[time-nuts] time transfer over USB

Jim Lux jimlux at earthlink.net
Tue May 14 14:19:51 EDT 2013

On 5/14/13 8:54 AM, Chris Albertson wrote:
> The problem is the PPS needs to be referenced to the same clock the PC
> is using as a basis for system time.     Yes, you could send the
> counter's value periodically but that has the same problem of sending
> the PPS, that is an un knowable delay.
> The good news is that good Moterola GPS receivers with 50 nanosecond
> PPS accuracy are available on eBay for $15.   Soyou don't have to mess
> with USB.

yes you do have to mess with USB, if that is the only interface available.

There is great value in figuring out how to accurately transfer 
time/sync/frequency over an existing interface.

Or, at least, to bound the accuracy achievable with various approaches: 
vanilla off the shelf device drivers, custom device driver, etc.

I think the isochronous approach probably has promise for the 1ms class, 
especially if you can figure a way to do two way.

Bulk transfer might work well on a system where nothing else is going 
on, so "best effort" translates to "now".

It's sort of like knowing how well you can do it over Ethernet.. the 
performance achievable depends a lot on what your hardware and device 
drivers look like, and sometimes, you're stuck with what you got, so 
it's useful to know when to stop struggling to do better.

It's also useful as a BS detector in marketing literature.. If someone 
came to me and said, I can give you 50ns timing accuracy over USB, I'd 
want to ask a lot of very detailed questions about how they claim they 
can do it, and an answer of "proprietary magic pixie dust" would not be 
very convincing.

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