[time-nuts] Need Time Help

David J Taylor david-taylor at blueyonder.co.uk
Tue Oct 4 05:04:25 EDT 2016

From: Hal Murray

david-taylor at blueyonder.co.uk said:
> You can use a GPS puck or equivalent with PPS output to get the sort of
> accuracy you need.  ..:

PPS output is only half the battle.  You also have to get it into the system
that needs it.

A modem control signal on an old fashioned serial port (not USB) is the
classic approach.  Unfortunately, many modern systems, especially laptops,
don't have serial ports.

GPIO pins on things like a Raspberry Pi also work well.

PPS over USB adds the USB polling interval to the error budget.  It will
probably work well enough if the goal is under 15 ms.


Agreed, PPS over USB isn't ideal, but possibly the extra jitter in the 
context of a Windows system isn't too bad.  On desktop systems I've been 
able to add a 3rd-party serial PCIe card and get excellent results.

On laptops I've usually been able to get well under a millisecond over 


apart from one rather cheap-and-nasty system which I won as a prize.  I 
wonder why they were giving that unit away!!  The secret is, perhaps, the 
new API introduced in Windows-8 (with some pushing from Dave Hart, I 
suspect) which allows much more precise retrieval of the Windows time, and 
is used by NTP when possible.

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Email: david-taylor at blueyonder.co.uk
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