What is the resistance of a big FPGA for VCCINT (unpowered)

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I've just got a brand new board with a Stratix II S180 on it. Before I power
it on, I checked the power supply rails for short-circuits. I get 20 ohms on
the 1.8V supply rail and 1.2 Ohms for the 1.2V. 20 ohms does not look like a
short but 1.2 is rather small. On those supplies, I only have logic
components and decap capacitors, the power supply is on another board.

So any advice before I push a few amps in it? Short or not?
(I must say that I'm somewhat stressed by the device price ;-)

Marc



Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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Do you still have a Stratix-II device unassembled?  If you've broken the
vacuum seal pack, you'd have to re-bake anyway.  You could check the
resistance on the unassembled parts you have.  I'd tend to worry only if the
lead resistance showed up as 1.2 ohms; you're concerned about solder shorts
under the BGA.  Solder shorts between power and ground (or between powers)
is a problem.  If you can bring up the board without configuring the FPGA, a
boundary scan could tell you if there are any signals stuck at VCC or
Ground.  The point where you might damage the chip is if the signal was
shorted to a rail and driven hard for an extended length of time.

How would you gain confidence in any board that has an expensive part on it?
If you aren't set up for boundary-scan or manufacturing defect analysis, you
flick the power on and off and check for excessive warmth.  You flick the
power on... and off and check for excessive warmth.  If you can check the
current draw of the board during the power-supply ramp, you only need
milliseconds to capture the trace on the digital scope and make sure it's
around what you expect.



Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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I believe this is normal for some of the devices when you measure with a
DMM. There was a thread on this topic in the past:

http://groups.google.ca/group/comp.arch.fpga/browse_frm/thread/a241d334f3abce3/56cd0d704eb6e10c?lnk=st&q=power+ground+resistance+group%3Acomp.arch.fpga&rnum13%&hl=en#56cd0d704eb6e10c

However, I have never encountered this behaviour in the devices I've used so
far...

/Mikhail



Re: What is the resistance of a big FPGA for VCCINT (unpowered)
Marc ,

My temptation is to tell you it is bad, but I will resist the temptation!

Seriously, a ohmmeter check may be dangerous:  many use a 9 volt battery.

Testing this way immediately violates the warranty (look at the absolute
maximum ratings in the data sheet).

9 volts will kill a 1 or 1.2 volt Vcc part!

I suggest that the very low core voltages, combined with the very large
static leakage at 90 nanometers, means you may no longer use an ohmmeter
to tell you anything.

Instead, a 1.2 or 1.0 volt current limited power supply is required.

Austin

Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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My ohmmeter walked away at work, besides - I'd need an ohmmeter to check my
ohmmeter.  Don't they have high impedance outputs even when the resistance
measurement goes to the 0-20 ohm range?  As long as the measurement -
through it's drive impedance - provided less than 1.2V through the 1.2 ohms
(1 amp?!), measuring the 1.2V rail should be fine.  Negative 1.2V could be a
different matter.

- John_H



Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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No.

Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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So every time I check for a connection (sub-ohm) you think I'm shorting
the battery?

Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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No.  There's a lot of room between "shorting the battery" and
"high impedance".

I suppose it comes down to "high impedance" not having a clear
quantitative definition.

Re: What is the resistance of a big FPGA for VCCINT (unpowered)

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Ouch! Hopefully it was not the case this time :)

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Good idea, I will make one for the next boards.

Thanks to all.

BTW I powered the boards. They did not blowup and the power supplies are at
their normal levels. :)

Marc



Re: What is the resistance of a big FPGA for VCCINT (unpowered)

|> Seriously, a ohmmeter check may be dangerous:  many use a 9 volt battery.

...but only as a supply voltage, not for testing.

Typically, the test voltage is below 2V on modern handheld instruments. My
4.5-digit Metex has 1.25V in all ranges, maximum current is about 0.5mA. The
diode tester runs with 3V, the max. current is about 1.3mA. I doubt that these
currents and voltages will in any way affect the chip, especially when the
operating (and leakage) currents are larger by a few orders.

--
         Georg Acher, snipped-for-privacy@in.tum.de
         http://www.lrr.in.tum.de/~acher
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Re: What is the resistance of a big FPGA for VCCINT (unpowered)
Georg.

Good to know!

Austin

Re: What is the resistance of a big FPGA for VCCINT (unpowered)

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Marc,

1.2 Ohms is fairly normal for this device. The EP2S90 is about 2 Ohms. You
should get worried if it goes below 1 Ohm. Had one customer see 0.2 Ohm -
turned out to be a short beween VCCint and PLL ground.

Best regards,


Ben


Re: What is the resistance of a big FPGA for VCCINT (unpowered)


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Thanks Ben,

In fact I did found shorts on another board (cf link below). They were all
between pads of the LLM21 decoupling caps and around 0.2-0.3 ohms. BTW I'm
somewhat upset with this, considering that I got the boards inspected
visually and by x-rays at the assembly shop. :(

http://www.fractalconcept.com/short-pb1.jpg

Marc



Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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Hi Marc,

Be thankful that you can actually see the short, and it's not under a
part :(

Because semi parts are active, their "effective" resistance is a
function of the applied "supply" voltage. To look for fab shorts with
semi parts loaded you need a very low voltage ohm meter, sub 0.2v so
that none of the forward diode junctions will actively conduct, and
even then leakage currents can be a problem.

You will probably have to build this from something that does a very
slow sweep from 0-0.5v (to allow caps to charge and equalize) and
plots the on board voltage sweep (sensed by a high impedance
amplifier) vs a known good gold board sweep.


Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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current limited to a few hundred ma, and plot voltage/current during
sweep


Re: What is the resistance of a big FPGA for VCCINT (unpowered)
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I once found a shorted part when I observed that the voltage rail in
question was about 0.3R in the general area but 0.2R across the power
pins of one particular part (QS3384, I think it was).  I pulled it off
and the problem went away.

I was surprised at the time that I was able to distinguish proximity
to the short with a plain old Fluke 10.  With a milliohmmeter it might
be even easier.

--
Ben Jackson AD7GD
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