[ntp:questions] Using Trimble TSIP under Linux

Rob nomail at example.com
Wed Oct 31 11:44:27 UTC 2012


Martin Burnicki <martin.burnicki at meinberg.de> wrote:
> Rob wrote:
>> David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
>> You need to add a circuit to stretch the narrow pulse into a 100ms
>> pulse.  The exact duration is not important.  Just arrange for a
>> monostable multivibrator that gets triggered by the rising edge
>> of the pulse and extends the pulse by R/C time.  Make sure the pulse
>> is shorter than 500ms or the autodetection logic will focus on the
>> wrong edge of the pulse.
>
> Do you really believe you get true microsecond accuracy out of a PPS 
> signal if the PPS input circuit detects the PPS slope only if the length 
> of the pulse is 100 ms or more?

No, I never claimed that.  I implemented a solution that can be used
by existing systems with existing hardware.  The solution is to use
an RS232 status line, a thread that sleeps on a change on that line,
and code that reads the system clock and puts it in an SHM segment.

This solution has several limitations, including the delay in the
level converter, the delay in the chip, the interrupt latency, and
the process scheduler latency.

However, it is orders of magnitude better than using the timestamps
obtained from serial data messages received from the GPS receiver
via the serial port.  And it does not require patching of the kernel,
re-compilation of the kernel for PPS support (not a standard feature
when I implemented all this), or tweaking of driver code in ntpd.

And it allows access to the gps data (position information) from the
same device that supplies the timing.   Useful in some mobile
applications.

It is not meant to be competition for the products that your company
sells.   Don't listen to the folks who claim that, or suggest this
solution as the answer to every question asked on this group.



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