[time-nuts] Serial port splitter s/w

[David J Taylor]

From: Chris Albertson
Sent: Wednesday, February 26, 2014 7:32 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Serial port splitter s/w

On Wed, Feb 26, 2014 at 11:15 AM, David J Taylor <
david-taylor at blueyonder.co.uk> wrote:

> The Raspberry Pi with GPS/PPS certainly beats any internet connection I've
> ever had delivered to this house, though:

Yes it will.   But the point of the server is to deliver time to other
computers.  The the measure is "goodness" is not the accuracy of your local
NTP server but the accuracy of the client computers that use it.     So if
I point my desktop computer at a set of 5 to 7 Internet pool servers or it
I point the same desktop to my local NTP server I don't se a lot of
difference.

With the notebook computer using WiFi I don't see any difference.

This is what I moment by saying it is hardly worth it if you happen to have
Ethernet-like connectivity all the way back to your ISP.  Of douse many
people don't and have to use DSL or Cable TV lines.

The ARM has some real potential for time keeping.   Inside the chip in
addition to the ARM CPU are two "RPU" processors.  Thee run at exactly
200MHz one instruction per cycle with very deterministic timing.  They have
access to all of the ARM memory and interrupts.   They are in effect two
micro controllers that are in addition to however many ARM cores there are.
  A really good project would be to move NTP's  PPS timing to a PRU.    But
as I said the weak link is getting time OUT of the NTP server, not into it.
  But again the ARM is great for real time because it has both the ARM
cores runnig Linux and the PRUs that you can program in assembly with
deterministic timing.

But as of today I don't think the PRUs are used for this but see below,
lots of potential uses for building things like TICs
http://processors.wiki.ti.com/index.php/Programmable_Realtime_Unit_Subsystem
=========================================

Chris, even with Wi-Fi connected computers, mostly running Windows, there is 
a huge difference between talking to a stratum-1 server on my LAN compared 
to running just Internet servers.  Our experiences differ, as I am on a 
cable modem connection from the UK's Virgin Media.  Folks need to measure 
what performance they are getting and choose their own best path, otherwise 
it's guesswork.

Cheers,
David
-- 
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Email: david-taylor at blueyonder.co.uk