Incorporating FPGAs on PCBs

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    I need to interconnect two or four FPGAs on a PCB, and I am looking
at the prospect of designing these boards myself. If any one has done
this, I would be grateful if you could provide some pointers,
especially links to websites that have this information. I would
probably be using the Xilinx Virtex II

    I don92%t know how to start this 96% but I have a few questions. Is it
possible for me to simulate the setup between FPGAs connected on a PCB
board. Or is it possible for me to bread board the FPGA 96% I have not
heard of this though. I have looked at the manual of the Virtex II,
and there are a large numbers of pins 96% I have yet to figure out which
pins I need to power at the minimum to get this to work. So I don92%t
want to start laying out a PCB Board immediately.

    I would be requiring significant on board communication 96% but I don92%t
think I need the Rocket IOs that are available with Virtex4 96% the
simple LVDS would do for me I guess. Is there a way for me to test
this aspect before actually putting it on the PCB??

    I have so far used FPGAs on the protyping board that comes with the
Spartan Kit from Xilinx. I have also used an Emulation machine with a
couple of FPGAs. In all of this I have never been concerned about the
external connections between FPGAs, so I am new to all of this.

Any help is welcome.

Thanks a lot.

Re: Incorporating FPGAs on PCBs

why would you want to use Virtex-II on a new board? They are more
expensive than the newer parts. Check out the various Spartan-3
variants and Virtex-5LX.

You need to supply power more or less to all power pins and you
definitely should use a board with at least 4 layers.


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Re: Incorporating FPGAs on PCBs
Thanks for pointing this out that the Virtex II might be costlier than
newer parts.

    I understand that powering up of the power pins should do 96% however
there are so many pins and it is possible for me to make a mistake. Is
there some tools that can verify this 96% or allow me to simulate this,
so that I don92%t make obvious mistakes. I don92%t think that the
developers of FPGA Boards do it without some kind of verification. I
am new to this stuff so I might be missing something obvious here.

Thanks again.

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Re: Incorporating FPGAs on PCBs
Actually we check it by eyes. You know, one designs others check! It
is a matter of experience I think.

Re: Incorporating FPGAs on PCBs
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Thanks for your response Enes. So do you guys ever breadboard your
stuff 96% or it is direct on the PCB?
Thanks again,

Re: Incorporating FPGAs on PCBs
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Although I am not so much experienced about these topics in my answer
I am referring to -experienced- people who designed boards in the
past. Also I must say that I have never been in a design which uses
multiple FPGA's but we designed FPGA+Microcontroller boards. As I
guess they are more or less the same thing.

We directly produce the PCB and correct the errors in the next
revisions. Besides power issues (misconnections) there are -I think-
more important topics such as using the IO standards. Misconnections
in the power pins can be corrected by examining the design multiple
times. For example by grouping the power pins in the netlist you can
easily route them without error. But if you do not specify your
standards before PCB design you can get stuck easily. Only a small
example, if you try to use _CC_ pins as output in LVDS -I am referring
to Virtex4- you certainly get into trouble but they work perfectly if
you do not use LVDS.

Unfortuantely I don't know a software checking for these errors at a
system level.

I don't know whether you know or not these issues but I hope it helps.

Re: Incorporating FPGAs on PCBs
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Thanks Enes for your information. This is exactly my problem. I have
designed PCB Boards with only microcontrollers and some memory chips 96%
but this is the first time I am doing FPGAs. So I have Zero
Experience. Also I am not aware of the standards 96% that92%s why I was
hoping for some kind of tool, before I actually get to putting it on
the PCB. I feel that this would be very difficult without a tool.

Thanks again.

Re: Incorporating FPGAs on PCBs
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I think it will be easier than you think :)

First try to define your needs than make meetings with your
colleagues. Or are you also the guy who writes codes for FPGA? If your
design is not too complex it will be easier.

Your needs are well-defined in data-sheets and also too many topics
are discussed in this group about I/O things.

I advise you to look at the pinout specifications first. Also look at
the DC and switching characteristics too. As an experience of mine, in
one of our designs we needed a 20 A rush-current for an FPGA. And
things like this...


Re: Incorporating FPGAs on PCBs
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Dear Enes,

    I am a student and looking into the possibility of taking this up for
my thesis. So actually I don92%t have any colleagues. Yes, I would be
also writing the code for the FPGAs myself  96% and I am going to try to
keep the design simple.

    However as I am alone, and without much experience in the area, I
would first like to get a feel of how complicated this would be. Is
there anyway I get hold of the stuff you are referring to: for e.g. I
did not expect there would be a 20 A rush-current on an PCB Board.
Also I don92%t have much experience with LVDS 96% though I know what it is
and have seen others use it. So is there some place I can dig for the
concerns that you are brining up.

Thanks again.

Re: Incorporating FPGAs on PCBs
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Now everything changes. I am also an Msc. student and currently doing
a VHDL work but, IMHO, doing something in a master (I don't know PhD)
in both hardware and software -I am looking at VHDL like software-
will be very tiring. If you try to observe some characteristics of the
FPGA's I don't have any idea about it but for me buy a kit and
continue your work on it.

Of course these are my thoughts. And if we talk about LVDS it is not
different from usual signals only two wires will be routed and you
will tell the FPGA that these pins are LVDS signals, that is instead
of a LVTTL or LVCMOS buffer you will use an LVDS buffer that's all.

I hope you success


Re: Incorporating FPGAs on PCBs


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You should look at VHDL like hardware in text form.

If you look at it like software, you will get confused.

-- glen

Re: Incorporating FPGAs on PCBs

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Hi Glen,

Yes you are right. Perhaps I could not tell what I think. As I
understand O.Olson has no FPGA experience.

I think producing a PCB with multiple FPGAs and writing FPGA codes and
verify that the system is working -only one person is doing all of
these- is a tiring job. Am I wrong?

Thank you.


Re: Incorporating FPGAs on PCBs
On Fri, 16 May 2008 02:29:52 -0700 (PDT), "O. Olson"

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Here is one approach to breadboarding with multiple FPGAs, which may
allow you to prototype your system before going to your own PCB.

Since you are using the Spartan kit, this board
is probably the most suitable component for the above approach.

They can also supply PC104 form-factor boards stackable the same way.

- Brian

Re: Incorporating FPGAs on PCBs
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Just to step back a tick, are you sure that what you need is
interconnected FPGAs?  If the design you're talking about could be made
to work with two Virtex IIs, then you can probably get it all into one
larger Virtex-4 or 5 and save yourself a whole lot of grief.

Rob Gaddi, Highland Technology
Email address is currently out of order

Re: Incorporating FPGAs on PCBs
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Thank you for your reply Rob.
    I am a student. Actually I do not know what the design is going to
be. We are trying to implement some sort of multiplexing/routing
strategy to increase/facilitate the interconnections between designs
on FPGAs.

Thank you for your suggestion though.

Re: Incorporating FPGAs on PCBs
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I built a board with 16 FPGAs on it years ago because there just
wasn't one big enough to do the job.

Last time I used the Xilinx 4000 series, I had to use 3 parts to fit
all my logic.

The hardest part is chip to chip communication.  Even with a master
(board) clock, you need to treat each chip as being in its own clock
domain.  And inter-chip communication is much slower than intra-chip.

The current spike as they all come out of program mode can't be
ignored either.  (At least I was not able to ignore it).  Programming
all of the parts off of one configuration device seemed to cause them
to exit program mode sequentially, distributing the spike a little.

Good luck,

If you program all of the devices off of one

Re: Incorporating FPGAs on PCBs
Some of the other posts seems to have touched on the issues I want to
discuss, but not in much detail.

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Someone did point out that this part is ancient technology and you
would be better served with a newer device.  Also, don't feel like you
need to limit yourself to Xilinx parts.  Altera parts and software are
just as good.  But you need to ask yourself a few questions.

Why do you think you need multiple FPGAs?

Why do you think you need to build your own hardware?

Both of these design decisions will result in extra work... work which
can interfere with getting results for your thesis.  You want to focus
on your work and not on the details of building hardware or getting
multiple chips to communicate or even load together.

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There are two kinds of simulation that you might be talking about.  If
you mean digital logic simulation, then yes, any HDL simulator can
simulate multiple FPGA chips just as well as a single chip.  A test
bench is typically used to tie together the different pieces of a
design and can tie together multiple chip designs.

If you mean to simulate the electrical signals between the chips, then
yes, that can be done too.  It will take a board signal simulator such
as Hyperlynx.  These cost some real money though.  I don't know if
there are educational versions available or not.

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Before designing your board, you need to decide exactly what it is
that you need to do with it.  To make a decision about the I/O, you
need to know what it will be interfacing to and the details of the
signals.  Don't just pick an interface because is sounds good.

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I would ask why you think you need to be concerned with the design of
an FPGA board?  There are lots of FPGA boards available and you can
get a dozen or so at less cost than designing your own board and
building it.  Most FPGAs that you will want to use are BGA packaged
which requires professional assembly.  Some amateurs have had success
with the larger pitch parts, but I don't recommend it.

Have you looked at the boards available?  Do you know how large a part
you intend to use?  Do you know what I/Os you need for the external
interfaces, if any?

For doing research work, I would follow the KISS rule.  Don't try to
design stuff you don't really need.

Re: Incorporating FPGAs on PCBs

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    Why not just a bigger FPGA ?

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    A message of hope : I just built a proto with a Spartan-3E 500, SDRAM,  
Ethernet MAC, had never designed a 4 layer board before, and it worked.  
Everything works. The only problem was the JTAG which turned out to be a  
faulty parallel cable. Ironically hand soldering the PQ208 was a lot  
easier (5 minutes) than debugging the JTAG (two days).

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    Bread board ? Nope. Fire up Altium, make a PCB, and use for instance, or get a readymade Spartan board from Enterpoint, etc.
    Actually you don't really need breadboarding... the FPGA will work. It's  
your switchmode regulator and other crap that won't work.

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    Powering all the power pins seems like a good idea...

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    As long as you have a FPGA and traces that go to it, you can make it work  
    Sleeping with the xilinx datasheets is a good idea too.

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