asic gate count

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Hi,
I am looking for some tool / ip that can give me approximate gate
count of mapper/demapper. Any helpful hint is greatly welcome.

Thanks,
Vijayant

Re: asic gate count

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To get a gate count ... you have to synthesize it, the synthesis will give
you an area and typical a "gate" is a 2 input NAND gate. So divide you're
total area by the area of the NAND gate and that is your gate count.

Mike



Re: asic gate count

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The traditional method for ASIC is to divide the number
of transistors by the number of transistors in a 2 input
NAND gate.  For CMOS, that is four.

Good or bad, that is the usual way.

-- glen


Re: asic gate count

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What is your method for determining how many transistors are in the design?
My synthesis tools only give me area.

Mike



Re: asic gate count
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Divide the area of your design by the area of a 2-input NAND with
lowest drive strength * 4.

Cheers,
Jon


Re: asic gate count
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Last I knew, they gave transistors, but that was some time ago.

For FPGA it is much harder to give a reliable count, and
you are asking in an FPGA newsgroup.

If it gives gate counts different types of gates and with a
little guessing on transistors/gate.   Is this for standard
cell, sea of gates, or something else?  I thought I used to
have pretty detailed information on the standard cell libraries,
including transistors for each library element.

-- glen


Re: asic gate count
I have a design on FPGA that is ready. However, we need to have some
mapping from fpga design to asic. I know that this will not be
accurate. But accuracy is not our concern right now. We just need
upper bound.  Also, we are also looking for some IP Core for ASIC so
that we can rough estimate.

Regards,
Vijayant



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Re: asic gate count
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Take the nuber of registers and combinatorial cells used for the fpga.
Start with the registers, inspect if you have register doubling in
synthesis for load balance and ask yourself if you need this register
doubling. In ASIC you have often load balancing with buffering.
For combinatorical logic you should have a look, what type of cells is
in your vendors library.
For a lot of technologies the tool will more likely use 3 input cells
instead of 4 input cells. This will roughly increase your
combinatorical cell count by 10%-30% (it depends on the design).
You should take into account, that you need to add something like 10%
buffering for global nets and timing correction that comes for free in
an fpga.

bye Thomas

Re: asic gate count
On Wed, 28 May 2008 11:24:03 -0700 (PDT), " snipped-for-privacy@gmail.com"

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One approach is to run it through the Xilinx tools and review the map
report (.mrp file). If you take this approach, I suggest eliminating
memory blocks (PPC if used) and DSP/multiplier blocks and re-running, to
understand how much of the gate count comes from these blocks.

- Brian

Re: asic gate count
Hi Vijayant,
every "rule of thumb" you are trying to use will give you a misleading
result. Even if you just want a maximum value.

The only way to get a nearly accurate number is to synthesize your
design with an asic synthesis tool using the desired technology library.

You may use a default synthesis at first, to get an idea of the size and
gate count. These results may vary depending on your design goals. If
you want to increase speed your design may become larger. If speed is
negotiable the design may become smaller with some area optimization
constraints.

However, the result of this synthesis will be an area value (most likely
in square micro meters) because the used gates (and flipflops) heavily
vary in size and transistor count. Unless you are using a sea of gates
technology that has only nand2-elements.

To give you an idea think about this:

If you have a simple 4to1 mux, this may be synthesized with a single
mux4 cell in some standard cell technologies. With a sea of gates
technology you need a bunch of nand2's for this function, plus some
routing resources.
So, how would you express the number of gates in these two cases?
The mux4 is just the solution with the minimum number of cells.
depending on your constraints the result might be any correct
combination of simpler gates.

Also, the gate count, however calculated is not relevant for production.
Only the area tells you how many chips can be fabricated on a single
waver. And the area changes with the used technology of course.
So 1000 gates in a 130nm technology yield less chips per waver than 2000
gates in a 45nm technology. (rough estimation, just to give you an idea)

So forget gate counts if you want to compare technologies.
Only use of gate counts is if you want to compare designs using the same
technology. And I mean the very same technology! (Just take a look at
some of the fruitless gate count discussions about Brand-A and Brand-X
FPGAs)

Have a nice synthesis
   Eilert


snipped-for-privacy@gmail.com schrieb:
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Re: asic gate count

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Just wanted to argue on your last point .. gate count is the easiest means
to compare technologies. It will stay relatively constant from tech to tech
and you will have the nand2 area for the technology you are in so you can
roughly compare the area of a design for different technologies using the
gate count for the design.

Mike



Re: asic gate count
On Fri, 30 May 2008 13:53:44 -0400, "Mike Lewis" >
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Only if you ignore the possibility of the use of high drive strength
gates. If your design is at the boundary of just meeting timing in a
process the actual gates used in your design will most probably
require higher drive strengths and your actual gate count will be
higher than what you'd have guessed by just looking at logic
requirements. Then when you port to the next lower feature process,
your relative gate count change will be much higher than what you
expect. As an example a NAND2X4 might be %50 larger than a NAND2X1 and
the former might be needed in one process where as the latter would do
in another.

Re: asic gate count
Mike Lewis schrieb:
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Hi Mike,
Muzaffer added a good point about drive strength, and another one is
that the area of different gate-cells (e.g. xor2) differ heavily from a
nand2. Even worse when looking at FFs-cells.
And, the libraries give you no multiplication factor based on the nand2
area, so you have to calculate all these for yourself for up to hundreds
of different cells. (easy???)
One more thing is that the use of more complex cells saves routing area.
how will you put that in your calculation?
And these are just points that show how misleading a gatecount-value
would be for comparing standard cell technologies. Comparing different
technologies like FPGA and standard cells would make even less sense.

The area calculated by the synthesis (or layout) tool gives you a value
you can work with.
And for FPGAs this would be the number of LUTs and FFs (plus some vendor
specific stuff if necessary).

And even these values for themselves can only be seen as one aspect of a
specific implementation. Because area can change heavily when you are
optimizing the very same design with different constraints.
The area may vary by a factor of two or three. (is 200% variation good,
even for a rough compare?)

In the end it's just another number. And like many others only useful if
you know more about the background of it.

And of course it depends on what you want to do with that number. (e.g.
Megapixels for digital cameras...nice to see them increase, but what
good is further increase when the pictures become worse due to noise?
The marketing guys love it anyway because the average custumer don't know.)

    Best regards
      Eilert

PS.: Just some funny story...
I'm preparing a lab course for FPGA synthesis and compared the
technology schematic views of XILINX ISE and Mentor Precision RTL.
ISE shows the equivalent logic inside a LUT as a bunch of 2 input gates
and inverters while Precision RTL uses big N input gates with bubbles.

It's all inside a LUT and uses the same resources, but confronted with
pictures of the same LUT drawn in these two styles needs some time to
tell that they show the same.

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