[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
> >> 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
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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