[ntp:questions] Linux PPS triggering with NMEA

MarkO orzech at nps.edu
Fri Apr 8 19:46:02 UTC 2005


We're currently running ntp version 4.1.1 at 1.786 on linux kernel version
2.4.18-4.  We have a Garmin NMEA GPS connected to the serial port and
sending a (default) 100ms PPS pulse on pin #1.

I have a couple very basic questions.  First, do we need to install
PPSkit in order to synchronize with the Garmin's PPS pulse?  And, if
so, can we use PPSkit 2.0.2 (for linux kernel 2.4.19) with our current
kernel, or do we need to upgrade the kernel as well?

I've run a few tests that lead me to think the answer to both above
questions is "Yes", but since I'm such a NTP novice, I was hoping to
get a confirmation.  In trying to get the PPS synchronization running,
I've set up the ntp.conf file as follows:

server 127.127.20.1 mode 1
fudge 127.127.20.1 flag3 1
fudge 127.127.20.1 time1 0.042
driftfile /etc/ntp/drift

Ten minutes or so after starting ntpd, I get the following output from
ntpq -p:

remote    refid      st t when poll reach   delay   offset  jitter
===================================================================
*GPS_NMEA(1)  .GPS.   0 l    1   64   37    0.000   31.270  27.697

and from ntptime, I get:

ntp_gettime() returns code 0 (OK)
  time c60147cc.bd2c9000  Fri, Apr  8 2005 11:11:24.738, (.738961),
  maximum error 725913 us, estimated error 19588 us
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 26811.000 us, frequency 153.481 ppm, interval 4 s,
maximum error 725913 us, estimated error 19588 us, status 0x1(PLL),
  time constant 2, precision 1.000 us, tolerance 512 ppm,
  pps frequency 0.000 ppm, stability 512.000 ppm, jitter 200.000us,
  intervals 0, jitter exceeded 0, stability exceeded 0, errors 0.


However, the time on our system seems to be consistently about 1/2
second behind the correct time (possibly triggering on the NMEA text
line rather than the PPS pulse?).  In a previous post to this mailing
list, Steven ("rtxo") suggested that you should see the line PPSSIGNAL
in the ntptime output if PPS is running properly.
(ref:
http://groups-beta.google.com/group/comp.protocols.time.ntp/browse_thread/thread/419f18b3a5c58e52/9ca0e3e5470738cd?q=pps&rnum=21#9ca0e3e5470738cd
)

Though our ntptime output does refer to the "pps frequency", it doesn't
have a "ppssignal" line.  

Thanks for any assistance!

-- Mark




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