[time-nuts] Thought experiment on a low cost timing board

David Andersen dga+ at cs.cmu.edu
Sun Feb 26 18:37:50 EST 2006

On Feb 26, 2006, at 6:24 PM, Paul Boven wrote:

> Hi everyone,
> Poul-Henning Kamp wrote:
>> You don't even need 32bits for that:
>> 	http://phk.freebsd.dk/pubs/timecounter.pdf
>> And doing it in hardware would be more expensive than in software,  
>> hardware
>> access is much slower than memory access.
> How about taking one of the bigger FPGA's, that can hold a complete
> stand-alone microcontoller, running a Unix-like OS? You could  
> implement
> several fast, wide synchronous counter/latches in the same FPGA  
> fabric,
> so there would be no need to bother with things like PCI busses. Does
> FreeBSD run on any of these chips?
> With a serial (for NMEA) and PPS input, and ethernet output, you'd  
> have
> an NTP-server-on-a-chip. Add a D/A-converter to discipline a TCXO, and
> you're all set. An FPGA with relatively few pins, hence no BGA, would
> suffice. Building such a device would be within the capabilities of  
> some
> of the more dedicated timenuts in here.

I'm going to echo the previous comments about complexity:  Egads,  
why?  NTP is relatively coarse-grained because of unpredictable  
delays in ethernet buffering and packet interrupt latency.  It  
doesn't require the precision you could get with custom hardware.   
Accuracy within a microsecond or so is more than good enough.  As  
Paul's own experiments showed, a Soekris box with a replacement XO  
can internally track GPS far better than any NTP slave will be able  
to synchronize to it.  It's cheap (~$220), nearly off-the-shelf,  
accurate to better than half a microsecond, and runs FreeBSD and  
Linux out of the box.

And there are some sexy new versions of the 4801 shipping soon with  
more memory, so you can keep an even bigger list of NTP peers. :)


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