[LinuxPPS] Struggling w/ PPS

[Nathan Mapes]

I've been working on this for a number of days and haven't gotten my jitter
down to < 1 ms.  I have a Garmin GPS 18 LVC attached to my Ubuntu 11.10
mini PC (w/ Atom processor).  From the logs I appear to be getting PPS
assert and clears.  The clock is syncing with the GPS, just not using the
PPS.  I compiled the 3.2.8 kernel checking off the PPS modules under
drivers.  I didn't compile ntp because the docs say my version is new
enough to have LinuxPPS support.  I've tried several variations for the
ntp.conf file.  They all do about the same.  Any help would be appreciated.

Thanks,

Nathan


@nano /etc/ntp.conf
....
server 127.127.20.0 prefer minpoll 4
fudge 127.127.20.0 flag1 1 flag3 0 flag2 0 time1 0.0
....


...
Feb 28 16:31:54 jack kernel: [ 2069.872985] pps pps0: PPS event at
1330446714.169615534
Feb 28 16:31:54 jack kernel: [ 2069.872992] pps pps0: capture clear seq #183
Feb 28 16:31:54 jack kernel: [ 2069.872998] pps pps0: PPS clear at 442468
Feb 28 16:31:54 jack kernel: [ 2070.672979] pps pps0: PPS event at
1330446714.969609831
Feb 28 16:31:54 jack kernel: [ 2070.672986] pps pps0: capture assert seq
#183
Feb 28 16:31:54 jack kernel: [ 2070.672992] pps pps0: PPS assert at 442668
...


@ntpq -c rv -p
associd=0 status=0415 leap_none, sync_uhf_radio, 1 event, clock_sync,
version="ntpd 4.2.6p2 at 1.2194 Fri Sep  2 18:37:15 UTC 2011 (1)",
processor="i686", system="Linux/3.2.8-stratum", leap=00, stratum=1,
precision=-20, rootdelay=0.000, rootdisp=102.484, refid=GPS,
reftime=d2f96179.d34aa493  Thu, Mar  1 2012  2:46:17.825,
clock=d2f9617b.cac9daba  Thu, Mar  1 2012  2:46:19.792, peer=19433, tc=4,
mintc=3, offset=23.324, frequency=1.726, sys_jitter=32.444,
clk_jitter=19.162, clk_wander=5.314
     remote           refid      st t when poll reach   delay   offset
jitter
==============================
================================================
*GPS_NMEA(0)     .GPS.            0 l    2   16  377    0.000   23.324
32.444


@/dev ls -l *ps*
lrwxrwxrwx 1 root root          10 2012-03-01 00:57 gps0 -> /dev/ttyS0
lrwxrwxrwx 1 root root           9 2012-03-01 01:58 gpspps0 -> /dev/pps0
crwxrwxrwx 1 root dialout 251,   0 2012-03-01 00:57 pps0


@ntpdc -c kern
pll offset:           0.010596 s
pll frequency:        -0.125 ppm
maximum error:        0.073324 s
estimated error:      0.025693 s
status:               0001  pll
pll time constant:    4
precision:            1e-06 s
frequency tolerance:  500 ppm
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