[time-nuts] time transfer over USB

Magnus Danielson magnus at rubidium.dyndns.org
Tue May 14 20:56:40 EDT 2013

On 14/05/13 20:19, Jim Lux wrote:
> 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.

Well, look at the GNSS sampler, it performs bulk transfer of a 
continuous clock but only uses 2 bits in the byte. Trivial to hook up 
the PPS to that.

If you do not want to hack into the GNSS sampler, there is a similar USB 
chip available on SEEED, check out the FT2232 (if I recall correctly).

We talk about relatively low advancement in hardware hacking to get 
access to the signals over USB. Sufficient software support exist to get 
you started.

If you could get some signal from the inside going out with some 
precision, it could be sampled too, and the delay can be cancelled out.


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