I know why it isn't centered... it's because it is taking a USB polling round trip to get the signal back.  The PPS happens somewhere between two polls every 1 ms, and it takes another 1 ms to come back.

On 24/03/2012, at 2:38 PM, Ron Frazier (NTP) wrote:

> I now have the PPS circuit working on the Sure board.  I have not soldered it yet.  I just taped a jumper wire between the PPS test point at the edge of the board and the DCD pin 1 on the RS-232 port.  The serial data is coming in through the Trendnet TU-S9 serial - USB converter, which is passing DCD.  I'm getting + .5 / - 1.5 ms offsets.  The PPS is nowhere to be seen on the statistics screen, but it is obviously working.  I don't know why it's not more centered around zero, and maybe that will change.  However, my total peak to peak range of offset variance is 2 ms, and that's coming through USB.  If I can maintain that level of accuracy, and it's consistent with UTC, then I'm very happy.  That's plenty good for my purposes.  I still may try to run it through a real serial port on another machine just for kicks.
> 
> Sincerely,
> 
> Ron
> 
> 
> -- 
> 
> (PS - If you email me and don't get a quick response, don't be concerned.
> I get about 300 emails per day from alternate energy mailing lists and
> such.  I don't always see new messages very quickly.  If you need a
> reply and have not heard from me in 1 - 2 weeks, send your message again.)
> 
> Ron Frazier
> timekeepingdude AT c3energy.com
> 
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