[time-nuts] multi input counter

Tijd Dingen tijddingen at yahoo.com
Thu Oct 6 21:15:09 EDT 2011


For a handy list of cheap fpga boards:

http://tristesse.org/CheapFPGADevelopmentBoards


I notice he's recently added the SOIO to the list. I've had my eye on that board as well for a while. If all you want is say 20 odd counters as you mentioned, then that even fits in the cheapest spartan-6 LX4 board. That board I would personally get for integration purposes, not development. For the latter the connectors etc are a little sparse. But once you're done and don't need all the switches and leds then it's quite nice for the price IMO.

For the bigger boards I'd say the nexys 2 (spartan-3E based) or nexys 3 (spartan-6 based) are good value for money. I have the atlys + nexys 2, and currently both are being hogged for counter related things. ;)

regards,
Fred



________________________________
From: Jim Lux <jimlux at earthlink.net>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Friday, October 7, 2011 1:02 AM
Subject: [time-nuts] multi input counter

since I rudely tagged onto someone else's search for a suitable counter, I'll restate my need here..
I want to set up an experiment  to characterize a bunch (several dozen?) of cheap XOs (e.g. SiTime parts) over temperature  and time and power cycles. I'm not looking for 1e-15 adev at 100 seconds  kind
of performance: maybe more like 1E-6 or 1E-7 ADEV at 100-1000 seconds (i.e. does the frequency of a 10MHz oscillator vary more than 1 Hz over 20 minutes?)

Something like a programmable MUX into one counter would work, but if you have 20 odd oscillators, making ADEV measurements for tau of 1-10 seconds on all of them would be tough.

If I were doing it in an FPGA, I'd just setup a bunch of counters and latch them once a second, then shoot the counts out a serial port in some fashion (might still wind up doing that).  Or, one could latch a single common counter with each of the unknowns divided down by, say, 10million. I think the two measurements are basically equivalent (one is measuring period, the other frequency, essentially).

Or, any of a variety of microcontrollers can do it.

Or, a combination of microcontroller + FPGA.

I think what I was hoping is that there's some already existing box that someone sells (or sold in the past) that does this. If not, I'll just build something. Probably the FPGA approach.. it seems simplest.. any suggestions from the assembled multitude for a inexpensive eval board that has an FPGA with suitable input pins for the output from those SiTime oscillators (and any other grungy oscillators I scrounge up)?

Something with, say, 32 inputs/pins brought out to a header on the eval board would be nice.  maybe the Spartan 3A or 3E for $200?  (I'll have to look at the data sheets)


or, given that I'm not looking for ultimate performance, are there any particular FPGAs to stay away from that are notoriously bad in this kind of timing application.


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