[time-nuts] Looking for off-the-shelf device to timestamp multiple PPS in...

EWKehren at aol.com EWKehren at aol.com
Wed Sep 28 18:21:14 UTC 2011

look at the attached description of the 5 channel counter system that I use 
 on my Dual Mixer it does what I call pseudo time stamp. Counters are 
always  counting. Code and write up courtesy of Richard McCorkle.
Bert Kehren
In a message dated 9/28/2011 12:23:49 P.M. Eastern Daylight Time,  
kevin at rosenberg.net writes:

Hello  esteemed time nuts,

I'm looking for device with an external clock input  as well as inputs
for multiple PPS signals that outputs timestamped  messages for each
input pulse via RS232. The initial need will be for two  PPS inputs.
I've considered a number of options, but I wonder if there a  more
prepackaged solution available since the device will for someone  less
technical than myself.

A bit more about the device. It needs to  be clocked with an external
reference. Ideally, taking 10 Mhz sine from the  PRS-10, but I can
easily enough convert the sine wave that to a square  wave.  Also, I
can use a TAPR ClockBox to convert to other reference  frequencies.

The purpose of the device will be to measure PPS periods  for at least
two oscillators over a course of a few months, sending the  data via
RS-232 to a logging computer for each input PPS input. The minimum  the
serial record will need to have is a time stamp of  seconds.nanosecons
and a label of which PPS signal triggered.

About  options I've considered,

1) A Soekris 4501 clocked by the PRS-10 to a  ClockBox, and then using
the TMR1IN and a GPIO input to time stamp the PPS  signal. However,
that'll require one 4501 for each PPS signal, so scaling  effort is

2) Writing a firmware for an AVR XMEGA that is  clocked from the PRS-10
and uses external interrupts and timers to output  RS-232
messages. Certainly less expensive than #1 option, but takes  more
time programming.

3) Using some other micro-controller with  external clock and times,
perhaps an ARM.

4) Using a dedicated time  counter for each PPS. However, reading the
manual for my PM6680/B as well  as a Agilent 53230A, that won't work
because in standard gating mode, some  time would be missed between PPS
signals. While the 53230A has a continuous  mode, that would require
the polling computer to accurately keep time of  the requests unless
the 53230A is collecting data under its 1,000,000  sample limit (only
~11 days at a PPS). Also, at $3500 per PPS input, a  53230A is the
most expensive to scale..

5) Using a FreeBSD or Linux  box with multiple RS-232 inputs and using
their PPS API.  However,  outside of some ARM boards, I haven't found
any boxes that can be clocked  by a external reference. And, I haven't
enough experience running FreeBSD  or Linux on ARM to know how well
this will work for me.

6) I'm quite  familar with using something like a Thunderbolt and its
PPS to make a  stable clock with NTP which could be used with an Intel
FreeBSD or Linux  box in #5. However, rather than NTP, the device
should be clocked only by  the external reference clock. The goal is to
see how the input PPS signals  deviate from the reference clock.

7) Use a box like in #5, and capture  the PPS from the PRS-10 as well
and using that to scale the other PPS  inputs that arrive around the
same time.

I've done web searches,  found some data loggers, but I haven't found
something that can run  continuously for months collecting data and
meet the other needs. I'd  really like to find a more prepacked
solution for the user of this device.  I'm surprised by my inability
find such a device for this seemingly  straightforward task.

Thanks for considering the issues. I feel I'm  overlooking some perfect
device that exists outside of just my  imagination.


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