[time-nuts] Raspberry Pi NTP server

[ASSI]

On Dienstag, 7. Juli 2020 18:27:01 CEST Petr Titěra wrote:
> Timing on USB need not to be so horrible. Below is stats from my server
> with GPS connected using FT232H chip (supporting high speed transfers on
> USB). Yes, the jitter is far greater than on other computer where PPS is
> connected directly but it is a lot less than that 500microseconds you
> get with common USB convertors.
> 
>       remote          refid     st t when poll reach delay offset jitter
> =======================================================================
> o127.127.22.0   .PPS.           0 u    7   16  377  0.000 -0.019  0.033
> *192.168.3.240  .GPSD.          1 u   24   64  377  0.377  0.187  0.026
> +192.168.3.246  .PPS.           1 u   28   64  377  0.643  0.181  0.028

The reason you're seeing this with the newer FTDI chips that support USB2.0 
highspeed rates is that the frame rate got increased 8x for highspeed USB, so 
the expected frame jitter is now 125µs when and if the interface as well as 
the full protocol stack support and enable it.  But you seem to have missed 
the point that Hal was trying to make: The jitter you are going to see has 
deterministic components and some of these can create bias when you try to 
filter with the usual assumption of a stationary zero-mean random sampling.  
In other words, you don't necessarily converge to the true time and where your 
filter tries to converge varies over time.


Regards,
Achim.
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