On Thu, 6 Feb 2014, rajputaja...@gmail.com wrote:
Dear Sir,
I am trying to synchronize AM335x processor based linux 3.2 with Motorola GPS
receiver using its Serial based NMEA frame and 1pps signal. I have connected
nmea frame to ttyO1 port. I am using buildroot based filesystem for NTP
configuration. NTP version used is 4.2.6p5.
I have configured ntp.conf file for nmea driver 20 for nmea and 1pps options.
(refer: http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html)
# Ajay - for nmea driver 20
server 127.127.20.0 mode 0 minpoll 4 maxpoll 4 prefer
fudge 127.127.20.0 flag1 1 flag2 0 flag3 1 time2 0.500.
flag3 1 - this sets the NMEA driver to use kernel PPS. So this is not a
seperate PPS signal. See below...
time2 is set to 0.600 since delay between 1pps signal rising edge and end of
nmea frame is approx 500ms.
i am issuing below command for nmea driver 20 before starting ntp service:
ldattach pps /dev/ttyO1.
[ 94.658905] pps_core: source OMAP-SERIAL1 got cdev (253:0)
[ 94.658935] pps pps0: new PPS source OMAP-SERIAL1
[ 94.663787] pps pps0: source "/dev/ttyO1" added
Also, while kernel booted it already had registered pps for kernel line
discopline as per below message:
[ 0.181427] pps_core: LinuxPPS API ver. 1 registered
[ 0.181427] pps_core: Software ver. 5.3.6 - Copyright 2005-2007 Rodolfo Giometti
<giome...@linux.it>
ln -s /dev/ttyO1 /dev/gps0 (for linking ttyO1 to /dev/gps0 for nmea driver 20)
(as per http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html)
and
ln -s /dev/pps0 /dev/gpspps0 (for linking pps0 to /dev/gpspps0 for nmea driver
20) (as per http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html)
Now, when i start the ntp service, i get below output for ntpq -p command:
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 0 l 4 16 377 0.000 4526.86 1269.17
Problem: Though i have configured my ntp.conf file for nmea and 1pps both
(using flag1 1), i do not see any entry for PPS in ntpq -p command.
if kernel has registered pps0 as pps source, why there is not PPS entry. I have
connected pps signal of 3.3v level to my dcd pin of ttyO1.
The PPS is 'integrated' in the GPS_NMEA source (via flag3). Eventually
you should get an 'o' next to it (if PPS is working). Here is what I have
on my system:
vom@ice:~$ ntpq -p | grep NMEA
oGPS_NMEA(0) .GPS. 0 l 9 16 377 0.000 0.000 0.001
Also, get the pps utilities and you can run ppswatch against your pps
device like this:
vom@ice:~$ sudo ppswatch /dev/gpspps0
trying PPS source "/dev/gpspps0"
found PPS source "/dev/gpspps0"
timestamp: 1391785256, sequence: 71, offset: 199993776
timestamp: 1391785256, sequence: 71, offset: 199993776
timestamp: 1391785257, sequence: 72, offset: 199999040
timestamp: 1391785257, sequence: 72, offset: 199999040
timestamp: 1391785258, sequence: 73, offset: 200002043
timestamp: 1391785258, sequence: 73, offset: 200002043
timestamp: 1391785259, sequence: 74, offset: 200002614
^C
Total number of PPS signals: 7
Maximum divergence: 200002614
You should also get some pps related output from 'ntpdc -c kerninfo'
vom@ice:~$ ntpdc -c kerninfo
pll offset: -2.55e-07 s
pll frequency: -3.515 ppm
maximum error: 0.007734 s
estimated error: 0 s
status: 2007 pll ppsfreq ppstime nano
pll time constant: 4
precision: 1e-09 s
frequency tolerance: 500 ppm
Also, the offset in NMEA output continously increases insteed of reducing.
Please run ppswatch and see if you are indeed getting PPS into the kernel.
Thanks & Regards,
Ajay
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