S3e slower than S3

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I wonder if anyone (Xilinx?) has actual information on Spartan3e fabric

I have done some actual measurements and as far of the results the
LUT propagation delay seems to be about 10% bigger than in S3?

This info seems to be obmitted in Spartan3e datasheets.
Oh well, it looks like will have to rely on our measurements when the
manufacturer does not publish the timing info.

I was hoping to see a little speed improvment so seeing 10% decrease
was a small surprise. Well I have not tested all the S3e I have yet maybe
other parts are actually faster than the one used for LUT speed testing.

Antti Lukats

Re: S3e slower than S3

Our information is that they are really no different.

Variations due to process can easily be +/- 10%.

As well, the first ES parts of any technology are hardly "process
controlled."  You gets what you gets.  And you are happy.

I have no idea if the first ES parts were fast or slow...


Antti Lukats wrote:

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Re: S3e slower than S3
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Hi Austin,

Ok, thats fair enough - I have 3 different S3e chips but I have not measured
all of them, the first one I did measure did show 10% LUT delay increase
compared to S3 slowest speed grade. Sure the 10% can be process
variations and I need to measure more devices in order to see the
actual difference if there is any.

I was hoping to see speed increase, and as it wasnt there so I asked
if there known the speed timing differences.


Re: S3e slower than S3

I would not expect any differences.

As well, all of the parts you have are likely to come from the same lot
(perhaps even the same wafer) and will not show any personality.

Those 12" wafers are huge, and the 3SE die are so tiny...


Antti Lukats wrote:

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Re: S3e slower than S3
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I have following chips for testing:


are all of them from the same wafer ??

ROTFL that would be nice design, make one wafer and cut smaller pieces for
smaller FPGAs

hm that may not be impossible actually... just make whole wafer full of the
fabric interlaced with IOB
then cut out different rectangles and get different sized FPGAs

ok, maybe not so reasonable :)

Antti Lukats

Re: S3e slower than S3
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No, they are not.  But they may all be from the same weeks, running the
same process, on the same equipment, and thus very similar.

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There are things that sound crazy, but then you end up thinking how to
do them.  This is one area where we are still looking for a way to make
all our parts at once, and just slice off the ones you need.

So far, there is no way to do this, but we are still thinking about it!


Re: S3e slower than S3
Antti, each of these devices comes from its own wafer.
We do not mix different device sizes on the same wafer.
Regarding speed:
Spartan3E never claimed to be faster than Spartan3. Both use the same
For S3E, low cost was the primary objective.
The S3E  data sheet says:
...to meet the need of cost-sensitive consumer applications...
...more logic per I/O...
...new features improve system performance...
...more functionality and bandwidth per dollar...

That's as close as any data sheet will ever come to stating:
"This is not meant to be a speed demon".
Peter Alfke

Re: S3e slower than S3
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Hi Peter,

no problems even there are missing 10% speed (maybe there isnt I have not
finsihed the testing)

I just was silently hoping to see higher in-fabric clock speeds

I was trying to figure out what are the max clocks that the S3e can work so
I tried it out, so for time being the fastest clock am able to use in S3e is
about 380MHz, maybe it would work a little higher also but I am not able to
produce a higher clock yet :(

at first measuremens I got readings like 899MHz but that was a hoax - bad
experiment. 380MHz is verified stable working - in S3 -4 speed the similar
number is 420MHz so here the 10% decrease

I was then looking at max-toggle rate for S3e, and timings specs and did not
find any that info so was wondering a little whats up

Antti Lukats

Re: S3e slower than S3
Antti, if you want to test or compare the internal fabric speed, you
can just build a ring oscillator inside, divide the frequency down in a
couple of flip-flops, and observe the frequency on the output pin.
Build the oscillator out of a reasonable chain with only one inversion
in it.
That way you can include or exclude certain routings, carry chains,
And the frequency counter gives you tremendous accuracy and resolution.
Peter Alfke

Re: S3e slower than S3
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hehe, ;)  this is the way I am doing it for some already !!!

this is great tool to measure things with virtually any accuracy .. I am
using Pentium cycle counter to measure frequencies on such FPGA systems that
do not have any known reference clock, I have a speical multichannel
jtag-bscan connected freqency measurement ip core and analyzer host

BTW it is included in the Spartan3e sample pack standalone utility to
measure the silicon oscillator :)

there are some tricks with the ring oscillator, for repeatable results it
should be (r)LOC ed to primitives that produces always same routing, also
some 'variants' tend to swing at too high frequency so high that it doesnt
get a single flip flop to toggle even if there is direct route and one
single load.

so I have different oscillators, a good one for Spartan3/e is a "2 LUT
delay" oscillator, it runs at around 400MHz in S3/e the output is divided by
2 in the same slice and only divied by 2 output is used.

on Virtex-4 this oscicllator is unuseable - runs to fast, so some more LT
delays need to be in chain to get useable frequency.


Re: S3e slower than S3
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If they did mix parts on a wafer, it would be complete parts of specific
sizes, not something that can be diced up in various ways.

That's called a shuttle run or multi-project wafer (MPW).  It's normally
done for test chips or very low volume production.  I wouldn't expect
that Xilinx would be doing that even for ES silicon, though they might
well do it for pre-ES design testing.


Re: S3e slower than S3

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Over a broad range of applications,  a Spartan-3E FPGA is _in general_
the same performance or mildly faster than a Spartan-3 FPGA using the
slowest speed grade for both.  These tests were done using worst-case
speed file numbers, which are more pessimistic than actual silicon.

Hopefully, this shouldn't be too surprising as Spartan-3 and Spartan-3E
FPGA are built on the same 90 nm process technology using the same
manufacturing facility.

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I would expect some variation comparing actual silicon on two different
devices.  The data sheet tells you the slowest silicon we're allowed to
ship, but the actual device likely is faster, especially at room
temperature and nominal voltage.

That said, if you compare the Tilo specification for the Spartan-3 and
Spartan-3E families in their respective data sheets, you will see about
150 ps of delay difference.

Spartan-3:  Tilo on a -4 device = 0.61 ns
http://www.xilinx.com/bvdocs/publications/ds099.pdf [Page 79]

Spartan-3E:  Tilo on a -4 device = 0.76 ns
http://www.xilinx.com/bvdocs/publications/ds312.pdf [Page 133]

I wouldn't focus on a specific parameter per se, as your path delay
will include other timing parameters as well.  For example, Spartan-3E
generally has faster flip-flop clock-to-output delays and faster
interconnect.  In the wash, Spartan-3E _generally_ is at or about the
same performance.
Steven K. Knapp
Applications Manager, Xilinx Inc.
General Products Division
Spartan-3/-3E FPGAs
The Spartan(tm)-3 Generation:  The World's Lowest-Cost FPGAs.

Re: S3e slower than S3
schrieb im Newsbeitrag
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Hi Steve,

it was really my fault - I have downloaded DS312.PDF zillion times and this
time when I was now looking for timing data I happened to open DS312.PDF
dated May 2005 so in that outdated version there was no timing data.

my measurements includes delay of 2 LUT, eg 2x Tilo + 2x interconnect
within the same switchbox (no routing only switchbox), the result of that
measurement indicated the Tilo increase exactly to the amount as as it
is stated in the datasheet. Actually a little less, so the interconnect
may really be a little faster.

would I have had the latest datasheet open when looking at timing data
I would not have been surprised, my mistake, in the feature I will try
to use only fresh Xilinx datasheets to avoid using outdated versions.

Antti Lukats

Re: S3e slower than S3

Do you know about the speedprint utility?  I don't know if it's part of
WebPack but it certainly is part of the mainstream tools.  On my unix
platform, all I need is a "speedprint xc3s50" and I get the results for the
faster speed grade device.  I can select a -4 speed grade with the -s switch
adding -s4 to the command line: "speedprint -s4 xc3s50" (if there is a -4 in
this Spartan3 size).

I like being able to get the piece-parts to some of my "best case" timing
values on critical logic without running through a full compile.  I can look
at different implementations (carry chain?  MUXF5?) for critical logic
without too much confusion.  At first glance the value names may be a little
confusing but there are ways to get some description on what they mean.

- John_H

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Re: S3e slower than S3
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Hi John

I dont trust any speed reports 100% so I am actually measuring actual
silicon and comparing the measurements.

Sure for academic comparison I could use timing report values too


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