[time-nuts] OT: 10 MHz data capture, help

Hal Murray hmurray at megapathdsl.net
Sat Apr 25 09:20:16 UTC 2009

> I have a old data device that is spitting out TTL data at 10 MHz.
> There's just a data line (no clock) but the edges clearly indicate an
> internal 10 MHz clock.

> I'd like to do a continuous capture of the bits, for up to tens of
> minutes, into a PC. That comes to about 1 GB of raw data. I can handle
> the decoding of the bits in software after the capture is done. This
> is a one-time experiment.

> What is the best/quickest/easiest way to capture data like this? I've
> looked at various USB or LAN logic analyzer and 'scopes but most seem
> to work on batches of data. I need a continuous capture. 

There are two issues with this problem.  One is clock recovery.  The other is 
getting a large chunk of data into memory and presumably on to disk.

This leads in to a question I've been meaning to ask for a while.  I've been 
looking for a low cost FPGA on PCI board.  This might be a wild goose chase, 
but I think it would be handy for a bunch of time-nut type applications.

If I had the board I'm thinking of, you would just plug your data signal into 
a SMA connector and grab the data.  It might take some FPGA hacking, but 
clock recovery at 10 MHz isn't a big deal and before long we'd have a good 
collection of FPGA code.

What I'm looking for is a FPGA on a PCI card with a front end where I could 
add my own interface circuitry probably on a daughter card.  I'm picturing a 
connector of some sort on the PCI card, probably standard 0.025 inch pins on 
0.1 centers, but I'd be happy with anything low tech and inexpensive.  I'd 
want it as far back from the front panel as is reasonable to leave room for 
my stuff.  I also want a couple of good mounting holes.  (and drawings and 
part numbers...)

The daughter cards would be flipped upside down.  Cooling would be poor but 
shouldn't be a problem with low power interface chips.  It would get exciting 
if you wanted to put a fast ADC out there.

I'd expect the vendor to make a few popular interface daughter cards 
available.  There are a couple of obvious ones.  One is N data lines with a 
clock on whatever ribbon cable type connector fits on the front panel.  
Another is several SMA connectors, one going to a clock input on the FPGA.

It might make sense to layout something on the front section of the board.  
As long as that section isn't stuffed it won't get in the way.  Whatever is 
likely to be most popular.

Does anybody know of an inexpensive FPGA card like that?

There are a handful of reasons why FPGA and PCI don't play together well.  
The main one is that old 5V PCI can generate 11V spikes from reflections and 
that is off scale with modern silicon technology, or at least the branch of 
it used in the FPGAs that I'm familiar with.

There is a 3V PCI spec, but (almost?) no mother boards implement it.  I think 
that's mostly a chicken/egg problem.  It didn't take off so nobody built cards
that depend on it so it didn't take off...  Lots of people make cards that 
run on
either 3V or 5V.  That's easy.

Does anybody know of any mother boards with 3V PCI slots?  I think the 66 MHz 
option for PCI needs them.

There are 2 ways that I know of to connect a FPGA to 5V PCI.   One is to use
FET bus switches.  The other is to use some other chip to connect to the PCI 
bus.  One possibility is a PCI-PCI bridge.  Another is an older technology 
FPGA.  A 3rd is one of the PLX chips designed to connect to external logic.  
I think they can all be made to work, but they seem ugly to me.

There are a couple more possibilites.  One is to cheat.  That means putting a 
scope on your PCI bus and noticing that even though it's a 5V bus, it's only 
got 3V signals on it.  You would have to get out the scope again every time 
you wanted to plug in a new card.  Ugly.

Another option is to use PCI-Express.  I'm not sure what is available for low 
cost FPGAs, but lots of mother boards now include PCI-Express slots.

These are my opinions, not necessarily my employer's.  I hate spam.

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