[ntp:questions] SNTP server + ntpd 4.2.4 client

Unruh unruh-spam at physics.ubc.ca
Mon Mar 17 18:07:21 UTC 2008

"Dennis Hilberg, Jr." <timekeeper at dennishilberg.com.invalid> writes:

>Unruh wrote:
>>> My system is running a Linux kernel patched with real-time support.
>>> I don't feel confident applying the PPS support patch on top of it.
>> No need. Just attach the gps as a refclock. The kernel does not need pps
>> support to use the refclock.

>The Linux kernel does not have built-in PPS support, so yes he would have to 
>patch and recompile the kernel in order to use the PPS provided by the GPS 
>device. Otherwise it will just be using NMEA time, which is not very 
>accurate for timing purposes. For Linux 2.4 there is the PPSkit, and for 
>Linux 2.6 there is LinuxPPS.

>Instead you can use the shmpps driver to use the PPS signal without patching 
>the Linux kernel. I use it and it works very well.

Precisely. As I said, your kernel does not need pps support to use the
refclock. I also use it and it works well. (well, I actually modified it,
so I have a purpose built interrupt service routine to service the parallel
port interrupt and read the clock. The shmpps then reads those times, and
sends them to ntp.  A bit of a rube goldberg but it works. 
I like the parallel port interrupt better than the serial but am not sure

>FreeBSD has built-in PPS support (no patch needed), but it's not enabled by 
>default. PPS support has to be enabled in the kernel config and the kernel 

And exactly what is that supposed to buy you? 
You need something which can be interrupted by the pps signal from the gps,
and read the system time on that interrupt. That does not require kernel
support (except of course being able to respond to interrupts-- and if your
kernel cannot do that, you forgot to switch on your computer.)


>Dennis Hilberg, Jr.     \  timekeeper(at)dennishilberg(dot)com
>NTP Server Information:  \  http://saturn.dennishilberg.com/ntp.php

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