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Re: Virtex4 running at 360Mhz DDR

All I am trying to point out is that the load is 6.25pF + 100 ohms, not
12.5pF + 100 ohms.

When folks wave their arms and state 12.5pF is the LVDS load, they are
miss-stating it.

Simple point.

And once you do the simulations, or look at the actual waveforms, you
realize that it is mostly just a beauty contest.  In communications
theory, excess bandwidth in the channel only adds to the error rate (due
to noise).  Some band limiting is a good thing.  Too much is a bad thing
(eg using the LVDS at 1.3 Gbs where it wasn't designed to be used, that
is where our MGTs are to be used).


Re: Virtex4 running at 360Mhz DDR

 Which of the following posts regarding Cin is more helpful
for both Xilinx and its' customers:

Austin [1]:
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Brian [2]:
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Austin wrote:
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 The only I/O capacitance number published in your datasheet is a
single-ended parameter called Cin (or if you prefer, C_comp from
the IBIS files).

Quoting this published datasheet Cin value is perfectly valid,
and does not require "correction".

 Comparing that number against the single ended Cin's of other
devices, or against a single ended spec, is also perfectly valid.

 I have never said the differential load is 12.5 pf; it is clear
from my posts that I understand this, and also understand that
the assumption of Cdiff_effective = 1/2 Cin_single_ended applies
only for the differential components of the signals on the Tline.

 I find it rather inconsistent that in past discussions of
Xilinx's newly onerous SSO limits for the current mode output
drivers, you've been quite insistent that real world paths are
NOT perfectly balanced-

 Yet when discussing the effects of high Cin, you posit that
everything is perfectly balanced back to a perfect source
termination, so that a 50-60% voltage reflection off of your
input pins is never a problem.

 If only all FPGA input buffers could live happily ever after
there in Austin's world, where all connections are ideal
differential point-point links, all drivers have perfect back
terminations, and no probing or multidrops are ever allowed.

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 And massive, coherent input reflections do not fit the AWGN
assumptions of most channel models, now do they?


p.s. As for your other post, I'll reply once I finish recovering
from a hard drive crash at home and can find my old files again.


Re: Virtex4 running at 360Mhz DDR
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Brian, LOL, I'm beginning to feel the same way. It's interesting that almost
all the PCB differential pairs I've seen are edge coupled striplines or
microstrips, very few are broadside coupled. Of course, usually with edge
coupled lines most of the coupling is to the ground plane, and very little
between the conductors. So, it's much more like two 50 Ohm lines rather than
a 100 Ohm pair. Not that it makes any difference, of course.
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Cheers, Syms.

Re: Virtex4 running at 360Mhz DDR
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Then why not publish them, along with a comparison of IBIS/HSPICE
simulations versus the real world measurements?

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Let's see if I've got this straight [1]:

A) Xilinx publicly posts in FPGA and SI forums touting their
   real world X vs. A package testing, and asks for feedback [2]

B) Forum users post some suggested measurements, which a
   certain Xilinx employee says they can make

C) Two months later, when asked when said measurements might
   be published, the very same Xilinx employee cops an attitude

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 That would be the same manual (UG077 v1.2) that mentions a
HyperTransport compliant DUT interface connector, without
pointing out that the the specified V4 FPGA Cin is 5x the
allowed HyperTransport max Cin for a 1 Gbps part ???

 As to why that matters: a HyperTransport test probe attempting
to monitor the input link to the FPGA can't function properly
because Cin reflections off the FPGA would prevent the probe from
properly clocking the data at the mid T-line probe sampling point.

 There are ways around this, but life would be easier if Xilinx
actually bothered to meet the spec in the first place.

  Lacking that, proper documentation of your part's shortcomings,
and how and when to work around them, would be appropriate.


[1] Speaking of those unable to perform the SI engineering that is
required of them : when might we expect publication of characterized
static DCI power and DCI impedance modulation limits for the five year
old Virtex2 FPGA family ?


Re: Virtex4 running at 360Mhz DDR


See below.


Brian Davis wrote:

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All I can say, is that they are coming.  Just takes awhile.  Right now
we have much more important things to do: tout our power advantage, our
static current advantage, our speed advantage, our MGT advantage, our
PPC advantage, our SI packaging breakthrough ...
Showing an IBIS simulation of a five year old interface is just not high
on our list -- too many customers use it, and are perfectly delighted
with it.  We do not want to be defocused and stop pointing out the areas
where we are clearly superior.
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I did. Yes.
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OK, so I was snippy.  I am told that the measurements will be done, but
again, it isn't a high priority.
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True:  we are not an ASIC/ASSP.  That is the one area where they win
(they can make these specs as tight as they please).  But guess what?
We are growing, increasing sales, and ASICs are not.  Our real
competition now is no longer other FPGA companies; it is the ASIC/ASSP
providers.  We can supply features and circuits on technologies they
can't (yet).  Who has 10 Gbs transceivers?  Who has the lowest power
405PPC?  Who has the lowest power/highest performance DSP48 blocks for
DSP applications?  We do, they don't.
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I claim in a real system, with a compliant transmitter, there will be
sufficient return loss matching to make the eye visible, and useful.
But, I agree, that in some cases, what you see is not what you get.
That can happen with a simple single ended input pin, and is definitely
true about 1Gbs, where observing it, breaks it (often).  I think that
there is a whole class of people out there who have to see it to believe
it.  OK.  But, they should get used to the fact that none of the test
equipment is really fast enough to show them what they want to see.  And
it is only getting worse.
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Already explained why we can't do that:  35 IO standards in one pin has
to make some compromises.
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We got all that.  That is what the user's guide is for.  That is what
the datasheet is for.  Should we place a billboard on 101 South that
states the IOB pin capacitance is ~ 12pF?  It is already in the
datasheet.  So is the MGT, PPC, DSP48, etc.  What do you think we should
spend time on?
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I think all this is now covered between data sheets, user's guides, and
technical answers on our website.  Let me know if there is something
missing between those three resources.

Generally speaking, if we don't specify it, then you are on your own to
use it there.  For example, if you chose to set the resistance to 100
ohms, to match a 100 ohm single ended line, we are not going to claim we
meet any standard (there isn't any), and we aren't going to spend time
characterizing all the silicon for it.  I believe we state the range of
the resistance from 40 ohms to 150 ohms, but when you use it at anything
other than 50 ohms, you are required to check it out (I would run the
spice simulations -- you may request impedances other than 50 ohms for
the spice models of DCI, 40, 50, 68, and 75 are the ones we have if I
recall correctly), as that is not any one of the 35 IO standards that we
designed the IOB to support.

A small change, such as using the DCI at 68 ohms instead of 50 ohms is
used by quite a few (to save power).  You can characterize it if you
need to, and if you feel there is a benefit you can derive, but unusual
usage of a feature in an area it was not intended to be used (not
specified), is not guaranteed.

Re: Virtex4 running at 360Mhz DDR

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  Perhaps it is time to make some pins less "jack of all trades, master
of none", and provide some with more focus ?


Re: Virtex4 running at 360Mhz DDR

It is something we agonize over everytime we look at a new family.

Should we add IO standard specific IOB's?  How many?  How are they to be

What should the IO/CLB ratio be?

Or, should we continue with the present plans (if it ain't broke, don't
fix it)?

What business did we lose because we could not meet a customer's
requirement?  How do we know we even lost any business at all?

We did add MGTs (and PPC's, EMAC's, DSP48's, ECC_BRAM's, FIFO_BRAM's,
etc), so it isn't like we are not looking at adding new things, or
mixing things up (the patented ASMBL architecture for example).

360 MHz, 720 Mbs DDR LVDS is now over five years old as something that
either X or A has provided with their devices.  One can argue the fine
points, but as a gross capability, it has been there for quite awhile.


Jim Granville wrote:

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Re: Virtex4 running at 360Mhz DDR
Wait a minute - don't oversimply the original design critera - 720Mbps
DDR LVDS is only a part of my question.  The design also needs to run
the internals at 360Mhz, and that include portion of the fabric, not
just DSP48, etc.  Five years ago, I don't think so.  Maybe in the lab
somewhere, but not as an available product.

Re: Virtex4 running at 360Mhz DDR
No problem.

That is what all of the wonderful features are for in V4 (SSIO, IODLY,
DCM, etc.).  All of the above go a long way to support the fabric.  Even
though the fabric will run at 500 MHz, it is far easier to mux it down
to 200 MHz, or 100 MHz (using the built in SSIO features) which makes
place and route easier, and also provides a lot of margin.

Just go buy the ML450 board (network interfaces), and you will get a
fully working platform to test out all of your ~ 400 MHz up to 500 MHz
DDR interfaces.


Re: Virtex4 running at 360Mhz DDR
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Hmmm, so why do they say in Table 1. "Virtex-4 IBIS Models do Not Have
Package Information"? How can they simulate the V4 package parasitics
without this? Are you saying they're up to no good? It'd be interesting to
see the Xilinx simulation from V4 LVDS output to V4 LVDS input.
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I disagree. The Cpin of course limits the rise time at the pin. But also,
and maybe worse, the capacitor at the input reflects a whole bunch of energy
back down the T-line. The bigger the cap, the more energy is reflected. Some
of this energy comes back again to the receiver after hitting the Cpin at
the Tx end of the T-line. This causes inter-symbol interference. (Were you
running your sim with a perfectly source terminated transmitter?)
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Stop changing the subject! You should be a politician! ;-) I did say your
Rocket I/Os were a solution! And I bet they don't have 12.5pF of
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I'm pretty sure you're agreed IBIS simulations are a good idea. Works in
simulation, should work in real life, right?
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Yep, Xilinx make great parts. They'd be even better with less Cpin though...
Bloody customers want it all! ;-)
Cheers, Syms.

Re: Virtex4 running at 360Mhz DDR
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Make up your mind Austin. On numerous occasions you have recommended
that people run simulation of I/O systems to see what should happen,
and you have recommened the IBIS models. To suggest that Altera does
not know how to run simulations is insulting. Enough!

Philip Freidin

Philip Freidin

Re: Virtex4 running at 360Mhz DDR

1.  They ignored the top comment lines of the IBIS model which instructs
them how to model the package (since package modeling is incorrect and
wrong in IBIS 3.2).

2.  They used an external resistor instead of the internal termination.

Run it right, or not at all.


Re: Virtex4 running at 360Mhz DDR
Hi Austin,

Well, things are getting a little less busy with my day job, so I finally
have time to start replying again... I figured I'd start with an easy one.

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Yes.  Stratix II has LVDS running at 1.3 Gbps reliably across process,
temperature, voltage.  Beautiful eye diagrams.  In simulation and on the
board.  And as noted here
( ),
we will be increasing the spec to 1.25 Gbps in an upcoming version of
Quartus II.

BTW, our simulations line up very will with board measurements.  We offer
accurate IBIS models that we proudly stand behind.


Paul Leventis
Altera Corp.

Re: Virtex4 running at 360Mhz DDR
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Does that mean in Stratix II I could run an internal clock at 625MHz and use
the I/O DDR to move data out at 1.25Gbps?
Thanks, Syms.

Re: Virtex4 running at 360Mhz DDR
Hi Symon,

There is a hard serializer/deserializer circuitry available for the
left and right LVDS I/O banks.  These SERDES blocks allow you to
deserialize/serialize by any factor between 4 and 10x.  For example,
you could bring in a 4x data bus running at 312.5 Mhz.  Or you can
bypass the SERDES block and use the DDR registers for a 2x SERDES.  Or
bypass completely for 1x... but not at 1.25 Gbps.  I don't know what
speed the SERDES/DDR I/O clock can run at or will run at when we update
this specification.  I'm sure it will be published at the time.

We also have dedicated Dynamic Phase Alignment (DPA) circuitry for
source-synchronous applications.  The DPA block enables you to
eliminate channel-to-channel and clock-to-channel skew.  It achieves
this by selecting the best clock phase to use for each I/O pair,
centering the sampling window in the eye.


Paul Leventis
Altera Corp.

Re: Virtex4 running at 360Mhz DDR

According to the data sheet, you can run the LVDS I/O up to 500 MHz in
the fastest speed grade part.  That would get you 1 Gbps.  More likely,
you would use the SERDES.  For example, at 130 MHz and using x8
serialization, you get 1.04 Gbps per pair.  Here is a link to the DPA


Re: Virtex4 running at 360Mhz DDR
Paul and John,
Thanks very much for your replies! So, for 1.25Gbps I'd need to use the
SERDES. I guess that means I have to use the PLL circuit to make the clock?
If I had more than 1 of these links, how easy is it to ensure that they're
all synchronised together. For example,
I want to send bits a_1, a_2, a_3, a_4 etc. on I/O LVDS_A
I want to send bits b_1, b_2, b_3, b_4 etc. on I/O LVDS_B
I use the serdes to do this. Can I ensure that a_n appears at (more or less)
the same time as b_n? I.e. that the shift registers in the two serdes are
I know, I should read the bloody manual more carefully, but I couldn't find
this on a first pass.
Thanks, Syms.

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Re: Virtex4 running at 360Mhz DDR
Hi Symon,

Sorry for taking so long to reply.

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That's what the SERDES block is for.  You just need to instantiate a
altlvds_rx (receiver) or altlvds_tx (transmitter) with the number of
channels you want in the link.  Each of the channels will share a common
PLL.  Therefore, they share a common clock, and the enable pulses derived
from that clock.

And if you want to give the manual another stab ;-), I've been told that
volume 2, chapter 5 of the Stratix II handbook, "High-Speed Differential I/O
Interfaces with DPA in Stratix II Devices" is helpful.
Figures 5-2, 5-11 and 5-12 are most applicable in this case.


Paul Leventis
Altera Corp.

Re: Virtex4 running at 360Mhz DDR
Many thanks!

Re: Virtex4 running at 360Mhz DDR
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According to our engineer who ran the sims, we did use on-chip termination
for both V4 and Stratix II.  I read the whitepaper again
( ) and I can't
find anywhere where it says we didn't use on-chip termination.

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Sorry to hammer on this again, but the above mentioned whitepaper does show
some beautiful eye diagrams for SII and some ugly ones for V4.  It also
shows how nicely our lab measurement (of 1.3 Gbps LVDS on Stratix II)
compares to the IBIS simulation.


Paul Leventis
Altera Corp.

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