[time-nuts] FPGA GPSDO (Was: Re: NTP jitter with Linux)

[Andrew Rodland]

Chris Albertson <albertson.chris at ...> writes:

> 
> On Fri, Apr 6, 2012 at 9:41 PM, Andrew Rodland <andrew at ...> wrote:
> 
> > Another option would be building something on an FPGA. This would be a
> > considerable stretch for me, since I've never done FPGA work, but if I build
> > from the ground up, I can have *very* tight control over things that are 
chosen
> > for me with a micro controller.
> 
> A compromise is to find a "soft core" for the FPGA.  This is a CPU
> implemented in FPGA and then it runs software just like a "real" CPU.
>  This would let you move your micro controller based be sign over to
> the FPGA quickly.   After that you can implement some specialized
> peripherals that do time stamping
> 
> How does the performance of the Arduino based NTP compare with what
> you could do with Linux on (say) and Atom or ARM processor?

Pretty well, I think. A PPS generated by my board shows an RMS absolute jitter 
of 174ns compared to a Spectracom 9183, which is almost impossibly good 
considering that all the timing is done off of a 2MHz clock.

As measured from the NTP side, the performance is diluted a lot by the fact that 
the W5100 ethernet chip has unknown and unpredictable delays, but it still comes 
back with a jitter of <20us (possibly better — the box I'm using to measure the 
NTP is a Linux machine that isn't really a timing champ. Wish I still had my 
net4801.)

Andrew