[time-nuts] Synchronization

[Bob kb8tq]

Hi

If you are moving blocks of data ( = pictures / video ) out over the same basic
USB interface. I suspect you will get into blocking issues. That sort of thing will
make the basic “frame rate” stuff a bit less important. I have that same “gotcha” 
in what I’m doing. The network will be a bit busy while I’m also trying to sent time.

Bob

> On Dec 5, 2019, at 2:21 PM, jimlux <jimlux at earthlink.net> wrote:
> 
> 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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