[ntp:questions] NTP Cheat Sheet

Bruce bmpenrod at comcast.net
Thu May 8 08:38:27 UTC 2008


Dave,

When introducing our CDMA technology, EndRun thought it wise to make the 
initial product emulate existing hardware that had already been 
interfaced to unix machines via the serial port and "gadget boxes."  The 
CDMA engine which is inside both the Ct and your Cntp emulates both the 
Truetime and Spectracom time strings, with 1PPS on DCD.  In addition, it 
emulates the Trimble Palisade event capture mode on the CTS input, with 
30 ns resolution.  One benefit to this driver is high resolution 
capability on the Windows platform, another is not needing to mess with 
kernels that don't natively support high-resolution 1PPS timetagging.

Users of the Ct can choose the emulation mode they like and use the 
standard drivers in the stock ntp distribution.  However, all of our ntp 
servers operate the CDMA or GPS engine in the CTS event capture mode 
with a heavily customized refclock driver.

While developing our first generation ntp servers (your Cntp is one of 
these) using off the shelf X86 embedded single board computers running 
linux, we found that longer poll intervals definitely exposed the 
instabilities of the uncompensated crystals used on these boards.  We 
found that with high measurement precision and holding the poll interval 
at 16 seconds, one microsecond level offset control could be maintained 
reliably, and have never experienced any loop damping issues.

Our current products contain our own purpose-built X86 core with clocks 
that are hardware phase locked to the main system disciplined 
oscillator, which could be TCXO, OCXO or Rubidium.  In these units, a 
shorter polling interval is really only an issue at startup for more 
rapid ntp lock because the kernel clock is rock solid between polls.

Bruce Penrod

David L. Mills wrote:
> Kevin,
> 
> Strange. I have an EndRun Cntp, bu it has an Ethernet interface. If 4 
> works for you use it. That's a pretty grotty driver with all kinds of 
> bandaids to deal with long forgotten radios; I wonder why EndRun chose 
> that driver...
> 
> Dave
> 
> Kevin Oberman wrote:
>> Dave,
>>
>> Thanks for the quick response, but I guess I failed to make one point
>> clear.
>> EndRun supplies only the hardware. It is the Ct which as only a serial
>> interface which is plugged into a standard COM port on the server. They
>> don't provide any software at all. The clock just provides the time
>> string every second (with an offset of about 10 ms. and jitter in the
>> area of 3-5 ms.) and a very accurate PPS which allows us sync within
>> about three usec. (The clock spec is better than ten usec, but three
>> seems the norm.)
>>
>> I am running 4.2.0-a using the stock TrueTime and PPS drivers. The
>> FreeBSD kernel has PPS support included, so any filtering in the
>> TrueTime driver should be of little or no impact.
>>
>> I am assuming than as long as I am using PPS and training the TrueTime
>> source, a minpoll of 4 is reasonable and I just wanted to make sure that
>> I am not wrong in doing this.




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