime=e1005453.fffff7a5  Thu, Aug 15 2019 15:59:47.999,
> rec=e1005454.debc0ef4  Thu, Aug 15 2019 15:59:48.870, reach=377,
> unreach=0, hmode=3, pmode=4, hpoll=4, ppoll=4, headway=0, flash=00 ok,
> keyid=0, ttl=0, offset=0.001, delay=0.000, dispersion=0.233,
> jitter=0.002,
> filtdelay=     0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00,
> filtoffset=    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00,
> filtdisp=      0.00    0.24    0.48    0.72    0.96    1.20    1.44    1.68
>
> ntpq> rl
> associd=0 status=0413 leap_none, sync_uhf_radio, 1 event, spike_detect,
> version="ntpd 4.2.7p397 at 1.2483-o Sun May  3 05:32:19 UTC 2015 (1)",
> processor="armv7l", system="Linux/4.1.12-v7+", leap=00, stratum=6,
> precision=-19, rootdelay=0.000, rootdisp=733.955, refid=SHM(0),
> reftime=e1005456.debbfb5d  Thu, Aug 15 2019 15:59:50.870,
> clock=e100545c.084cd64c  Thu, Aug 15 2019 15:59:56.032, peer=10147, tc=4,
> mintc=3, offset=0.000921, frequency=0.047, sys_jitter=310.013202,
> clk_jitter=0.000, clk_wander=0.000

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

============================================================================
Ross Alexander, (780) 675-6823 desk / (780) 689-0749 cell, rwa at athabascau.ca
                         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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