[time-nuts] Linux PPS clues?

[Chris Albertson]

On Wed, Oct 19, 2016 at 11:15 PM, Ilia Platone <info at iliaplatone.com> wrote:

> I need to know how much precise this system can be. How much resolution
> can I obtain with a cheap receiver (maximum quantization frequency)?
> Formulas are well accepted.



Even a cheap receiver will have error that is orders of magnitude smaller
then the resolution that the linux kernel can work in.

You should expect the system time to have error on the order of about 10
microseconds

The PPS signal error comping out of events low cost GPS receiver has error
oon order of say a few tens of nano seconds, that is 100 to 1000 times less
error

The major source of error is not GPS , but is the interrupt latency
uncertainty. But this is not bad at all, you can expect roughly a 10
microsecond uncertainly in the time stamps more or less.

But a lot depends on how you handle the second GPIO interrupt.  the GPS
interrupt is handled inside a kernel level handler.  The handler snap shots
the system clock and stores it in RAM, then sets a flag that the user space
process checks.    Does your GPIO interrupt have a kernel level driver to
snapshot the system clock ir is this happening in a user space process?  If
the later expect worse performance.    For best performance copy and paste
the Linux PPS code and then re-build the Linux kernel with your new driver.


Getting event time stamps much better then this requires some purpose built
hardware outside of the computer.

Chris Albertson
Redondo Beach, California