[time-nuts] Synchronization

[jimlux]

On 12/5/19 5:16 AM, Bob kb8tq wrote:
> Hi
> 
> One timing issue could be that USB is not that great for timing. Since it is
> packet based it introduces jitter in the link. Running NTP on a “traditional”
> RPi is unlikely to produce numbers below a milisecond.


USB only guarantees 125 microsecond timing (8kHz) - that's the basic 
"frame rate" for isochronous channels, and was designed years ago to 
make it possible to do telephone quality audio in real time.

It might well be that USB 2 and USB 3 have shortened the timing windows 
as well as increasing the speed, but I wouldn't be counting on it.  One 
might also have specific USB implementations that are better, but you're 
depending on a "feature" of the chip, not part of the spec.  I did see a 
mention on wikipedia about some interrupt latency of about 1 microsecond 
in high speed transfers, which is substantially better than the 125 
microsecond frame timing.

There are people who claim to have low uncertainty USB timing. Fierce 
electronics was one I found using google, but they appear to use custom 
USB hubs.