[time-nuts] DMTD to MMTD

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Thu Feb 18 00:29:59 UTC 2010

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Bruce Griffiths
> Sent: Wednesday, February 17, 2010 4:20 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] DMTD to MMTD
> Lux, Jim (337C) wrote:
> >>> I don't know if there's a FIFO in front of the UART (e.g. what if you get simultaneous zero
> >> crossings).. but I would expect there is.
> >>> The "hard work" is in the zero crossing detector ahead of the FPGA. (and perhaps in the latching
> of
> >> the ZCD inputs into the FPGA).
> >>> Given how long ago it was made, that FPGA isn't a huge one.
> >>>
> > You've got 8 channels, each zerocrossing at about 200-300 Hz (the difference frequency is 123 or 124
> Hz, so you get twice that many zero crossings), or about 1600-2400 messages/second.  At 6 characters
> per message, that's about 10,000 characters per second, so you'd need a fairly fast UART to keep up.
> (OTOH, the article mentions dropping characters..)
> >
> >
> A LAN, USB or Firewire interface may be more appropriate all are easy to
> implement.

Yes, but faster rise times and potentially more EMI.  The FTL guys are pretty obsessive about stray noise sources. 

As far as interfaces.. if I were choosing, I'd use LAN (like pair a Rabbit with the FPGA.. the Rabbit does the UI as a static webserver kind of thing), although USB is ok, but ties the box to one computer.  USB is cheaper though. 
Firewire/1394 is a dying standard... If you need speed, then GigE would probably be a better choice.

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