[time-nuts] Low cost GPS module for < 100ns timestamping error
hmurray at megapathdsl.net
Sat May 3 01:47:04 EDT 2014
tnuts at toneh.demon.co.uk said:
>> In general, low cost oscillators make pretty good thermometers.
> True, but it's short term stability that matters here - over 10s of seconds
> the temperature shouldn't change much - especially if a bit of insulation
> is used around the oscillator.
Ballpark is 1 PPM per degree C. That's 1 microsecond/second. You are
interested in low ns, so you have to at least think about temperature
changes. Will your box be outdoors in the sun? What happens when a big
black cloud comes over?
> I'm not sure I understand this - I need to record the time that an event
> occurs so I need an accurate time reference. What can I store to
> post-process? The time reference drift and jitter relative to the local
> oscillator before and after the event?
I was assuming that you would capture the "time" of your events and also the
"time" of several/many PPS events surrounding your events. By "time", I mean
the counter value from the counter/timer module.
I'm not familiar with the chip you are interested in, but the ARM SOC chips
I've worked with have a switching matrix between the input pins and their
collection of IO modules. I would try to set things up so the PPS signal can
be used by both counters. (maybe using 2 pins) The idea is to switch the
event counter to the PPS signal to find the offset between the counters.
You also have to monitor the serial port so you can label the time of a PPS
pulse and/or do sawtooth correction. You also need status info. How good is
the fix? ...
What are you going to do if/when the GPS fades out? (or gets confused by
One of the reasons to collect some data is so you know what happens just
before and/or just after your GPS unit decides it can't get a good fix.
These are my opinions. I hate spam.
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