[time-nuts] time transfer over USB

Chris Albertson albertson.chris at gmail.com
Tue May 14 11:54:53 EDT 2013

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.

On Tue, May 14, 2013 at 12:40 AM, Peter Monta <pmonta at gmail.com> wrote:
> So a USB-based GPS should:
> - maintain a cycle count of its local crystal oscillator (e.g. with a timer
> peripheral)
> - report this count when requested
> - timestamp PPS edges from the GPS module, and report these timestamps when
> requested
> This would seem to be enough to gradually converge to a good estimate of
> (USB_host_time - GPS_PPS) across the noisy USB link.


Chris Albertson
Redondo Beach, California

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