Question on Metastability

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Using a VirtexE part.  I have two banks of registers.  One is actually a
32-bit counter running real slow at 1KHz (1mS time tag).  The other is a set
of data registers that run anywhere from 100Hz to 100MHz.  I need to
register the time tag set of registers into the variable frequency registers
at predetermined... but variable intervals.  I can't do a majority vote with
either clock because I don't know which will be running faster and I only
have a couple clock periods to live with as far as delay.  I draw a blank on
this one, does anybody have any ideas to this predicament?

I tried doing this with simple registering from one set to the other... And,
as expected, I run into metastability problems several times per second.


Re: Question on Metastability
If I understand you right, you need to copy the 32-bit counter ontent
(which changes very slowly) into a fast-changing (fast-clocked)
register at asynchronous times.
I am sure that your problem is not metastability, but rather timing
uncertainty between the 32 register bits. My suggestion is to run the
32-bit counter as a Gray counter. Since only one bit changes at any
time, a not-totally-synchronous transfer never results in a serious
error. Afterwards, you can convertthe gray code back to binary.
Binary-to-Gray conversion uses one XOR per bit, and is very fast.
Gray-to-binary involves a ripple chain, and may be more challenging at
100 MHz, but I suppose you can do it off-line. Or you pipeline the

As is often the case,,mtastabilty gets blamed here for a completely
different (and more solvable) problem.
Peter Alfke, Xilinx Applications

Re: Question on Metastability

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Yeah, I agree - I normally just lump them together although I shouldn't.

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I never thought of a gray counter.  I'll look into this as it may be a
better solution.  I can always pipeline the conversion - I do have a dozen
or so cycles to play with before I need to get the result out.


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Re: Question on Metastability
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What about dual port memory, or perhaps a special FIFO, given the
extreme clock ratios ?  [and the gray counter as well, for safety..]

Re: Question on Metastability

This worked quite well.
I left my 32-bit binary counter alone (hey, I'm lazy!) -- added a binary to
gray code converter on the output.  Registered that using a higher clock
that was used to derive the 1KHz clock - so I get a nice, registered gray

With the higher frequency clock, I register the gray code at the required
time interval.  Perform a gray code to binary conversion to get back to my
original value.   Luckily, I have several clock periods worth of time to
bring back the original binary value as this piece of the puzzle isn't real
Now all I have left is a 1-bit ambiguity - which is quite reasonable.


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Re: Question on Metastability
You could run both your 32-bit counter and the other set of registers off a
fast (e. g. 100 MHz) clock. Detect the edge of the 1 kHz counter clock
signal and make it a clock enable for the counter together with the 100 MHz


Re: Question on Metastability
I agree with Stephan. Making the whole design synchronous is the best
I was overly concerned with pointing out that the problem has nothing
whatsoever to do with metastability, and I overlooked the simpler
Synchronous is the way to go, whenever possible.
Peter Alfke, Xilinx (from home)

Re: Question on Metastability

But - I have an oddball case of a clock.  One always runs at 1Khz.  The
other *May* run as fast as 100MHz but usually runs slower -- sometimes as
slow as 100Hz.  If I knew my variable clock was always fater than the 1KHz
clock, the edge detection would work fine.

Incidently, I already have a circuit similar to what you have in one of my
other designs which does have a guaranteed fast clock.


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Re: Question on Metastability

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The obvious solution would be to use a third clock of known higher
frequency.  If this isn't avaliable, you could add hardware to detect
which clock is running faster, but that may be more work than it's
worth, especially if your variable rate clock changes frequently.

This would be a good time to add 2 cents worth on the virtues of
having a free-running clock in an FPGA (as were in early Xilinx
parts).  Many newer parts have internal oscillators for configuration
but the clocks aren't available to the routing fabric.  I've seen
some threads here about building oscillators in the FPGA from
internal LUT delays and the like.  I wouldn't like to depend on that
sort of design due to variances in process and temperature.  For
future designs, however I would suggest always having a fixed clock
of sufficient frequency to feed a DLL (unless you can't afford the
crystal which would be unusual in most designs containing an FPGA).


Re: Question on Metastability
<snip> This would be a good time to add 2 cents worth on the virtues of
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However, those variances actually self-track, so a Logic Element +
Routing  derived clock can be self-margining.

Lattice have added a usable clock to their MachXO, so this might
catch on....

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There is another class of Osc, called the Calibrated On Chip -
these are now common in small Microcontrollers, and achieve
in the regions of 1-2-3% stability over Vcc and Temp.

Ideal would be both; The first type would be easy for the FPGA
vendors to offer as simple IP blocks, but the CalOsc is
needs special HW, and a mindset change in the FPGA sector, so
is less likely.....


Re: Question on Metastability
The original question has been answeed: I still stand behind my
suggestion in #2 in this thread.
A self-contained stable oscillator is a difficult (if not impossible)
proposition. All transistor parameters have unacceptable variation over
temp, voltage, and processing. Metal resistivity and capacitance are
fairly stable, but how do you make an oscillator out of that?
Peter Alfke

Re: Question on Metastability
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Depends on what you mean by stable and acceptable.

Certainly to RS232 standards is achievable today.

For some background reading, look at the Philips LPC9xx family,
and the Atmel AVR series. They have impressive On-Chip osc

Also look here (even more precise) :

they show Silicon Oscillators, spec'd to ~1%. and targeting
ceramic resonator applications.
They also have Spread spectrum Silicon oscillators.

More esoteric are the still experimental 'machined silicon' oscillators
-  more special process handling, but precisions getting close to
low end quartz crystals.

There are many apps, where even 2% is more than precise enough,
and others where +/-20% would be fine.


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