rpi3 clock drift

[Ross Alexander]

On Wed, 27 Nov 2019, IanLepore wrote:

> Date: Tue, 26 Nov 2019 15:48:54 -0700
> From: Ian Lepore <ian at freebsd.org>
> To: James Shuriff <james at opentech.cc>, Bakul Shah
>       <bakul at bitblocks.com>
> Cc: "freebsd-arm at freebsd.org" <freebsd-arm at freebsd.org>
> Subject: Re: rpi3 clock drift
> Message-ID:
>       <486c0bc46deeefb8338f50fe6dfdf7951786e56c.camel at freebsd.org>
> Content-Type: text/plain; charset="UTF-8"
>
> On Tue, 2019-11-26 at 22:34 +0000, James Shuriff wrote:
>> kern.timecounter.tc.ARM MPCore Timecounter.frequency: 54000000
>
> 54 MHz seems like a very strange frequency for a clock in an arm
> system.  Of course, everything about an RPi seems to be strange and
> non-standard.
>
> Can someone else with an rpi3 (which keeps good time) confirm that
> that's the usual number?
>
> -- Ian

Mine does reasonably well; it's disciplined to a local stratum-0 (a
pi2 with a GPS hat supplying PPS).  The timecounter values are

     rpi3:/var/db/ntp # sysctl kern.timecounter

     kern.timecounter.fast_gettime: 1
     kern.timecounter.tick: 1
     kern.timecounter.choice: ARM MPCore Timecounter(1000) dummy(-1000000)
     kern.timecounter.hardware: ARM MPCore Timecounter
     kern.timecounter.alloweddeviation: 5
     kern.timecounter.timehands_count: 2
     kern.timecounter.stepwarnings: 0
     kern.timecounter.tc.ARM MPCore Timecounter.quality: 1000
     kern.timecounter.tc.ARM MPCore Timecounter.frequency: 19200000
     kern.timecounter.tc.ARM MPCore Timecounter.counter: 1547324379
     kern.timecounter.tc.ARM MPCore Timecounter.mask: 4294967295

and the ntpq info looks like

     rpi3:/var/db/ntp # ntpq

     ntpq> lpee
         remote           refid      st t when poll reach   delay   offset  jitter
     ==============================================================================
     *debian2.bogons  .PPS.            1 u   39  512  377    0.759    5.328   3.179

     ntpq> rl &1
     associd=63559 status=961a conf, reach, sel_sys.peer, 1 event, sys_peer,
     srcadr=debian2.bogons, srcport=123, dstadr=192.168.2.6, dstport=123,
     leap=00, stratum=1, precision=-19, rootdelay=0.000, rootdisp=1.099,
     refid=PPS, reftime=e18933b7.00541adb  Wed, Nov 27 2019 10:41:11.001,
     rec=e18933be.acaf7c6a  Wed, Nov 27 2019 10:41:18.674, reach=377,
     unreach=0, hmode=3, pmode=4, hpoll=9, ppoll=9, headway=17, flash=00 ok,
     keyid=0, offset=5.328, delay=0.759, dispersion=14.153, jitter=3.179,
     xleave=5.695,
     filtdelay=     0.87    0.76    0.78    0.77    0.79    0.79    0.71    0.80,
     filtoffset=    6.01    5.33    4.48    3.45    2.52    1.53    0.75    0.56,
     filtdisp=      0.00    7.95   16.10   23.85   31.71   39.74   47.61   55.62

     ntpq> rl
     associd=0 status=0618 leap_none, sync_ntp, 1 event, no_sys_peer,
     version="ntpd 4.2.8p12-a (1)", processor="arm64",
     system="FreeBSD/13.0-CURRENT", leap=00, stratum=2, precision=-21,
     rootdelay=0.759, rootdisp=23.748, refid=192.168.2.3,
     reftime=e18931ac.ad6bd044  Wed, Nov 27 2019 10:32:28.677,
     clock=e18933ef.81828f21  Wed, Nov 27 2019 10:42:07.505, peer=63559, tc=9,
     mintc=3, offset=5.327665, frequency=-7.309, sys_jitter=0.000000,
     clk_jitter=1.121, clk_wander=0.013, tai=37, leapsec=201701010000,
     expire=201912280000

     ntpq> quit

This box is drifting a bit right now because I've got a 'make -j 3
buildworld' running; the cheapo little resonators that the Pi folks
use instead of crystals drift madly with thermal stress.  I can see
the house furnace cutting in and out by graphing the loopstats file
:).  While idle it's usually good to less than half a millisecond.

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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