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.
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