[ntp:questions] Garmin GPS 18LVC Setup but questions on best way

Unruh unruh-spam at physics.ubc.ca
Wed Dec 31 17:57:25 UTC 2008


George R. Kasica <georgek at netwrx1.com> writes:

>On Wed, 31 Dec 2008 00:20:45 GMT, "David J Taylor"
><david-taylor at blueyonder.neither-this-part.nor-this-bit.co.uk> wrote:

>>Unruh wrote:
>>> "David J Taylor"
>>[]
>>>> With just the reference clock type 20, I get the accuracy needed.
>>>> The PPS line from the GBS-18 LVC is wired to the DCD line of the
>>>> serial port.  In my ignorance, I don't see why you even need the SHM
>>>> driver, but as I said before, I'm no expert!  I don't see why my
>>>> system picks up the PPS from just the type 20 driver, and yours does
>>>> not.
>>>
>>>
>>> Because you have the PPS kernel patch installed or it was
>>> automatically
>>> instaled in BSD? The OP does not and does not want to.
>>
>>Thanks.  Yes, I had to recompile the FreeBSD kernel to include this patch. 
>>I must have thought at the time that it was mandatory, or was advised to 
>>do this.

>I'd be happy to put it in here, but doing so with the Fedora Core 9
>RPM based code is not something I'm familiar with. I'm used to working
>with straight source code for kernels ie make make install type steps
>(don't take those literally please I know the steps) or just leaving
>the RPMs alone. I just am not sure how or if the PPS patches I've seen
>will apply to the kernel version here on the FC9 box or how it will
>affect the system-obviously breaking it is not an option - well, it
>always is but not a good one since it archives a ton of weather data
>:). 

I think it will gain you nothing over the shm option, except you will be
sitting there worrying and watching your computer compiling a kernel.

However, has anyone done tests on the serial port interrupt to see how much
latency there is in that? Ie, If I use, say, shmpps and I trigger the serial
control line at time t, what is the time t+dt that shmpps reports as the
time of the trigger? And what is the fluctuation in dt? On the parallel
port interrupt service routine I have done that and the result was about
0-2usec. (in fact I suspect most of the noise in at least the short term
fluctuation of ntpd is from that source). But shmpps and gpsd have a whole
layer of serial port driver interposed between the interrupt and the
report. Does that made a difference?



>If someone can detail what needs to happen I'd be happy to run it that
>way, though as of right now I'm not sure what it would gain me over
>the shm driver option. FYI the kernel I'm running is:

>2.6.27.7-53.fc9.i686 #1 SMP

>and loaded rpms are:

>kernel-devel-2.6.27.7-53.fc9.i686
>kernel-headers-2.6.27.7-53.fc9.i386
>kernel-firmware-2.6.27.7-53.fc9.noarch
>kernel-2.6.27.7-53.fc9.i686
>kernel-doc-2.6.27.7-53.fc9.noarch

>-- 
>===[George R. Kasica]===        +1 262 677 0766
>President                       +1 206 374 6482 FAX 
>Netwrx Consulting Inc.          Jackson, WI USA 
>http://www.netwrx1.com
>georgek at netwrx1.com
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