| Re: [chrony-users] gpsd, pps and chrony |
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On Mon, 4 Apr 2011, Ed Wildgoose wrote:
On 04/04/2011 17:19, Bill Unruh wrote:
On Mon, 4 Apr 2011, Ed W wrote:
Also I would like to investigate contributing 2 servers to the ntp pool.
I would like to try and use chrony for these... I understand this
hasn't been done previously, any thoughts?
My impression is that the pool people ONLY allow ntpd in the pool. And
they
test to make sure that you are running ntpd (eg using ntpq I suspect).
Hmm, well I guess we will see whether they are buying or selling...
However, if they are being strict on choice of software, then using an
"unreleased" software version is a strawman which is easy to put up.
There is a bunch of cool stuff in the current git and I think it would
be great to get a release out fairly imminently?
Nope. Hearing the name "chrony" will drive Mills to distraction:-)
Oh, my freetime project at the moment is to try and get the GPIO
interrupts working as a kernel PPS module on the Alix board (hoping to
get higher accuracy than using a serial port...) This coupled with an
old Garmin 18 appears to allow for the possibility of a nice Stratum 1
time server for around the $100 ish mark... Could be cool! Will keep
you informed how it goes...
The problem is not the serial port interrupt. It is as fast as any other I
believe. The problem is the time between the interrupt being serviced by the
kernel, and the program (gpsd) being notified that an interrupt has occured
and then timestamped. If you rewrite the serial interrupt service routine to
timestamp the interrupt as soon as it gets it, it should be much much better.
I wrote a parallel port interrupt service routine to do that. You still get
usec level wander (ie it takes about 1-2 usec for the system to service the
interrupt and do the time stamping-- tested by having a program generate an
interrupt by putting out a signal on a parallel pin to the parallel interrupt
line (ACK) timing when that output went out and then having the interrupt
service routine timestamp the incoming interrupt.) but that is pretty good. A
Garmin 18 is not capable of better than that anyway.
For a cheaper version, get the sure GPS board. It is supposed to give about
20ns timing on the PPS line, rather than garmin's 500-1000ns
Cheers
Ed W
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William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
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