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.
Hi Jim,
On 07/10/11 01:02, Jim Lux wrote:
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?)
That is what you are going to measure :)
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.
Tricky.
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.
You would be able to use the standard 3E development-board for sure.
If you run your 10 MHz into the clock input, you could run a
time-stamper clock with it and then use a suitable divide down counters
for each time-stamped clock.
Consider that you have 7 digits of time-stamp value and 8 digits of hex
code for time-stamped inputs and a carrige return, you need to output 16
bytes. Considering that it takes 32 read-outs a second in peak. This
means 512 bytes a second or a rate of 5120 baud needed, so 9600 baud
output will do just fine. The time-stamper clock can count in BCD to
make conversion cheap (and easy to code, just slab 0010 on top of
digits). A 64 bit wide FIFO is needed to store time-stamps, as they can
come at high burst rate but low average flow.
It's doable and not too much work to cook up. I did something similar
before, but I just can't find the code lying around. Sad, since it had
the serial transmitt code and FIFO stuff more or less what we need.
Oh... look! No CPU!
Cheers,
Magnus
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@earthlink.net
To: Discussion of precise time and frequency measurement time-nuts@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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and follow the instructions there.
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. ;)
The Nexys2 is great - it has the I/O support you'd need for this
application, and not a lot of other expensive peripherals that you don't.
I've prototyped some pretty hardcore stuff with it.
They include full schematics in their manual.
-- john
On 10/6/11 4:24 PM, Magnus Danielson wrote:
Consider that you have 7 digits of time-stamp value and 8 digits of hex
code for time-stamped inputs and a carrige return, you need to output 16
bytes. Considering that it takes 32 read-outs a second in peak. This
means 512 bytes a second or a rate of 5120 baud needed, so 9600 baud
output will do just fine. The time-stamper clock can count in BCD to
make conversion cheap (and easy to code, just slab 0010 on top of
digits). A 64 bit wide FIFO is needed to store time-stamps, as they can
come at high burst rate but low average flow.
It's doable and not too much work to cook up. I did something similar
before, but I just can't find the code lying around. Sad, since it had
the serial transmitt code and FIFO stuff more or less what we need.
That's basically what I was thinking.. 9600 bps is also slow enough
that you can do something on the receiving end like open the file, add a
line, close the file, so it makes rolling over files every hour or
whatever easy.
The experiment I have in mind will last about 2 months, with the batch
of oscillators cycling in temperature either on a 100 minute cycle or a
24 hour cycle. (Low orbit or earth/mars rotation period)