[ntp:hackers] Reduce jitter of refclocks (connected via USB)

Volker Strauß Volker.Strauss at gmx.de
Tue Mar 15 19:49:59 UTC 2011


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Hi Martin,

15.03.2011 10:27, Martin Burnicki:

> First, please keep in mind that the accuracy provided by the DCF77 AM
> signal is also only in the same range.

OK, I know, but I assume the jitter of the internal clock is not as high
as the jitter measured on my system...?

> The device is USB 1.1, but supports microframing i.e. 125 us rather than
> 1 ms. Unfortunately the OS device drivers don't use this if they see a
> 1.1 device. If you connect the device via a USB 2.0 hub, though, the
> microframing is used and thus the additional error is reduced to 125 us.

Good to know. At the moment I've no USB hub, but I will try that if
there's one present (maybe I've to buy one ;) )


> Please note the Meinberg USB devices do not simply send a serial ASCII
> string via a built-in serial-to-USB converter. They use the same binary
> data structures as the PCI cards, and can be accessed from applications
> using the same API calls as the PCI cards. This requires the Meinberg
> driver package for Linux.
>
> The original approach of the driver package was to feed the timestamps
> to ntpd via ntpd's parse driver, in which case at each second changeover
> an IRQ is generated by the PCI devices, and a data packet is sent
> automatically by USB devices.
>
> The drawback of this approach is the latency occuring from the moment
> the second changeover occurs (and thus the IRQ is generated or the USB
> packet is sent) until the timestamp from the refclock arrives in the
> parse driver where an associated timestamp of the system time is taken.
>
> Volker, the development version of the Linux driver I've sent you
> supports also a different approach where the mbgsvcd daemon which comes
> with this driver package lets the kernel driver read both a timestamp
> from the refclock (PCI or USB) and an associated system timestamp. The
> pair of timestamps is then fed to ntpd via ntpd's shared memory driver.
> This approach avoids latencies and execution times which lead to a time
> offset even if a PCI card is being used as reference.
>
> The low level routine which reads a timestamp from an USB device is
> shared among the Linux and Windows version of our driver package, but
> contains conditional code for Windows which reduces the jitter from the
> underlying USB layer. We are planning to port this also to the Linux
> version, but this has not yet been implemented and tested.
>
> Anyway, under Windows this already provides pretty good accuracy with
> the TCR51USB device which receives an IRIG signal and thus yields much
> better internal accuracy than a DCF77 AM receiver. I'm sure this is also
> true for Linux when it has been implemented.
>
> To test the new approach you simply need to configure the shared memory
> driver rather than the parse driver for the USB device, and start the
> mbgsvcd program.
>
> In ntp.conf, replace the lines
>
> server 127.127.8.0 mode 2 minpoll 4 maxpoll 4 noselect
> fudge server 127.127.8.0 ...
>
> etc. by the new lines
>
> server 127.127.28.0 minpoll 4 maxpoll 4 iburst
> fudge 127.127.28.0 refid shm0
>
>
> If you run mbgsvcd in the foreground then it prints the determined
> offset between the USB device time and the system time on the console.
> Again, keep in mind the accuracy is also limited due to the DCF77 signal.

Sounds good. But it runs on Linux... I tested the configuration and it
worked. However, the result (-> jitter) wasn't better than before. It
seems that I have to wait until it's implemented also for Linux.

Thank you for the information!

Best regards

Volker
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