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.
Cheers,
Magnus
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