Incorporating FPGAs on PCBs

O. Olson wrote:
> Hi,
> 
> 	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 don�t know how to start this � 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 � 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 � I have yet to figure out which
> pins I need to power at the minimum to get this to work. So I don�t
> want to start laying out a PCB Board immediately.
> 
> 	I would be requiring significant on board communication � but I don�t
> think I need the Rocket IOs that are available with Virtex4 � 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.
> O.O.

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

> Dear Enes,
>
>         I am a student and looking into the possibility of taking this up =
for
> my thesis. So actually I don=92t 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....

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

-enes

On May 16, 10:34 am, Rob Gaddi <rga...@technologyhighland.com> wrote:
>
> 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

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.
O.O.

On May 16, 2:29 am, "O. Olson" <olson_...@yahoo.it> wrote:
> Hi,
>
>         I need to interconnect two or four FPGAs on a PCB, and I am lookin=
g
> 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 don=92t know how to start this =96 but I have a few questions. I=
s 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 don=92t
> want to start laying out a PCB Board immediately.
>
>         I would be requiring significant on board communication =96 but I =
don=92t
> 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 th=
e
> 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.
> O.O.

BTDT.

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,
G.

If you program all of the devices off of one

Enes Erdin wrote:

(snip)


> 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.

You should look at VHDL like hardware in text form.

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

-- glen

On 17 May=FDs, 03:35, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:

> You should look at VHDL like hardware in text form.
>
> If you look at it like software, you will get confused.
>
> -- glen

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.

--enes

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


On May 16, 5:29 am, "O. Olson" <olson_...@yahoo.it> wrote:
> Hi,
>
>         I need to interconnect two or four FPGAs on a PCB, and I am lookin=
g
> 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

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.


>         I don=92t know how to start this =96 but I have a few questions. I=
s 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 don=92t
> want to start laying out a PCB Board immediately.

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.


>         I would be requiring significant on board communication =96 but I =
don=92t
> 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??

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.


>         I have so far used FPGAs on the protyping board that comes with th=
e
> 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.

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.

> 	I need to interconnect two or four FPGAs on a PCB, and I am looking

	Why not just a bigger FPGA ?

> 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

	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).

> 	I don’t know how to start this – 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 – I have not

	Bread board ? Nope. Fire up Altium, make a PCB, and use for instance  
protoexpress.com, 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.

> heard of this though. I have looked at the manual of the Virtex II,
> and there are a large numbers of pins – I have yet to figure out which
> pins I need to power at the minimum to get this to work.

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

> So I don’t
> want to start laying out a PCB Board immediately.
>
> 	I would be requiring significant on board communication – but I don’t
> think I need the Rocket IOs that are available with Virtex4 – 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??

	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.