Building a board with Spartan 3 FPGA.

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Hi, I am trying to build a very simple Spartan3 - based FPGA board.
The idea is to have FPGA + Flash PROM & Jtag header and I/Os.
Nothing else.

I am running into a problem:
Using my DMM, VCCAUX & VCCO seem to be shorted to GND.
At this point I am absolutely sure that they are shorted through the
Xilinx chip.

Does Xilinx chip need to be programmed with some kind of default
configuration, before it can even be powered on?
In other words, should there be some kind of *safety configuration*
programmed into the Flash PROM, prior to applying power to Xilinx FPGA?

If you have any references, links, suggestions, guidelines for putting
a single FPGA chip on a board with nothing other than the basics for
programming it, I would be grateful if you point me to them.

I copied my design from Digilent Spartan3 Starter Kit, getting rid of
all the *starter kit* peripherals there and just leaving the
essentials: JTAG header, Flash PROM, FPGA, expansion  I/Os

Thanks!


Re: Building a board with Spartan 3 FPGA.
no.
DMM is not good to measure but before config the VCCxx pins should take
some normal current, eg not short circuit.

doing a s3 proto is rather simple, just connect the GND, VCCxx, JTAG
and pull PROG_B thats it

Antti


Re: Building a board with Spartan 3 FPGA.
If VccAux appears to be grounded in a powered board, your problems will
never be with the device.

If VccAux never comes up, there is no configuration to do anything negative
to your board.

A newly-powered Spartan3 comes up in an innocuous state until the device is
programmed at which point it does what it's programmed to do.

Check your soldering.  Check your gerbers.  I've seen problems with both
despite the best attempts of engineers and design software.


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Re: Building a board with Spartan 3 FPGA.
-----------------------------------
doing a s3 proto is rather simple, just connect the GND, VCCxx, JTAG
and pull PROG_B thats it
------------------------------------
That's exactly whats on my board + Flash PROM.

On a fully working Spartan3 board from Digilent,
when the board is not powered up:
the VCCO and VCCAUX appear to be shorted to GND (using DMM).
Of course, this could be due to low impedance between supplies and GND,
due to high Iq through the FPGA. But the board is not powered at all.

How do I debug the shorts if I can't even test for them?


Re: Building a board with Spartan 3 FPGA.
1) Before you assemble, check for supply and ground shorts.
Too late?
2) Do a visual inspection of soldered TQFP leads with a stereo microscope
If you're doing a BGA instead, it's a much different issue.
Xray inspection of assembled boards would be needed to troubleshoot BGA
shorts; this inspection is available from most professional assemblers.

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Re: Building a board with Spartan 3 FPGA.
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Its not too late, I have multiple boards for this.
Without FPGA  (TQFP144) chip on board, there aren't any shorts.
I have checked connections with stereo microscope, no solder bridges.

I think the next thing I will try is to assemble everything, but the
FPGA
and verify that everything without FPGA is fine.
Trouble is that FPGA will be harder to solder with all the stuff on the
board already.
But doable.

Thanks!


Re: Building a board with Spartan 3 FPGA.



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Have you verified where pin 1 is?  I got tripped up a while ago with Xilinx
using a center-of-side pin 1 on CPLD packages but a corner pin 1 on FPGAs.
Really dumb mistake, but pretty easy to do.

Jon


Re: Building a board with Spartan 3 FPGA.
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I second this as a necessary check.  The file I looked to for Xilinx CPLD
orientation guidance (last time I specified orientation for a tech) had
superfluous information.  The text was shown along with a circle (or two).
There was a specific cirle locating the pin 1 location that wasn't pointed
out as *the* unique identifying feature and was not readily visible on the
part without the right shadowing.



Re: Building a board with Spartan 3 FPGA.
http://xilant.com/content/view/35/2 /

EASY - the board as above DOES work, I used it for SPI indirect
programming testing

Antti


Re: Building a board with Spartan 3 FPGA.
EASY - the board as above DOES work, I used it for SPI indirect
programming testing
-----------------------------
Very nice but where is the schematic for it?

Thanks!


Re: Building a board with Spartan 3 FPGA.
When you say "shorted", how many ohms (without power applied) are you
seeing between VCC* and GND?  Is it a dead short (i.e. a couple of
ohms max), or is it in the ~15 ohms or more category?  If the latter,
it is probably not shorted,  and may just be the unpowered resistance
of the power system (load and/or regulator).  If it is a dead short,
and it does not exist without the devices on the board, then there are
a few options.  Solder bridges or faulty components, or you may have a
mistake in the pinout of one of the devices, and have a device ground
pin tied to the power plane, or vise-versa.

Andy


Re: Building a board with Spartan 3 FPGA.
No, they really are shorted, 1.2ohms.

I will try verifying the pins again.
Then soldering everything except the FPGA & Flash PROM to see what the
state of the pins is.

It would be very nice to compare to a really simple board/schematic
however.

Pavel


Re: Building a board with Spartan 3 FPGA.
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It's probably nothing to worry about. Here are a couple of excerpts from a
similar discussion on the SI mailing list:

The reason is because of leakage current in transistors. As process
technology progresses (180nm -> 130nm -> 90nm ...), the leakage current in a
transistor gets bigger. That is, a transistor, even when it is "off", will
have some current flowing through it. Also, you can pack more transistors
onto a chip. Therefore, you end up with tens to hundreds of millions of
leaky transistors, which leads to a low resistance when measured with an
Ohm-meter.

Because IC components are nonlinear, how much resistance you measure
strongly depends on the ohmmeter's applied voltage.  (Most ohmmeters don't
apply a voltage, so don't expect a fixed value from your meter.)

When measuring semiconductor junctions with an ohmmeter, the voltage
developed (i.e., resistance) across a DUT  will vary with the applied
current, therefore, a different reading will be indicated depending on the
scale (current source value) used.

/Mikhail



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